The pictures and diagrams in this module are concerned with river systems. The examples are from the UK and they provide good illustrations of drainage basin features and hydrological processes which lead to the development of distinctive landforms. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere" (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.A river system is an important part of the hydrological cycle in the UK. Water enters the system in the form of rain or snow that is precipitated from the atmosphere. Some of the water returns quickly to the atmosphere by evaporation and through transpiration by plants. That which remains on the ground surface, plus that which percolates underground, steadily moves through the river system until it is discharged into the sea. As the water runs through the river system it interacts with the land, wearing down hills and mountains and carving out valleys. The work done by a stream or river is directly related to the force of water flowing along its channel. This energy is related to the amount of water supplied by atmospheric processes and the gradient of the channel from source to mouth. Where stream energy is high, the river can be a powerful agent of erosion, wearing away its bed and banks and transporting a huge acquired load downstream. Where stream energy is low, deposition of the load occurs and the river bed builds up. River flow is affected by seasonal variations and local storms. A stream frequently overflows its channel (floods) following heavy rainfall or rapid melting of deep snow. Small channels may also dry up completely during drought. The character of the surface across which the stream is flowing has a direct bearing on river landforms. Where rocks are hard, landform building processes act slowly. However, a river quickly exploits lines of low resistance. Major valleys are often associated with outcrops of weak rock or geological faults. Perhaps the most important influence on river systems today are human beings. They takes water supplies from stream channels for a variety of reasons and their activities can considerably alter the natural system.
The Appearance of a Small River Basin
UK Wales River Basin Hydrology Catchment Channel Dendritic
This picture shows almost the whole of a small stream catchment in the early stages of its development. It is situated on a west facing slope in the Black Mountains of mid-Wales. Note the narrow "V"-shaped valley, steep gradient, interlocking spurs and dendritic pattern of tributary channels.
Sub-Parallel Drainage Channels
UK Scotland River Basin Hydrology Channel Sub-Parallel
On a steep slope surface water generally takes the most direct path downhill. However, surfaces are rarely uniform in character and the surface flow soon becomes concentrated into distinct channels. These appear as roughly parallel watercourses. More progressive development of one channel compared to another, probably due to a higher flow rate, causes capture by the more dominant channel lower down the slope. This results in the development of a sub-parallel drainage pattern.
A Cascading Waterfall
UK Wales River Torrent Waterfall Plunge Pool Erosion Vertical
The picture shows a spectacular waterfall, Sgwd-yr-Eira in the Brecon Beacons National Park, South Wales, and its associated plunge pool. Here the river Hepste crosses a resistant band of Millstone Grit. The cap rock lies above a weak band of shale which has been weathered and eroded in such a way that people and animals can pass behind the vertical cascade of water. A well developed plunge pool lies at the base of the falls.
Spring Sapping at a River Source
UK River Source Spring Sapping
The source of many British rivers is often undistinguished. It may perhaps be a mere marshy hollow where water-saturated soil causes an overflow sufficient to initiate a surface flow. "Spring sapping" as shown in this picture is a reflection of the gradual headward creep of a stream source. As surface run-off is directly related to rainfall, and since rainfall was considerably greater at times in the past, many streams now appear as springs some distance downstream of their former source.
A River Channel - More than Water!
UK River Channel Load Suspension Solution Bed Debris Water Boulders
A river is much more than a channel of moving water. It contains a suspension load of broken rock particles, a solution load of dissolved minerals and a much heavier bed load of pebbles and boulders. The latter is only moved along in small jumps by saltation or when extreme flood flow provides the necessary force to move it. The river will also be capable of transporting a natural biological load. The picture provides points for discussion and emphasises that the appearance of a river channel will change from time to time.
A " V"-Shaped Stream Valley
UK River Valley Torrent Spurs Steep
Where a river has lots of energy due to its position high above base level, the gradient will be steep and the river will be actively cutting down into its bed. The picture shows a typical narrow "V"--shaped torrent tract cut into part of the Longmynd plateau in Shropshire. Note the direct course of the stream, the initial development of interlocking spurs, the steeply graded channel and the steep sided valley.
A Pothole in a River Bed
UK River Valley Torrent Turbulent Pothole Erosion
The picture illustrates a feature often found in the upper parts of a drainage basin where stream gradient is high, rocks are hard and rainfall is abundant. Potholes can also occur in the lower parts of a basin which has been subjected to rejuvenation (see elsewhere in this module). Strong turbulent flow causes swirling pebbles to act like drills, excessively deepening and widening hollows in the rock forming the river bed. The corrasion pot is approximately half a metre in diameter.
Potholes in the Rheidol Gorge
UK Wales River Gorge Pothole Erosion Torrent Vertical
The picture shows a scene in the valley of the river Rheidol to the north of Devil's Bridge in mid-Wales. The process of corrasion has created huge potholes which have coalesced to produce a spectacular gorge just a couple of metres wide but several metres deep.
A Breached Corrasion Pot
UK River Pothole Erosion Vertical
Continued enlargement of a corrasion pot leads to the sides being breached, as in the example shown here. The pot has been drilled into Carboniferous limestone. The process of enlargement was assisted by chemical attack by weakly acidic water. The joint plane on the right of the pot has certainly been dilated in this way.
A Fault-Guided Valley
UK River Erosion Vertical Valley Fault
Certain weaknesses in the rock structure of an area across which a river is flowing are prone to rapid excavation. This is particularly where stream flow is strong, as in upland torrent tracts and places where rejuvenation has taken place. This picture shows a river which has cut a deep steep sided valley along the line of a geological fault.
Features of a Developing River Valley
UK River Valley Winding Gravity Fault Straight Slopes
In this picture the stream, like many others, has exploited a line of geological weakness, a small fault. Although the fault line itself was probably straight, the stream is now winding and the valley sides show signs of gravity collapse and mass movement. The development of valley side slopes is a very important factor in determining the character of a river valley.
Natural Collapse of a Straightened Man-Made Channel
UK Somerset Fen River Channel Straight Collapse Meander Banks
It is not natural for a river to follow a straight path. If, for some reason, part of the bank is weakened it collapses and debris can fall into the channel. Not all of this is carried away and this makes the thalweg (line of strongest flow) move towards the opposite bank which in turn becomes weakened by undercutting. Eventually the channel will take on a meandering form with alternating zones of erosion and deposition respectively on the outside and inside of bends. Such features are well illustrated by this picture of part of the Somerset fens. The straight cut channel has been cut by man in an attempt to improve drainage of the surrounding lowland. Despite the very gentle gradient and the sluggish moving water, parts of the bank have collapsed and if the fallen debris is not removed from the channel it will not be long before meanders begin to develop.
A River in its Valley Tract
UK River Valley Tract Meander Widening Erosion Scar Mature
This picture of a stream valley in upper Wharfedale clearly demonstrates the interrelationship between the development of meanders and widening of a valley. Note the well-defined meander scars where the river is attacking the valley sides on the outer side of a meander. In instances like this, lateral erosion becomes more dominant and the river enters the distinctive "valley" tract.
Vertical and Lateral Erosion along a River Valley
UK River Valley Tract Erosion Vertical Lateral Meander Scar
The small stream in this picture shows good examples of both lateral erosion (valley widening) and vertical erosion (deepening). Such features are typical of streams that are still quite high above base level in areas where fluvial processes have been operating for a reasonable time. The stream gradient is less steep than in its torrent tract. Small terraces and abandoned meander scars mark where the channel flowed before attaining the lower level.
Meanders on the River Wye in Mid-Wales
UK Wales River Meander Tract Valley Erosion Deposition Plain Flood
This view of the River Wye in mid-Wales can be found a mere 20km from its source. In this region rainfall is high and the river is eroding both vertically and laterally. The increasing dominance of lateral erosion is illustrated by the strong undercutting on the outer bank of the well developed meander. The point-bar on the inside bank of the curved channel is also well defined.
Erosion and Deposition along a Stream Course
UK Stream Channel River Erosion Transportation Deposition Flow
Along the banks of many small streams and rivers there is usually ample evidence of active processes causing continuous change to the landscape. In the section of a river valley shown, a shallow channel winds across a flat valley floor. In places the stream flow is turbulent because the water is not very deep and there is considerable friction with the rocks forming the river bed. Most of the stream energy has been deflected to the outside of a curve. There the channel has been scoured out more deeply and undercutting of the bank is seen. Lack of stream energy on the inside of the bend has led to the deposition of water-worn pebbles. These accumulate here unless they are disturbed by intermittent floods. In time the stream will move from left to right of the picture.
A Small (Misfit) Stream Draining a Large Valley
UK Wales River Valley Meander Misfit
The picture shows a meandering stream, the Afon Castell, occupying a broad valley on the western flanks of Plynlimon in mid-Wales. A narrow channel, small meanders and a small meander belt can be observed. There is strong scientific evidence to support the theory that stream channel width is closely related to precipitation and run-off. Rainfall is high in this region, the rocks are hard and impermeable, and the stream seems to have been able to develop its valley without interference. However, the valley occupied by the stream seems to be considerably larger than one would expect for a stream the size of the present day Afon Castell. The reason for this "misfit" is that in previous times, i.e. the Ice Age, rainfall and run-off would have been greater and the stream channel and its valley would have been wider. The wide channel no longer exists but the wide valley remains.
Meanders on the River Severn in Shropshire
UK Severn River Meander Tract Valley Erosion Deposition Plain Flood
The fluvial landscape shown here near Buildwas is mature and characterised by a fairly deep and extremely sinuous channel winding (from left to right) across a broad flood plain. There is plenty of evidence of erosion on the outer sides of meanders and deposition on the inner sides of bends. There is also a clear indication that at sometime in the near future the river will change its course. Can you identify where this will probably happen?
A River Flood Plain and Ox-Bow Lake
UK Thames River Plain Flood Ox-Bow Meander Bank-Full
The picture shows an ox-bow lake, highlighted by the bank-full flood level of the river Thames near Cirencester. At the bank-full stage a river is most powerful in its action.
A Meander Cut-Off
UK River Erosion Meander Cut-Off Valley
The river has produced a meander to the right of the picture. Recently, however, it has rationalised its course by cutting across the neck of its meander. The resulting cut-off will, in time, dry up and become a marshy depression. The essential feature here, however, is that the meander has visibly widened the valley bottom by eroding the rock which forms the right-hand valley side.
Former River Courses
UK Wales River Channel Course Change Meander Terrace
The picture shows part of the lower Rheidol valley in West Wales.Continuous evolving of meanders and lowering of successive valley floors have combined to produce the river terraces and abandoned meanders.
The Flood Plain of a Small River
UK Thames River Plain Flood LevΘe Silt Deposition
The plain tract of a river basin is frequently affected by flooding. This is due to the channel being unable to contain the amount of water moving along it, and because the land close to the river is usually low-lying. The region prone to flooding is called the flood plain. When flooding occurs, the river covers the latter and, as the flood subsides, fine silt transported by the flood water is deposited on the plain. The material that is deposited during a flood is balanced by that lost to normal gravity movement and fluvial processes at other times of the year. The coarser part of the flood load is dropped along the margins of the river channel where it builds up raised banks known as levΘes. The picture shows small scale flooding of a tributary of the Thames in the Cotswolds. Note the flood plain and the levees firmly fixed by the roots of the trees. You can also see the abortive attempts of a local farmer to contain the flood.
The Flood Plain of a Large River
UK Avon River Plain Flood LevΘe Drowned
Extensive flooding in a river valley with a larger channel and broader flood plain is illustrated by this picture. It shows the river Avon near Tewkesbury in flood. At this point the flood plain is over a kilometre wide and much of the low-lying land alongside the river is drowned under several feet of water. The main channel follows a curve near the edge of the flood plain between the bushes in the foreground and the levΘes in the centre of the picture.
No Play Today, Playground Flooded
UK River Flood Disruption Playground
High water levels and consequent flooding in densely populated lowland regions can cause serious disruption to everyday life. It will be several days before flood waters from the nearby River Thames subside and West London children can use the playground again.
A Braided River Channel
UK River Channel Detritus Blocked Load Flood Braided
In parts of northern Britain most rivers have acquired abnormal loads. This is due to the mass of detritus dumped in their valleys by waning ice sheets and meltwater streams. The larger fraction of the river's load is only moved downstream at times of flood and, for the most time, the bed remains choked with rubble, causing the stream channel to split and follow a braided course. This picture is of the river Swale in the northern Pennines.
River Cliffs and Valley Terraces
UK Wales River Valley Terrace Cliff Erosion Lateral Vertical Change
The small stream in the Marcross Valley, South Wales, has incised into the valley bottom "head" deposits of alluvium, and is vigorously engaged in widening its channel. The stream has meandered from side to side, steadily paring back the valley sides and forming a number of distinct river cliffs. Almost level terraces above the river cliffs are remnants of an earlier valley floor which existed before the stream cut down to its present level.
Incised Meanders
UK Wales River Meander Incised Rejuvenation Change Capture
The river Teifi in West Wales formerly meandered across a flood plain about one kilometre wide. Rejuvenation occurred due to its capture by the River Rheidol which was graded to a lower base level. This caused rapid downcutting when the knick point at the point of capture retreated upstream. The incised meanders seen in this picture were formed as a result. At this point the Rheidol has cut about 50 metres into the old Teifi surface.
The River No Longer Flows this Way
UK River Valley Wind-Gap Change Capture Infill
This picture is a view of the landscape near Devil's Bridge. It shows the wind-gap marking the position of the old Teifi valley before it was captured by the river Rheidol. Thick accumulation of peat bog chokes the former valley which has also received a substantial amount of infill of weathered rock. The rock moved downslope from the surrounding hills under gravity. This process would have operated much faster during the Ice Age when vegetation was scarce and processes of weathering and erosion were more active.
The River Wye at Symonds Yat
UK Wye River Valley Gorge Downcutting
One of the things that can affect drainage channels which have been in existence for a long period of time is tectonic uplift of the land across which the river is flowing. If the uplift is slow and steady the river will cut down into the rising landscape and maintain its course. The lower Wye region has risen several hundred feet since the river established itself there. Today the river flows through a deep meandering gorge where once there was a broad flood plain.
Features at the Mouth of a Small Stream
UK River Stream Mouth Delta Base Level Deposition
The mouth of a river basin determines the base level for that basin. All of the land higher than that point will be subject to fluvial processes which aim to reduce the whole catchment to base level. The nature of the basin mouth can vary. Sometimes the river flows directly into a lake, another river or the sea with little or no observable change. In other cases distinctive landforms develop. In this view of a small stream emerging from a nearby hillside into a bigger channel a good example of a delta can be seen. The delta is the result of deposition. Material, washed down from the hillside by the small but steeply graded stream, has been dropped due to a lessening gradient. Also, meeting with a large but slow moving body of water has added to a fall in energy level. Large deltas are rare in the UK but are quite common in other parts of the world.
A Model River Basin
UK River Basin Hydrology Catchment Watershed Stream Order Diagram
Rivers form an important part of the hydrological cycle in which water is perpetually transferred between atmosphere, land and sea. Each river drains a catchment area that is unique to itself. The catchment reaches the highest altitude at points furthest away from the river mouth. A watershed exists between adjacent catchments. Water from precipitation "A" (rainfall and snow) collects all over the catchment and eventually flows out of the system at the river mouth "B". Some of the precipitation is also lost through evaporation "C" or taken up by plants to be returned to the atmosphere by evapotranspiration "D". Precipitation will generally be greater in the higher parts of the catchment "E". Here surface water collects in small low order channels and these join together to form the high order channel "F" of the main river. At lower altitudes the river channel will be slight, the rocks in the valley will be well weathered and soils will be deep. Here water flow will be by slow percolation through the ground water sub-system "G". Streams in the diagram are numbered according to the system devised by Strahler. High order streams are few. The highest order reached by UK rivers is usually 5 or 6. Base level for the system is the altitude of the river at its mouth. Processes operating within the river basin combine to try to reduce the whole of the catchment surface to base level.
Flow Details of a Typical UK River
UK River Flow Hydrology Hydrograph Rainfall Discharge Diagram
The chart compares flow rates (discharge) with rainfall (precipitation) throughout the year for a typical UK river. Several important relationships can be identified:"A" A surge in flow during the winter following a period of continuous, heavy rain. "B" A low flow rate in mid-winter affected by freezing conditions."C" During the spring and early summer, rainfall and flow increase together. However, the increase in flow is less than expected due to a rise in evapotranspiration rates when plants become more active. "D" A big gap (lag) between the peaks of rainfall and flow due to infilling of ground and soil water stores depleted by summer losses."E" Steady autumn rains lead to a rapid increase in flow as plant activity subsides and ground and soil water stores become saturated.Care should be taken when studying a hydrograph of one particular year only. One year's rainfall and flow can be quite different from the next.
Meander Development in a Straight River Channel
UK River Channel Meander Point-bar Thalweg Diagram
It is not natural for a river to follow a straight path. The sequence of diagrams ("A"-"B"-"C") illustrates how a straight stream-course changes due to processes which operate within the natural channel. For some reason part of the river bank is weakened and it collapses. Debris falls into the channel. Not all of this is eroded and carried away immediately and so it causes the thalweg (line of strongest flow) to move towards the opposite bank. This, in turn, becomes weakened by undercutting leading to further collapse. Eventually the channel takes on a meandering form with alternating zones of erosion and deposition.The thalweg is indicated by the dotted line. "X" is a collapsed bank which leads to the downstream development of a point-bar on the inside of a meander bend.
Describing Meanders
UK River Channel Meander Pool Riffle Wetted Perimeter Diagram
The model diagram summarises many of the features associated with a typical meandering stream. The river has significantly widened its valley by lateral erosion but downcutting is still taking place. The basin surface is covered with well-weathered material which is gravitating slowly towards the channel. Meanders often show geometric form and various measurements can be taken when there is a need for a detailed study of a river system. The river has channel width "w" and depth "y". Important to river channel studies is its cross-sectional area and the character and length of the wetted perimeter "x-x'". Sections at "A", "B" and "C" show how the cross-section of the channel varies according to its position in relation to meanders. Deeper sections of the channel (pools) usually coincide with the outer side of a meander where strong surface flow concentrates stream energy "d" beneath the bank. This causes erosion "e". Where stream energy is low and flow is slack, as on the inside of a meander, deposition occurs. Between meanders, river energy is more evenly distributed throughout the channel. At this point, flow is less strong but it can often be more turbulent (a riffle) as there is more friction with the bed of a shallow stream. Often a raised bank or levΘe "g" is developed along the edges of the channel. This is due to deposition of the coarsest part of the river's load during recession of a flood. The remaining flood load is deposited across the flood plain "f" or discharged via the river mouth. The river is most powerful at the bank-full condition i.e. when it fully occupies the channel and is just about to spill over on to the flood plain. Other features shown on the diagram are meander radius "R", meander wavelength "L" and width of the meander belt "M". Measurements taken from several streams suggest that the meander belt is usually 12 to 16 times greater than the width of the river. Hence small streams have small meanders and large streams have large meanders. Some very mature river valleys have meandering meander belts.
The Development of an Ox-bow Lake
UK River Meander Ox-bow Lake Diagram
Uniterrupted meandering of streams often leads to the formation of ox-bow (cut-off) lakes. The diagram shows a sequence of events that leads to the formation of such a feature. Ultimately the river breaks through a narrow neck of land "X" between adjacent meanders and follows a more direct course. "A" shows the line of strongest flow (the thalweg). "B" highlights the banks which are subjected to erosion. "C" shows former banks that have become isolated from river action. "D" identifies areas of deposition in slack or stagnant water. Isolation of the lake from the main channel is further encouraged by deposition of coarse stream load in the slow moving water near the entrance to the cut-off. "P" is a reference point which allows an appreciation of how the lake relates to meander progression. This kind of lake is only a temporary feature of a river landscape. It will soon fill with silt and vegetation or it will be destroyed by the development of later channel patterns.
34
1.03
Geography
The Variety of the UK Coastline
2602
The pictures and diagrams in this module are concerned with the nature of coastlines. The examples are from the UK and they provide good illustrations of coastline features and marine and subaerial processes which lead to the development of distinctive landforms. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere" (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.The coastline is a zone of great interest. It enables us to see rocks and structures that are not normally exposed. It also shows us the wide range of processes that operate there as a result of interaction between land and sea. On the shoreline the force of waves and changing tidal levels combine to form distinctive landforms. Where wave energy is high the sea will erode the land and cliffs will be dominant. Where wave energy is weak deposition will result and the land may encroach on the sea. Waves are formed by friction as winds blow across open sea. Wave energy depends on wind strength, the depth of water and the distance of water across which the wind is blowing (fetch). Waves can be local in origin or, in some cases, are associated with oceanic disturbance due to cyclonic storms in the South Atlantic. Waves from the latter source usually have a longer wavelength and are called "swell" waves. Tides are the oscillations of water caused by the gravitational attraction of the moon and sun. When constricted in narrowing channels they can result in strong local currents. The most important influence of tides relates to continuous wetting and drying of the inter-tidal zone between high water and low water. Strength or weakness of different rocks and structures that outcrop along the coast have a direct bearing on the type and rate of landform development. There can also be seasonal variations, for instance, storms tend to be more frequent in winter. On the cliffs sub-aerial weathering processes will be dominant. Gravity collapse features will occur when natural stability is disturbed. Most coasts experience a mixture of erosion and deposition and there is a close relationshilp between the two processes. Erosion of cliffs in one place is usually balanced by deposition of beaches in others. The coastline is a zone where change takes place in a relatively short time scale but there are factors that can affect coasts in the longer term. Sea level is known to fluctuate due to climatic effects and because of isostatic readjustment of the earth's crust. A relative rise in sea level results in drowning of coastal landforms. Relative uplift creates raised beaches. Human activities on coastlines frequently relate to measures taken to protect the land from invasion by the sea. Sea walls and breakwaters are a common sight in vulnerable areas. In recent years there has been an increase in interest to conserve natural, and often unique, coastal habitats that are threatened by development.
The Coastline - a Zone of Conflict
UK Coast Conflict Erosion Deposition Change Equilibrium
A coastline is a zone of conflict between land and sea. At any point in time its form reflects both the processes operating there at that time and processes which may have operated in the past. Erosion, transportation and deposition are processes that cause change. They interact in an attempt to produce a stable coastline where land and sea are in equilibrium. That is where neither land nor sea is advancing at the expense of the other.
Corrosive Sea Spray
UK Coast Corrosion Splash Salt Cliff Limestone
Sea water is a corrosive liquid which will destroy natural and man-made materials. Sea spray is particularly destructive when it saturates permeable rocks. Salt crystals grow inside minute cracks and spaces and result in mechanical breakdown. On limestone cliffs around the UK coast sea spray creates a distinctive "splash zone" across the foreshore where the rocks are deeply pocked and fretted by corrosion. The irregular features shown on the cliffs at Ogmore-by-Sea are almost entirely the result of splashing.
Vertical Chalk Cliffs Near Beachy Head, Sussex
UK Sussex Coast Cliff Chalk Erosion Shore Platform Undercutting
Along sections of the coast where wave attack is dominant, the sea is encroaching upon the land and cliffs are usually formed. The height of the cliff depends on the level of the land surface being attacked. Frequently the cliff top terminates at a regular height corresponding to some former erosion surface, for example the 137 metre platform much in evidence in southern and western Britain. The actual form of the cliff is a reflection of rock competency. Hard rocks resist erosion and weathering and are associated with steep slopes. The chalk cliffs shown in the picture rise almost vertically from the shore, reaching a height of over 100 metres. Wave attack at the base of the cliff is responsible for undercutting. This leads to gravity collapse and the cliff retreats inland. Chalk is not a particularly hard rock, but continuous rapid erosion at the base of the cliff ensures that sub-aerial processes have little effect and the cliff profile remains steep. On the seaward side of the cliff a growing shore platform uncovered at low tide marks the base of wave action. Wide shore platforms are capable of absorbing a great deal of wave energy, in time, leading to the conditions of equilibrium.
Undercutting and Collapse of Chalk Cliffs
UK Coast Dorset Cliff Undercutting Erosion Collapse Chalk Change
Undercutting of this chalk cliff in Dorset has caused collapse. The resulting rock debris provides short term protection for the base of the cliff, but in two or three years time this particular pile of rubble will have become comminuted and be removed, thus allowing wave attack to resume.
A Stepped Cliff Produced by a Change in Sea Level
UK Coast Cliff Change Erosion
Equilibrium along coastlines can be disturbed by changes in sea level. The section of coast shown in this view is cliffs on the Gower peninsula in South Wales. The stepped feature was produced by a fall in sea level (or uplift of the land). A former shore platform is exposed at the base of the upper cliff and, in time, this will be eroded away as the sea undercuts it at the lower level. Note that the upper and older cliff has a more subdued slope due to the effects of sub-aerial processes and the non-removal of weathered debris.
Remnants of a Submerged Forest at Borth
UK Wales Coast Change Beach Submerged Ancient Forest
At Borth in West Wales the remains of a 10,000 year old forest are exposed on the beach at low tide. This indicates that sea level has risen (or the land has subsided) in this area in the past. The stumps of the trees were buried in sediment and have remained relatively well preserved until being exposed by marine processes operating at low tide.
A Landslip at Blackgang on the Isle of Wight
UK Isle of Wight Coast Cliff Landslip Change Erosion Unstable
Less competent rocks are prone to the effects of mass movement on a large scale since slopes tend to be unstable very quickly. Sub-aerial processes are almost equally as effective as wave action in producing coastal landforms in such situations. Change can be rapid where well weathered and broken rocks are transported away by the sea, leaving new sections of the coast freshly exposed to attack. If the debris was not transported away, the accumulation of weathered material would lead to conditions of equilibrium.
Folkestone Warren Landslip
UK Coast Landslip Folkestone Collapse
Near Folkestone on the south coast of England soft Cretaceous sandstones and clays have collapsed in a huge landslip. This was due to a combination of marine processes acting at the base of the cliff and sub-aerial processes acting on the rocks of the upper cliff. The feature is far from stable and the railway which crosses the landslip has been frequently repaired following further movement. One stormy day there was a major disaster here when a passenger train became derailed crossing the landslip shortly after it had begun to move.
A Geo Cut Along a Fault Line in a Coastal Cliff
UK Coast Cliff Geo Erosion Weakness
Any weakness in geological structures exposed in coastal cliffs is readily exploited by wave action. In the picture a narrow geo has been cut along the line of a small fault. In similar situations caves may be created by the enlargement of weak zones where wave energy becomes concentrated in confined spaces.
Cliff Features at Stair Hole near Lulworth in Dorset
UK Coast Cliff Erosion Weakness Blowhole Wave Energy
Here the sea has just begun to break through a barrier of resistant limestone outcropping parallel to the shore. It has readily excavated the softer sands and clays behind the barrier to the front of the picture. Large joints, bedding planes and faults determined where the sea first breached the limestone. Air trapped in confined spaces explodes violently through blowholes (weak zones in the roof or walls of caves or geos) and, as indicated by the incoming storm wave, this erosion process is still effective.
Differential Erosion of a Cliff Face
UK Coast Cliff Erosion Differential Monocline
The influence of rock structure is very important in the evolution of coastal landforms. In this picture one can appreciate the differential erosion that has taken place. The folded limestone strata appear to be much more vulnerable to attack by the agents of weathering and erosion than the adjacent horizontal beds. The upper surfaces of anticlines, and in this case a monocline, have wide joints and cracks open to the elements.
Durdle Door in Dorset, a Wave Eroded Arch
UK Dorset Coast Cliff Erosion Arch Wave Notch Energy
The picture shows a classic example of a wave eroded arch. This feature was caused by wave action at the base of a cliff-like narrow promontory. It was probably created by the enlargement of caves on opposite sides of the promontory. Note the wave-cut notch showing where the waves are active and how wave energy is concentrated more strongly on the end of the promontory than it is in the bay.
This view of Bolt Head gives an impression of the development of landforms along coastlines of erosion where the rocks are competent. Weaknesses in the base of the chalk cliffs have been attacked by wave action. A cave is seen on the right; this is an enlargement of the well-defined wave-cut notch where there is some weakness at the base of the cliff. Further to the left is a small arch formed by the enlargement of two caves on either side of the headland. On the extreme left is a stack, part of the former cliff that has become isolated due to the processes of marine erosion.
Remnants of Land Eroded by the Sea at Lizard Point
UK Cornwall Coast Stack Stump Erosion Wave
The coastline in this view has retreated some distance from its original position. Stacks and stumps are the remains of cliffs and arches that resisted erosion for a time as the rocks were very hard. Wave energy is however very strong on headlands and promontories and even the hardest rocks are eventually worn down.
A Shore Platform at Clarach in West Wales
UK Wales Coast Erosion Waves Shore Platform
The shore platform is an important feature associated with coastlines of erosion. It marks the basement of wave action and slopes gently seawards from points a few metres below the height of the highest tides. The platform is for the most part exposed at low tide. Although the platform is generally sloping, it may appear as a very rugged surface with local differences in geological detail expressed in its surface profile. The picture shows part of the shore platform near Aberystwyth. The ridges and hollows represent differences in the resistance to erosion of Silurian mudstones and shales. These are dipping at a steep angle. When parts of the resistant ridges break off they fall into neighbouring depressions on the platform. Here, mobilised by passing waves, they act like grindstones, cutting even deeper into the softer parts of the platform.
A Deep Scour-Hole on a Shore Platform
UK Coast Shore Platform Scour-Hole Erosion
This picture illustrates the powerful erosion capabilities of waves armed with hard rocks and boulders. The concentric patterns within the deep hollow on the shore platform are due to differences in colour of stratified layers of mudstone.
Sorting and Transportation of Wave Eroded Debris
UK Coast Shore Platform Erosion Transportation Boulders Sand
Wave action tends to be very strong where a shore platform is narrow in width. Storm waves vigorously attack the cliffs and parts of the cliff collapse on to the shore platform. At high tide waves wash over the loosened boulders, rolling them around and causing further break up. The wearing process is further encouraged by attack from small broken fragments of rock that are picked up by the water and hurled at the bigger rocks by incoming waves. When the worn rocks are small enough they are transported away by the sea to a new part of the coast. Sand only remains in this high energy environment in a few sheltered positions beneath large boulders.
Natural Defence Against the Waves
UK Coast Beach Sand Boulders Protection Breakers
Beach material is mostly derived from the erosion of cliffs. It can consist of coarse boulders and pebbles which are only moved around when wave energy is high, or the finest sand which is moved whenever seawater splashes over it. The finest material will also be moved by the wind. However, as most beaches consist of a massive amount of material, it would take a particularly violent storm to have any serious effect on its existence. Large boulders in fact are pretty good at protecting weak areas of the coast from wave attack. The picture shows the way in which the energy of small plunging breakers is dispersed as the waves cross a boulder-strewn beach.
The Shore Platform at Ogmore-by-Sea
UK Coast Erosion Shore Platform Folds Planed
The wave-eroded shore platform in this picture is almost 500 metres wide and shows how a sequence of shallow anticlinal and synclinal flexures have been planed off by marine action. The truncated folds appear as sinuous ledges, offset by small fault lines which traverse the platform from left to right. The platform is a dramatic illustration of cliff recession but incoming waves lose a great deal of their energy when crossing it at high water. Cliff retreat will lessen as the platform widens. Coastlines with a very wide shore platform are almost at equilibrium.
A Slope-over-Wall Cliff
UK Wales Coast Cliff Erosion Slope-over-Wall Periglacial
A "slope-over-wall" cliff is a feature which is quite common along the coasts of western Britain. This landform is a result of interaction between marine action and periglacial processes. In the first instance the normal pattern of marine erosion cut the cliff with an undercut notch and shore platform. The coast was then transformed by a glacial (Ice Age) phase with lower sea level. The old cliff profile was modified by sub-aerial processes (weathering and mass movement). The lower part of the cliff was covered by periglacial deposits which moved quickly downhill across non-vegetated surfaces. The sea then cut a new shore platform at a lower level. In the final stage, sea rose back to its former level after the Ice Age. Loose periglacial deposits were removed and then transported by wave action to other parts of the coast or to be deposited offshore. A new cliff is now being cut and the slope-over-wall profile will remain until it eventually is destroyed by wave attack beneath the retreating cliff face. Undersea processes are likely to cover up most of the Ice Age cliff. This picture shows a good example of a slope-over-wall cliff which has evolved in this way at Wallog, south of Borth in West Wales.
A Broad Bay-Head Shingle Beach in Cardigan Bay
UK Wales Coast Beach Deposition Shingle Cusp Tide
Individual beaches vary considerably in their size, form and character. Each is, however, dependent upon factors relating to the supply of beach material; the origin and nature of constituent materials, transportation factors and sometimes man's interference with natural systems. The beach shown in the picture is a broad bay-head beach composed mainly of shingle. The shingle varies in grade according to its position on the beach. Although much of the material was derived from erosion of the distant cliffs, close analysis showed that there were many pebbles of foreign origin. The latter had probably been transported into the area by ice during the Pleistocene Ice Age and were deposited as drift and moraine before being reworked by marine processes. A cross-section of the beach shows several distinct ridges or berms associated with constructive wave action at the level of the most recent high tides. The level of highest berm is usually related to wave action during the last major storm event. Also shown in the picture are some well formed beach cusps. These are indicative of interaction between swash and backwash of successive waves and the concentration of energy at regular intervals along the beach. Cusps are temporary features of a beach landscape and are easily modified by storm waves.
Constructive Wave Action on a Beach
UK Coast Deposition Beach Shingle Sand Cusp Wave
The picture shows a closeup of beach features associated with constructive wave action. The beach is located in a small bay sheltered between two headlands, one of which can be seen in the distance. Fine sand has been deposited by wave action at the head of the beach. Closer to the shore which is just below high tide level is a band of shingle showing well marked beach cusps. It is easy to see how the cusps are related to the swash of incoming waves which tend to pile up slightly coarser material in curved formations. Fine material is removed by the backwash of the waves.
A Coastal Spit; the Ro-Wen Spit Near Fairbourne
UK Wales Coast Beach Erosion Long-Shore Drift Transportation Deposition Spit Re-curved
This picture shows part of the Mawddach river estuary. A combination of storm beach construction, long-shore transportation of beach material and wave refraction at the entrance to the estuary have produced the re-curved coastal spit feature. Note the oblique approach of the incoming waves. These provide long-shore movement along the main body of the spit away from the viewpoint and extending northwards into the estuary. The area on the landward side of the spit is being built up by accumulation of blown sand and vegetation.
Chesil Beach; a Storm Beach of Unusual Dimensions
UK Dorset Coast Beach Storm Shingle Tombolo Lagoon Fetch Wave
Waves approaching the Dorset coast from the south west have a tremendously long fetch and often originate from storms in the distant South Atlantic. Such waves are very powerful. Chesil Beach, shown here, connecting the former island of Portland to the mainland, clearly illustrates their constructive capability. Millions of tons of shingle have been moved around by wave action to create an embankment some 20 kilometres long, 100 metres wide and up to 30 metres high. On the landward side of the beach is a freshwater lagoon known as the Fleet. Originally this was part of a bay but is now isolated from the sea by the storm beach. Eventually the lagoon will silt up and fill in with vegetation. Spits or bars connecting islands to the mainland or to other islands are known as tombolos.
Sand Dunes at Ynys-Las, near Borth, West Wales
UK Wales Coast Dune Sand Marram Wind
Where beaches of fine sandy material face exposed coasts or where there are offshore sandbanks exposed at low tide, wind action is important. The wind, particularly strong north westerlies, picks up dry sand and carries it inland. Although in some cases transportation may be several kilometres inland, most of the sand is deposited immediately on the landward side of the shoreline in the lee of obstacles such as shingle ridges. The piles of loose sand deposited in this way are called mobile dunes. The dunes can become fixed by marram grass which is well adapted to surviving in this difficult habitat. The picture shows mobile and partly fixed dunes. Through the centre a large blow-out has been created by storm winds blowing through an area which was unvegetated. The latter probably resulted from the joint actions of wildlife (burrowing rabbits) and careless humans. Good examples of dune development can be seen at Margam, Formby, Cotham and Wells-next-Sea.
The Estuary of the River Mawddach in North Wales
UK Wales Coast Estuary River Tidal Deposition Silt Mudflat
The coast of Britain, particularly in the west, has many examples of broad shallow estuaries. In these, at low tide, mudflats and considerable accumulations of sand and silt can be seen. These features are a result of deposition in the slack waters of the estuary. Deposition is encouraged by interaction between river and tidal currents and by flocculation of clay minerals in salt water. Flocculation is the tendency for clay particles to cling together and settle under gravity.
Saltmarsh Vegetation in the Mawddach Estuary
UK Wales Coast Estuary Saltmarsh Sedimentation Silt
A slight change in sea level (lowering) or rapid infilling of an estuary can promote the growth of saltmarsh vegetation such as rushes and reeds. Once the plants have become established they increase local evapotranspiration rates, allowing the silt to dry out more quickly. The roots of the plants provide a means of fixing the silt and thereby encouraging further sedimentation. Environments like the one shown are a haven for wildlife and, as such, are frequently protected from development or destruction of the natural ecosystem.
Borth Bog; a Reclaimed Coastal Wetland
UK Wales Coast Wetland Marsh Bog Deposition Drainage Reclamation
The activities of man have drastically changed this example of a low-lying coastal wetland. The huge expanse of former coastal saltmarsh has been artificially drained and reclaimed from the estuary. Man-made drainage ditches and water pumps have been used to lower the water table. Marsh vegetation has been burned off and the area re-seeded with grass for animal grazing. The decline of natural wetland habitats has been a subject of great concern to conservationists in recent years.
Preserving the "Holiday" Beach: Bournemouth
UK Dorset Coast Beach Tourism Sand Groynes Sea Wall Promenade
Parts of Britain's coastline have been the subject of a lot of human activity. Beaches like the one shown in this picture are an important tourist amenity and need to be managed carefully to protect them from both natural hazards such as storm erosion and from over-use by holidaymakers. Groynes have been constructed to try and check the long-shore movement of sand. The cliffs of soft Tertiary sandstones were once a source of beach sand. They have now been graded and stabilised. This is to lessen the risks of mass movement induced by sub-aerial processes acting on the steep unstable slopes. The base of the earlier cliffs is now protected by a concrete sea wall and promenade. Losses of sand from the beach are replenished with sand brought in from elsewhere (by lorry) as required.
Rough Sea on the Channel Coast
UK Kent Coast Beach Sea Storm Groynes Waves
Most of us think of the seaside as a place of enjoyment with little danger. That is because we tend to visit coastal resorts at times of the year when it is not usually stormy. Storms are of course frequent throughout the period from October to March. This wintry scene at Hythe on the Channel coast shows storm waves pounding the beach. The latter is being attacked by obliquely approaching plunging breakers which erode the shingle and transport it towards the east. Groynes or breakwaters have been constructed in an attempt to control such movement.
A Diagram Illustrating Long-shore Drift
UK Coast Diagram Drift Long-shore Beach
The diagram explains the phenomenon of long-shore drift. This is commonly found on Britain's beaches and is responsible for them being referred to as rivers of sand. The white arrow indicates the general direction of the incoming tidal surge which causes waves to approach the shore obliquely. Sometimes the direction of the prevailing wind reinforces this situation. The incoming wave washes loose material up the beach at an angle. After the wave has broken pebbles move back down the beach at right angles to the shore. In time a pebble will follow a path from "L" to "M" along the beach.
A Diagram Illustrating the Development of Dungeness
UK Coast Foreland Cuspate Dungeness
Storm beaches invariably become aligned perpendicular to the direction of the prevailing wind. Sometimes this causes features to grow out from the original coast. The landform which develops is called a cuspate foreland. A good example is at Dungeness on the channel coast. The diagram shows the development of successive shingle ridges across a former bay. "A" is the oldest ridge and "H" is the most recent. "1" is a zone of erosion and "2" a zone of deposition. "3" relates to the maximum fetch of storm waves and "4" the direction of the prevailing wind.
The Formation of a Slope-over-wall Cliff
UK Coast Cliff Erosion Slope Wall Periglacial Change
The diagram illustrates the formation of a slope-over-wall cliff. This landform is a feature of erosion and is commonly found along the coasts of western Britain. It results from the interaction between marine and periglacial processes dating from pre-glacial times. In Stage 1 the normal pattern of marine erosion has produced the following features:"A" the cliff with an undercut notch."B" a wave-cut shore platform."C" marks the former profile. In Stage 2, a glacial phase with lower sea level, the old cliff profile "A'" has been considerably modified. It was no longer attacked by marine processes except at a much lower level where a new cliff "E" is forming. The cliff slope was only acted on by sub-aerial erosion. The lower part of the profile "D" has become infilled with periglacial deposits. In Stage 3 sea level has risen again and the loose periglacial material has been easily removed. A new cliff is being cut by marine action and the slope-over-wall feature has been created at "F". The eroded debris will have been transported along the coast to a low energy environment or will have been dumped on the sea bed at "G".
The General Character of Britain's Coast
UK Coast Character Map
The general character of Britain's coast and some of the factors which influence it are shown on the accompanying map. The key is as follows:1. The direction of fetch of long-travelled prevailing winds.2. The path of the incoming tidal surge. Tidal currents eventually meet at "X". Channelling of tidal currents into the funnel of the Severn estuary "Y" produces a high tidal range and creates the Severn bore.3. Coastlines where storm waves are frequent.4. Coastlines where generally hard and resistant rocks outcrop.5. Coastlines where chalk outcrops.6. Coastlines where deposition is dominant.7. The southern limit of raised beaches associated with post-glacial isostatic uplift. South of this line drowned river estuaries or "rias" are found.
35
1.02
Geography
Past Glaciation and the UK
2603
The pictures and diagrams in this module are concerned with glaciated landscapes. The examples are from the UK and they provide good illustrations of glacial and periglacial features and provide an appreciation of the processes which lead to distinctive landform development. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere" (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.Today about one tenth of the Earth's surface is covered by ice but nowhere in the UK does ice occur as a permanent feature of the landscape. However, there is plenty of evidence that ice covered much of Britain and northern Europe in the recent past. During the height of the Pleistocene Ice Age almost the whole country was buried under a huge ice mass that extended as far south as a line from the Bristol Channel to the Thames Estuary. Even the land south of the ice front was affected by conditions of extreme cold (periglacial conditions). Sea level was lower and most of the English Channel was dry for a time. Great thicknesses of ice accumulated in the mountainous parts of the country where precipitation was higher. Huge glacial troughs extended down from the mountains and spread across adjacent lowland plains. The ice moved very slowly, perhaps only 50 metres a year, but as it went it eroded and shaped the underlying surface. Loose material of all shapes and sizes fell into the glaciers and was slowly carried miles downstream. Valleys became deepened, slopes steepened and spurs truncated. Where gravity was less and the temperature sufficiently high to melt the ice, considerable volumes of ice-transported debris were deposited. Glacial deposits changed pre-glacial landscapes just as dramatically as eroding glaciers. The source of glacier ice was not just confined to the UK. Huge ice sheets spread south from Scandinavia bringing with them a variety of rock fragments previously unknown to the UK. As the climate ameliorated the ice sheets fed a multitude of powerful meltwater streams. Processes of stream erosion, transport and deposition were greatly enhanced by high energy channel flow and a plentiful supply of sand and pebbles. The nature of river valleys in the UK, the character of the UK coastline, and many examples of rock weathering and gravity collapse of steep slopes owe a lot to patterns and processes which were active in the Ice Age.
The Ice Age Almost Re-Created
UK Cumbria Glaciation Ice Erosion Lake Frozen
This picture shows the view looking up the valley now occupied by Derwent Water towards the distant Cumbrian Mountains. The wintry scene was captured in February and, with the frozen lake surface and distant snow covered mountains, it is easy to draw comparisons with what the landscape was probably like during the Ice Age. At that time, of course, there would not have been any trees and a massive glacier would have filled almost the whole of the valley.
The Lake District Landscape, a Legacy of the Ice Age
UK Cumbria Glaciation Moraine Boulder Clay Ice Erosion Deposition Lake
This view of part of the English Lake District in Cumbria is typical of highland Britain. It shows plenty of evidence that glacial processes were responsible for shaping the landscape. Distant mountains supported snowfields, and these fed the glaciers which moved slowly down the valleys. As the glaciers melted they deposited moraine and boulder clay on the valley floor. In other places the enormous weight of ice, armed with trapped rocks and boulders, scraped deep into the landscape. This process polished surfaces and left huge hollows occupied by modern day lakes. The view shows a wintry scene a few kilometres south of Keswick.
Great Langdale, a Glacial Trough
UK Cumbria Langdale Glaciation Trough Valley River Misfit
The picture provides an oblique aerial view of Great Langdale in the Lake District. It shows the character of a typical glaciated valley. Note the "U"-shaped cross-section, the relatively flat valley floor supporting better quality pasture (the inland) and the steep valley sides with rough grazing (the intake land). It is easy to imagine the valley once filled with a massive glacier curving away towards the distant lake. Glaciers move very slowly compared to streams. The cross-sectional area of the glacier needs to be about 10,000 times greater than that of a stream draining the same area. Today's stream, therefore, looks tiny (a misfit) when compared to the valley.
Ice Smoothed Rocks and a "U" Shaped Valley
UK Cumbria Glaciation Erosion Ice Smoothed Valley Scree U-Shaped
Huge quantities of ice moved outwards from the highland snowfields, modifying earlier river-carved valleys on its way. Less resistant obstacles and surface soils in its path were picked up and carried along within the slowly moving glacier. The rocks of the channel floor were polished and smoothed by the combined action of the ice and rocks it was carrying. As ice moves much more slowly than running water, the cross-sectional area of the ice channel needs to be something like 10,000 times greater than the cross-sectional area of a stream draining the same catchment. For this reason ice occupied the valley to a considerable depth. Widening, deepening and straightening were commonplace. Spurs that projected into the earlier river valleys were truncated. Tributary valleys graded to the earlier river level now hang high above the floor of the glaciated channel. The ice eventually melted as the climate improved, leaving steep unsupported valley sides. Screes forming at the base of these steep slopes and material deposited by the waning glacier tend to modify the typical "U"-shaped cross-section of the ice channel. The picture shows ice-smoothed rocks in the foreground; these formed when ice spilled over a rock barrier before passing along the formerly glaciated valley. The valley is now occupied by Watendlath Beck in the English Lake District.
Steep Slopes and Impeded Drainage
UK Wales Snowdonia Glaciation Slopes Lake Valley Drainage
The picture provides an impression of the high incidence of steep slopes and impeded drainage in a glaciated upland region. The view is of Llyn Gwynant in Snowdonia. The steep slopes resulted from excessive downcutting by valley glaciers. Sometimes the glaciers cut deep hollows into the landscape and these hollows are now partly occupied by lakes. When glaciers retreated at the end of the Ice Age, irregular masses of moraine were deposited across the ground surface. This then impeded drainage and created more lakes and areas of marsh.
Scree, Ground Moraine and a Roche MoutonΘe
UK Glaciation Erosion Smoothing Moraine Scree Roche MoutonΘe
This picture shows a typical scene in an upland region that has been glaciated. There is a steep valley side partly concealed by scree, and plenty of grass-covered ground moraine. In the foreground is a weathered remnant of ice erosion - a roche moutonΘe. Close examination of the shape, alignment and surface striations of roche moutonΘes provides evidence to help reconstruct patterns of ice movement.
A Cirque in Winter
UK Cumbria Glaciation Erosion Cirque Snow Arete
This wintry scene in the Cumbrian Mountains highlights the form of a north east facing cirque. To some extent it illustrates what probably happened during the Ice Age. Snow was driven by the prevailing south-westerly wind and affected by eddies as it passed over mountain tops. It collected on the sheltered side of the mountain and soon became compacted and compressed due to successive accumulations. The weight of the thick mass of overlying snow caused the underlying layers to melt but, as the environment was extremely cold, the melted snow inmmediately re-froze to form blue ice. This ice then began to slide downhill aided by gravity. It polished, deepened and steepened existing valleys as it went. Coarse angular rocks, loosened by freeze-thaw processes acting on the higher parts of the mountain, became embedded in the ice. Most of the material fell into the large crevasse structures that opened up in summer. It was then carried along by the moving ice as sub-glacial moraine. In the post-glacial situation several distinctive features can be seen; the steep concave cirque headwall; scree that has collected beneath steep slopes; and a sharp ridge, called an arete, between the cirque headwall and an adjacent valley.
Llyn Cau, a Lake Enclosed Within a Cirque
UK Wales Snowdonia Glaciation Cirque Headwall Lake Tarn Scree
A good example of a cirque (corrie) is provided by the deep hollow occupied by Llyn Cau on Cader Idris in Snowdonia. The summit shown in the picture is some 350 metres above the level of the lake. Huge fans of scree have accumulated at the base of the steep headwall and have partly infilled the lake. In the foreground the hummocky terrain marks deposits of moraine which enclose the lake on the open side of the cirque.
Moraine and Scree Around Llyn-y-Gadair
UK Wales Snowdonia Glaciation Cirque Scree Lake Tarn Change
Llyn-y-Gadair lies to the north of Cader Idris in Snowdonia. The lake is here viewed from the summit of the mountain. The scene shows a typical tarn sitting in an ice-eroded hollow at the foot of a very steep glacier headwall. The lake is partly infilled with scree. It is enclosed with a lip of terminal moraine and a polished rock step on the open side. Sedimentation processes acting around the edge of the lake are causing a slow but gradual reduction in the size of the tarn.
A Nivation Corrie, Cwm Amarch
UK Wales Snowdonia Glaciation Cirque Nivation Landslip Periglacial
Not all cirques are created by the process of ice erosion. In the case of Cwm Amarch there is very little evidence of ice-smoothed or ice-gouged rock, neither is there much moraine. This is, in fact, a cirque-like hollow created by successive landslips under severe periglacial conditions. The hollow faces south and is cut into relatively soft shales. These collapsed under the influence of gravity when saturated with an excess of meltwater during the summer.
A Rock-Strewn Valley Floor
UK Glaciation Rock Ground Moraine Fall
The picture shows what a typical glaciated valley floor looks like. Not only does the valley floor drop in a series of very steep steps, but it is also littered with rocks. Most of these are angular and sharp-cornered, suggesting that they have moved only a short distance from their point of origin. Some of the rocks will have accumulated from the erosion of moraine. The finer components of the latter are easier to remove by transporting agents, leaving larger boulders concentrated on the surface. Others have simply fallen under gravity from surrounding rock outcrops, having been loosened by frost action.
The Nature of Moraine
UK Glaciation Moraine Deposition Boulder Clay Unstratified
This picture is a close-up showing the nature of moraine. When glaciers and ice sheets melted they frequently left an unsorted mixture of angular boulders, gravelly material and fine clay piled unevenly (unstratified) on the ground.
The Transportation of Rocks by Ice
UK Yorkshire Glaciation Transportation Ice Erratic Erosion Deposition
In some instances boulders and rocks became embedded in glacier ice and were then transported considerable distances from their source. They were even carried across prominent relief barriers. The picture shows a large Silurian gritstone boulder which has been transported by ice. It was deposited on top of younger Carboniferous Limestone at Norber in North Yorkshire. The more resistant boulder has protected the limestone on which it is sitting. The surrounding limestone has been affected by the normal processes of denudation and it is possible to estimate how much the land surface has been lowered since the Ice Age. Unlike running water, ice can flow uphill and over obstructions (provided the upper surface of the ice sheet slopes in the direction of movement). Rocks which are transported a long way from their source are called erratics. On the east side of Britain erratics have been examined and proved to come from Scandinavia. Rocks from Northern Ireland and Scotland are also found on beaches in Wales.
Freeze-Thaw Debris and Ice Smoothing
UK Glaciation Weathering Scree Ice Smoothing Freeze-Thaw Moraine
The picture shows several features typical of glaciated upland valleys. In the foreground, partially covered with hardy grass, is a good example of scree or talus. This is the result of freeze-thaw processes prising away large angular blocks from exposed rock faces. Such processes were particularly effective during the Ice Age, acting on exposed rock surfaces which penetrated the covering of snow and ice. Lower down the slope is a rocky outcrop that has been smoothed and polished at its point of contact with the ice. Deep in the valley below is a tarn, a moraine ridge, a rock step and considerable amounts of ground moraine.
Steep Slopes and a Landslide
UK Wales Snowdonia Glaciation Deepening Valley Slope Landslip
Glaciers were capable of cutting very deep and steep-sided valleys. In any ice-eroded landscape there will be examples of slopes that are over-steep and not at equilibrium with their environment. In such cases rock falls and landslips are not uncommon. In this picture a huge scar can be seen on the opposite side of the valley. This has resulted from gravity movement of a large mass of relatively soft mudstones and shales. They were left behind on an unstable slope when a valley glacier melted. The landslip blocked the valley, impeded drainage and led to formation of the lake, Tal-y-Llyn.
Tal-y-Llyn, a Formerly Glaciated Valley
UK Wales Glaciation Valley Lake Alluvial Fan Delta Truncated Spur
The picture shows a good example of a formerly glaciated valley. Features due to ice erosion can be seen - truncated spurs and a hanging valley. There are also features formed by other processes acting on the newly glaciated landscape - the lake dammed by a landslip, the alluvial fan and the delta.
A Hanging Valley and Post-Glacial Alluvial Fan
UK Wales Glaciation Erosion Hanging Valley Deposition Alluvial Fan
Hanging valleys result from the deepening of main valleys by eroding glaciers. A tributary valley originally graded to the level of the pre-glacial landscape will have had its lower end completely truncated by the passage of ice along the main valley. It now appears to hang on the side of the deeper main valley and waterfalls and torrents often cascade down to lower levels, eroding powerfully as they go. When stream flow is checked as it finally meets the flat valley floor, huge conical fans of fluvial deposits build up. These are called alluvial fans. The one shown here extends out into Tal-y-Llyn lake. Alluvial fans can grow and completely block drainage along a valley. Buttermere and Crummock Water in the Lake District resulted from a single lake that was split into two parts by an alluvial fan. It is possible that the one shown here will have a similar effect on Tal-y-Llyn at some time in the future.
Cwm Amarch; a Hanging Valley and Alluvial Fan
UK Wales Glaciation Erosion Hanging Valley Alluvial Fan Fluvial
The picture provides a closeup of features commonly seen along the edge of a formerly glaciated valley. The hanging valley is the result of removal of the lower part of a tributary valley. The latter drained into the main valley at a much higher level before the Ice Age. When the glacier, which moved from right to left across the picture, cut deeply into the landscape it effectively rejuvenated the tributary valley. Much of the material removed by the small but rejuvenated stream has been transported a short distance by fluvial processes. It has then been deposited in an alluvial fan where the stream meets the low energy environment of the main valley floor.
Moraine Crossing a Scottish Glen
UK Scotland Glaciation Deposition Moraine Valley
In the middle distance of this picture can be seen a ridge of hummocky ground crossing an almost flat-floored Scottish glen. The valley bottom is almost certainly the bed of a former Ice Age lake, now drained. The soil appears to be relatively fertile as it supports good grass. The hummocky ground, however, is less fertile and is covered with gorse and shrubs. It marks the position where a glacier paused for a time during its retreat at the end of the period of extreme cold. The pause allowed a substantial amount of moraine, released from the melting ice, to accumulate in one place where the ice front crossed the valley.
Fluvio-Glacial Deposits; an Esker in Scotland
UK Scotland Glaciation Fluvio-Glacial Deposition Esker Gravel
Eskers are a feature of meltwater (fluvio-glacial) deposition. Within and beneath any ice sheet or glacier, particularly in their zones of melting, streams will be flowing. These erode powerfully and acquire a considerable load of debris from the glacier. This debris frequently lines the sub-glacial channels much in the same way as it would line a river channel. When the ice finally melts away, all that remains are winding ridges of partly sorted gravel. These are called eskers. They mark the former paths of streams that once flowed beneath the ice. The example shown here is near Muirkirk in southern Scotland.
Hummocky Terrain Due to Glacial Deposition
UK Scotland Glaciation Deposition Boulder Clay Drumlin
Ice sheets fed by glaciers from higher regions frequently spread across adjacent lowland regions. They carried a huge load of eroded material with them and when the climate warmed up and the ice began to melt deposition began. Finely ground material became trapped beneath the ice moving slowly towards the ice front in the zone of melting. A combination of frictional forces and energy changes related to ice flow led to the development of mounds of boulder clay. These were arranged in a distinctive en-echelon (basket-of-eggs) pattern called Drumlins. The features shown here are to the south of Callander in Central Scotland.
Freeze-Thaw Frost Wedges in the Chilterns
UK Chilterns Glaciation Periglacial Freeze Thaw Frost Wedge
In zones close to the edge of great glaciers and ice sheets the land may not have been in contact with ice itself. Nevertheless, conditions of great cold would have persisted there, relieved only by momentary periods of summer melting. Such conditions were responsible for the formation of frost wedges in certain parts of southern England. Cracks in chalk rocks were gradually enlarged and almost at the same time filled in with reddish coloured coombe rock deposits. The examples shown here are exposed in a motorway cutting in the Chilterns. They are about 4 metres across at the top and extend to a point some 8 to 10 metres below the original surface.
Periglacial Head Deposits
UK Glaciation Periglacial Deposition Involutions Head
During the Ice Age, unprotected by vegetation, huge masses of debris gradually spread downhill. These eventually came to rest as head deposits. Changes in hydrostatic pressure were caused by indifferent freezing and thawing under periglacial conditions. This led to considerable movement and distortion in loose weathered materials. Head deposits show poor stratification and are frequently distorted into shapes known as involutions.
Beach Materials and Cliff Shape, Legacy of the Ice Age
UK Wales Glaciation Coast Cliff Beach Slope-over-Wall
Much beach material along the coast of West Wales has been derived from re-worked glacial and fluvio-glacial deposits. Today it occurs as the smaller component of beach rocks, the larger part of the material being derived from marine erosion of local cliffs. The shape of the cliff itself is yet another indication of glacial events. During the Ice Age a great deal of water was retained in the extended polar ice caps. Sea-level was lower than today. Cliffs that existed before the Ice Age were no longer attacked by the sea and were graded to more gentle slopes by sub-aerial processes alone. The grading of the cliff during the glacial phase can be seen by the slope at the top. This now ends abruptly at the near vertical wall of the modern cliff. The shape of the cliff is known as "slope-over-wall".
Recessional Moraine in Cardigan Bay
UK Wales Glaciation Deposition Moraine Sea
Jutting out into Cardigan Bay for over a kilometre is a narrow natural causeway exposed only at low tide. The causeway is made up of assorted large boulders, many of which are not native to the locality. The boulders have been identified as moraine and the origin of the feature is glacial, although there has been some modification by marine action. During the Ice Age sea-level would have been lower and much of the area of water seen in the picture would have been a relatively flat lowland plain. The ice spread into the area from beyond the distant mountains. Fluctuations in the flow and melting of the ice allowed the buildup of ridges of ice-transported debris along the ice front. Sometimes the ice front paused sufficiently long enough in one place for this to happen; at other times it pushed loose materials ahead of it during a temporary re-advance.
The Glaciated Coastline of Western Scotland
UK Scotland Glaciation Coast Change Valley Island Isostasy Fiord
The effect of glaciation on the coast of Western Scotland is very complex. During the Ice Age glaciers spread down from the nearby highlands and coalesced into ice sheets along the coast. Sea level was, at that time, about a hundred metres lower than today and the glaciers cut deeply into the land. When the ice finally melted. the sea flooded into fiord-like inlets. Due to isostatic readjustment after the massive weight of ice had disappeared, the land rose by several metres. Parts of the former glaciated valley floor now stand out as islands. The combination of deep water, where glacier ice gouged deeply into the landscape, along with shelter provided by the islands, has created several good natural harbours. The one shown in the picture is Oban.
Dry Valleys Originating from the Ice Age
UK Glaciation Permafrost Periglacial Dry Valley Loam Nivation
When soil and sub-soil were subjected to permafrost conditions during the Ice Age, normally pervious rocks were rendered impervious for a time. Surface drainage dominated where the surface thawed out during short summers. Under such conditions, dry valleys were cut in the chalk and limestone landscapes of southern England. Nivation processes were operating on steep north and east facing slopes. Because of this, large quantities of chalky debris were washed downhill and mixed with the clay of adjacent lowlands to form fertile loam soils.
Severity of Weathering under Periglacial Conditions
UK Glaciation Weathering Periglacial Rock
The severity of weathering under periglacial conditions can be judged from the appearance of these lava rocks exposed on a mountain top in North Wales. The conditions prevailing in such a locality today probably occurred at much lower altitudes, possibly right down to sea-level, during the Ice Age. The ground surface is almost wholly weathered rock with just a few patches of soil in sheltered places. Vegetation cover is restricted to a few hardy plants, mosses and lichens.
A Glacial Overflow Channel
UK Wales Glaciation Drainage Overflow Meltwater
Glaciers did not only confine their activity to pre-existing valleys. In some cases both the ice and meltwaters spilled over low interfluves into adjacent valleys. These overflow channels were deepened and widened and appear like ordinary valleys. Sometimes a stream or river which succeeded the glacier, or an Ice Age meltwater stream, continues to follow the path cut by its maker. There could well be an example of an abandoned valley nearby. On other occasions the overspill valley may be the feature abandoned by the modern drainage system. The picture shows an old overflow channel near Corris in North Wales. It contains two tiny streams, one draining towards the viewpoint and the other draining in the opposite direction.
Evidence for the Existence of an Ice Age Lake
UK Yorkshire Glaciation Drainage Lake Change
The picture shows the dark fertile soils and flat landscape that betray the former site of Lake Pickering in North Yorkshire. The Yorkshire Wolds in the distance formed the southern edge of the lake. Drainage of the Vale of Pickering has been considerably altered by the effects of glaciation. The flow of the lower part of the River Derwent has been almost completely reversed. Before the Ice Age the river flowed out to sea near Filey. Its exit was then blocked by ice and fluvio-glacial deposits, creating a huge lake. This overflowed via spillways to the south. Eventually the Derwent became established along its present and considerably longer course, passing through several steep-sided gorges before entering the Humber estuary. (See elsewhere in this module for a map which explains the above situation). Similar circumstances prevailed in other parts of Britain. For example, the River Severn was diverted south from its former mouth in the Dee estuary due to the cutting of an overspill gorge from Lake Lapworth near Ironbridge in Shropshire. The Warwickshire Avon was formerly a tributary of the River Trent but it now follows a westerly route to the Severn. The Vale of Evesham is the site of the Ice Age Lake Harrison.
A Map of Britain During the Ice Age
UK Glaciation Ice Pleistocene Advance Map
The map shows the extent to which Britain was covered by ice during Pleistocene times. It shows the situation at the maximum extent of the ice advance. This occurred at the peak of the Lowestoft phase about 400,000 years ago. "A" shows the extent of ice cover. "B" the location of land which would have suffered extreme conditions of cold and periglacial activity. "C" shows areas of open sea. At this time a vast amount of water would have been trapped in the extended polar ice caps and sea level would probably have been some 65 metres lower than today. The English Channel was partly dry and the North Sea almost completely covered with ice moving south from Scandinavia. "D" shows the position of the ice front at the peak of the less-extensive Gipping advance. North of this line the older moraines were re-worked. The Gipping followed the warm, Hoxnian, inter-glacial period during which the climate was almost sub-tropical. Altogether there were four advances and three inter-glacial phases recorded in Britain. "E" shows the areas to which the ice retreated during the last stages of glaciation. Some authorities support the theory that we are currently enjoying warm conditions associated with an inter-glacial phase and that, in time, the ice sheets will extend southwards again.
How Ice is Transported Over Mountains
UK Glaciation Erratic Transport Erosion Deposition Ice Diagram
In some instances boulders were picked up and carried great distances by ice. Sometimes they were transported to the other side of prominent relief barriers. It is hard to imagine how this could have happened but the diagram illustrates how boulders might have been moved from west to east across the Pennines of northern England. Blocks of Shap granite "x" occurring only on the western side of the Pennines were picked up and later deposited as erratics on the lowland plains of the east coast "y". Snow and ice "N" probably accumulated to a greater depth on the western side of the country due to on-shore prevailing winds "P" and orographic uplift. As long as the upper surface of the ice sheet was graded downwards towards the east ice will have moved in that direction regardless of the nature of the underlying terrain.
A Diagram Illustrating the Formation of Cirques
UK Glaciation Cirque Formatiion Diagram Tarn
The diagram illustrates the process of cirque (or corrie) formation at valley heads under glacial conditions. Snow, driven by the prevailing wind "W" and encouraged by eddies "e" collects in the lee of the relief feature. It soon becomes compressed under the weight of successive snowfalls, thaws then immediately re-freezes into blue ice. This ice then begins to slide downhill along pre-existing valleys, deepening them and creating steep valley sides as it goes. The base of the cirque is strongly eroded by coarse, angular rocks which become embedded in the ice and act as cutting tools. During the summer the bergschrund crevasse opens and rocks fall into it from the cirque headwall. Much of this rock debris is carried along by the glacier as sub-glacial moraine "m". In the post-glacial situation several distinctive features can be identified. "H" is the almost vertical cirque headwall. "S" is scree which has collected beneath the headwall. "T" is a tarn which has formed in the hollow gouged out by the ice. Sometimes there is a lip of terminal moraine partly encircling the tarn. "G" is ground moraine strewn across the valley floor. The map shows some of these features as they actually occur in part of the Cairngorms in Scotland. Most cirques face north east or east. "U" refers to upland summits, "R" to cliff-like slopes with bare rock, "M" ridges of moraine and "O" is a former glacial overflow channel.
A Landscape During and After the Ice Age
UK Glaciation Diagram Landscape Change
The diagram shows various aspects of how a landscape might have looked during the Ice Age (left) and compares it with how the same landscape appears today (right). The post-glacial landscape illustrates many features common to typical glaciated valleys in northern and western Britain. It is based on the area around, and to the south of Derwentwater in Cumbria.
Drainage Diversion Brought About by Glaciation
UK Glaciation Drainage Diversion Change Pickering Overflow
The sequence of maps illustrates a good example of the way in which drainage patterns can be modified by glaciation. The course of the River Derwent has been considerably altered due to the effects of glaciation. The key to the maps is as follows:S - ScarboroughF - FileyM - MaltonA - AmpleforthP - Pickering1 - Forge valley2 - Newton Dale3 - Malton gorge4 - Ampleforth gap
34
1.02
Geography
Limestone and Chalk in the UK
2604
The pictures and diagrams in this module are concerned with limestone and chalk scenery. The examples are from the UK and they provide good illustrations of the nature and field occurrence of these rocks including aspects of landform development. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.Lime rich rocks are found at various geological horizons in the UK.The older limestones outcrop mainly to the north and west of the Tees-Exe line in highland Britain. Exceptions to this are the Magnesian Limestone, which occurs in a narrow band stretching south through Yorkshire to Nottingham, the upfolded blocks of Carboniferous Limestone in the Peak District and Mendips and small isolated inliers of Silurian Limestone in the West Midlands. The bulk of any limestone is essentially the mineral calcite with the chemical composition, calcium carbonate. Most of the surface landforms of limestone areas are related to the peculiar chemistry of this mineral. Normally calcium carbonate is insoluble, but when combined in the same environment as water and carbon dioxide, it converts to soluble calcium bicarbonate. The latter is readily carried away in solution by excess water. Under certain conditions this chemical reaction is reversible leaving lime-rich deposits. Solution processes on the surface of limestone outcrops and along joints and bedding planes within the rock mass produce some unusual phenomena which are well represented in Carboniferous Limestone landscapes.Limestones are formed in various ways but can be simply classified into organically formed types and inorganically formed detrital limestone. The commonest forms, although being very different in appearance, are all organically formed. Silurian Limestone and Carboniferous types are often formed from the accumulation of lime-rich skeletons of dead marine life cemented together by a lime-rich matrix. Jurassic Limestones are sometimes oolitic in character, consisting of numerous tiny spheres of lime cemented together and resembling the roe of fish. Chalk is an almost pure white limestone made up of the microscopic remains of minute sea creatures. Limestones are sedimentary rocks laid down in a stratified manner. Where the rocks dip in a particular direction cuesta formations with prominent dip and scarp features occur. The outcrops of the younger limestones in Britain are all located to the east of the Tees-Exe line in lowland Britain, with the exception of a small outcrop on the shores of Dornoch Firth in Scotland.Within the Jurassic limestones, which are for the most part oolitic incharacter, there are three important axes to which the succession thins. These axes, caused by local uplift at the time the limestone series was being laid down, are: the Mendip axis, the Moreton-in-the-Marsh Axis and the Market Weighton axis. The latter was also effective during deposition of chalk in Yorkshire. Both the Jurassic Limestone and chalk occur as gently inclined strata dipping mainly to the south-east with prominent scarps facing north-westwards. More complex "Alpine" folds in the south of England have resulted in the anticlinal axis of the Weald with its inward-facing scarps and the adjacent synclinal basins of London and Hampshire.Chalk cuestas are noted for the occurrence of dry valleys. There are several theories on the formation of dry valleys which were all cut at a time when the water table was at a higher level than today.
A Solution Basin
UK Cumbria Rock Limestone Karst Solution Carboniferous Erosion
The picture shows an example of a solution basin seen at Hutton Roof Crag, Cumbria. These basins are sometimes referred to as Kamenitza. They result from solution of limestone by water collecting in minor surface hollows on the exposed rock. Typically they occur on more or less horizontally bedded limestones and are up to about 70cm in diameter. They have flat bottoms and steep, fretted sides which may overhang in places. Usually organic deposits (algae, spores and mosses) and inorganic material line the bottom (the black layering seen in the picture). This provides a source of carbon dioxide which aids solution attack. Sometimes the fine deposits form an effective seal. The margins then become the focus of solution activity and the basin is extended laterally. In the Craven area basins 3 to 5cm deep are known to develop in less than 10 years on Carboniferous limestone.
Limestone Pavement
UK Yorkshire Rock Limestone Karst Pavement Clint Grike Runnel Erosion
The view is of a Carboniferous limestone outcrop near Austwick, Yorkshire. It shows a dissected pavement developed on gently dipping strata. Pavements are attributed to the stripping of limestones during glaciation. Solution erosion on the exposed bedrock subsequently gives rise to a complex arrangement of runnels (karren) over the surface. Detailed morphologies of pavements are influenced by a number of factors. These include the frequency of occurrence of joints, the dip of the bedrock and the nature of the limestone lithologies. Solution-enlarged fissures between blocks of limestones are called grikes (German - kluftkarren), and the tabular surfaces clints (flachkarren). Grikes narrow towards their base and are sometimes several metres deep. Their sides may be etched with runnels. Where grikes intersect, deep wells may form (karrenrohren). Grikes provide sheltered habitats where humus and soil accumulate, enabling plants and small shrubs to become established. The surfaces of the clints show drainage runnels (rundkarren) with smooth troughs and crests. Such forms develop beneath a humus or vegetation cover. Their orientation normally follows the surface slope of a clint. Usually they form short, dendritic networks terminating at the grikes.
Solution Runnels
UK Rock Limestone Karst Solution Runnel Erosion
The picture shows a steeply dipping limestone outcrop at Hutton Roof Crag in Cumbria. Solution erosion by surface water drainage can form impressive networks of grooves or runnels. The limestones dip at 20░ and over. This has facilitated the development of long runnels, which are sometimes over twelve metres in length. The runnels deepen and widen when traced down-tip. They terminate in grikes whose progressive enlargement has helped to dismember the drainage patterns. The majority of the runnels have rounded crests (rundkarren). The sharp crested forms (rinnenkarren) that can also be found are attributed to weathering in the free atmosphere.
A Swallow Hole
UK Rock Limestone Karst Disappearing Stream Solution Erosion
The picture shows a good example of a swallow hole associated with a bedding plane cave, Rowten Cave, Yorkshire. Owing to the permeability of karst rocks, rapid and marked infiltration occurs everywhere. Stream courses are reduced in number and are often affected by the total or partial loss of water underground. The swallow hole (also referred to as a swallet or sink) shows water disappearing into a small, shallow, gently dipping cave. Such sinks are common in north-west Yorkshire. They result from mechanical and solution enlargement of openings along bedding planes.
The Origin of Limestone
UK Rock Limestone Karst Silurian Fossil Oolitic Chalk Geology
The picture depicts an interesting piece of Silurian limestone. It shows more or less exactly what the sea bed may have looked like when the rock began to form about 420 million years ago. In this case fragments of Brachiopods, Trilobites and other dead marine creatures accumulated in lime-rich mud on the sea floor. The processes of lithification converted these into a hard limestone rock. Carboniferous limestone was formed in a similar way. Oolitic limestones, not shown here, consist of numerous tiny spheres of lime that have grown about some minute fragment of a broken shell. The term Oolitic is derived from the Greek word Oolos, meaning egg. The texture of the rock resembles fish roe. Chalk is another form of organic limestone, made up of the minute remains of microscopic sea creatures like Foraminifera. A further type of limestone is that formed from grains or fragments of a previously eroded lime-rich rock. The latter is called a detrital limestone.
Reef Limestone
UK Rock Limestone Karst Silurian Reef Coral Algae Bryozoa Geology
The picture shows an exposed quarry face in Silurian limestone near Much Wenlock in Shropshire. Although the limestone shows some stratification this is interrupted by reef-like knolls. This type of organic limestone had its origins in warm, shallow seas where marine life was extremely abundant. The ancient ecosystem supported a variety of reef-building organisms including Corals, Algae and Bryozoa. This quarry originally supplied lime to be used as a flux in the iron furnaces at nearby Coalbrookdale. More recent uses include the spreading of crushed limestone to help improve pastureland on hill farms.
Malham Cove
UK Pennine Rock Limestone Karst Carboniferous Fault Scar
Malham Cove is a well known landmark in North Yorkshire. Here the face of the Pennine fault has been exposed due to erosion by the head streams of the River Aire. The massively jointed nature of the hard, grey Carboniferous limestone is emphasised by organic staining. The tendency for water to percolate from joint and bedding planes attracts moss and algae which produce the staining. A solution-eroded limestone pavement can just be seen at the top of the cliff. Note the limestone scree extending down the steep hillsides into the river valley. The river emerges from a spring at the base of the cliff. Large exposures of Carboniferous limestone are sometimes called "scars".
A Disappearing Stream
UK Rock Limestone Karst Drainage Underground Swallow Hole
The view shows a stream disappearing underground into a swallow hole. The feature has occurred along the contact between Carboniferous limestone and the overlying shales of the Millstone Grit series in North Derbyshire. The stream can be followed across the impermeable shales in the foreground, marked by the longer grasses that thrive in the wetter conditions. It then appears to open out into a boggy area before disappearing into the swallow hole at the foot of the distant steep slope. The rock in the distance is limestone and the swallow hole has been enlarged by solution processes.
Bar Pot, a Collapse Depression
UK Rock Limestone Karst Carboniferous Subsidence Pot
Collapse depressions usually occur as a result of cavern breakdown. Beyond a certain thickness, the roofs of limestone caves are unstable and can easily fall in on themselves. In the case of Bar Pot, Yorkshire, this has produced a deep chasm with vertical walls. Large blocks of Carboniferous limestone, remnants of a former plateau surface, line the bottom. Most collapse depressions are characterised by high depth-width ratios. It is not always easy to decipher the origins of some collapse features since, with time, steep cliff-like walls may assume conical or bowl-shaped forms due to mass movement and infilling.
A Karst Spring
UK Rock Limestone Karst Spring Carboniferous Hydrology
Karst springs represent points at which percolation and conduit waters reappear at the surface. Sometimes they are large and permanent and drain extensive areas. Many emerge from caves and some cut spring-sapped notches or valleys into their headward slopes. The spring shown in the picture is at the source of Jenkin Beck, Ingleborough, Yorkshire. It drains a catchment in Carboniferous limestone. During wet periods the waters back up within the underground system and flow takes place through the higher level outflows seen near the top of the picture. Records of the physical variations (in colour, water temperature resistivity and discharge) of the spring waters are often made. Chemical changes (in calcium carbonate and magnesium carbonate concentrations) are also noted. This enables conclusions to be drawn concerning solution erosion and the nature of ground water movements.
Winnat's Pass, a Collapsed Cavern
UK Rock Limestone Karst Carboniferous Cavern Collapse
Near Castleton in the Peak District, Winnat's Pass marks the position of a former underground drainage system. The huge underground caverns belonging to this system became so enlarged that their roofs collapsed to form the near-vertical walled gorge. Massive reef knolls contribute to the character of the walls of the gorge. The rock is Carboniferous limestone.
Intermittent Drainage in Kingsdale
UK Rock Limestone Karst Carboniferous Intermittent Drainage
The picture shows a valley with discontinuous surface drainage, Kingsdale, Yorkshire. Kingsdale, with its craggy sideslopes and glacially modified form, displays many of the features of a typical Yorkshire dale. Besides these morphological traits, it is also characterised by intermittent surface drainage. During much of the year the valley is dry along a 3km length above Keld Head. Rising waters pass through the permeable alluvial formations lining the valley bottom into sinks and joints in the underlying Carboniferous limestone. Wet weather brings flooding of the sinks in the river bed and the restoration of the surface flow. Water flowing in the beck, immediately below Keld Head, is seen in the centre of the view. On the west side of the valley the Great Scar Limestone (170m thick) outcrops in a series of benches. The foot slopes are mantled by extensive screes of weathered material. Whernside is visible rising above the valley to the north-east.
Minerals in Limestone Areas
UK Rock Limestone Karst Carboniferous Vein Mineral Lead Geology
Carboniferous limestone rocks are often associated with old mineral workings. Galena (lead sulphide), barytes (barium sulphate) and fluorspar (calcium fluoride) have been mined from long narrow veins or rakes. The latter were mineralised by hydro-thermal solutions associated with Hercynian earth movements. The picture shows part of the Dirtlow Rake in the Peak District. It is being re-worked for the gangue of baryte, fluorite and zinc blende left behind by 18th and 19th century lead and silver miners.
Dry Stone Walls
UK Rock Limestone Karst Carboniferous Stone Wall Enclosure
Upland Carboniferous limestone regularly presents an almost treeless landscape of dry-stone walls and stone-built villages. The village in this view is in North Derbyshire. The small rectangular fields originated from enclosure in the 16th and 18th centuries. Stone for building the walls was all obtained locally. The lane in the foreground appears much wider than the road that now occupies it. This probably indicates that it was an old drover's lane along which farmers herded their stock to market.
A Cement Works in the Hope Valley
UK Rock Limestone Karst Carboniferous Cement Lime Mining
Carboniferous limestone is a valuable raw material for industry. Large works producing cement, agricultural lime and roadstone are common sights wherever the rock outcrops. The example shown in this picture is the Hope cement works in the Peak District. Nearby clay deposits favoured the location alongside an enormous limestone quarry. The flooded clay pits can just be seen in the low-lying area beyond the cement works. Only a small part of the limestone quarry can be seen in this view. Also, chalk is extensively quarried for cement making in other parts of the country, notably near London along the Thames estuary. The UK produces about 15 million tonnes of cement a year.
A Limestone Quarry
UK Rock Limestone Karst Carboniferous Aggregate Pollution
One of the main objections to quarrying is that it frequently creates untidy landscapes. Rocks like Carboniferous limestone occur in areas of outstanding natural beauty such as The Peak District National Park, and there is a conflict of interests between the quarry operators and tourists. Quarrying supplies valuable raw materials; lime for the chemical industry, cement making and farming, and aggregate for road construction, railway ballast and building. The picture shows a typical limestone quarry scene at Eldon Hill in Derbyshire. Note the unsightly screening plant and piles of crushed aggregate. The white dust generated by quarrying and processing activities has a considerable polluting affect on the local area.
Stratified Lias Limestones
UK Rock Limestone Lias Coast Cliff Undercutting Strata
The picture shows a 30-metre-high cliff of Lias (Jurassic) limestone at Llantwit Major. The almost horizontal bedding planes of the limestone strata can be clearly seen. Sub-aerial weathering processes are acting on the upper part of the cliff and there is evidence of marine action at its base. In one place the cliff headland has been dangerously undercut by wave action and is liable to collapse. In another place a section of the cliff face appears to have become detached and is ready to fall. In the foreground a conical pile of boulders marks an earlier rock fall. Wave action appears to have moved limestone rubble away from the cliff site and only the largest boulders remain on the shore platform. Note the distinctive buff colour of the Lias limestone.
The Cotswold Scarp
UK Cotswold Rock Limestone Jurassic Cuesta Scarp
The picture shows a panorama of the Cotswold scarp looking towards its highest point (over 300 metres) at Cleeve Hill near Cheltenham. Here the scarp is steep, more or less continuous and extensively wooded. It receives a fair amount of relief rainfall carried in by moist, south-westerly winds blowing up the Severn Estuary. The scarp is the uptilted edge of a thick succession of Jurassic limestones. The latter dip gently towards the east beyond the crest of the Cotswolds on the skyline. A distinctive assymetrical ridge of this nature with a steep scarp and gentle dip slope is called a cuesta.
Cotswold Outliers
UK Cotswold Rock Limestone Jurassic Scarp Outlier Erosion
This view is from the Cotswold scarp in the region of Gloucester. It looks down to the Severn lowlands upon which small outliers of Jurassic limestone occur. The outliers are the isolated hills rising from the plain in the middle distance. Their occurrence is due partly to down-faulting and partly to the limestone's ability to withstand erosion. It also proves that the limestone rocks formerly extended considerably further to the west of their present location.
The Cotswold Dip Slope
UK Cotswold Rock Limestone Jurassic Dip Slope Cuesta Soil
The dip slope of the Cotswold cuesta is characterised by long gentle slopes separated by broad, often streamless, valleys. Land use is dominated by mixed farming, with an emphasis on arable crops such as wheat and barley. The fallow fields show up well amid the green fields on account of the rich red-brown colour of the soil. This is typical of soils developed on limestone. Ridge-top roads and nucleated market towns with a history in the woollen trade are a feature of the Cotswolds. This view is to the north-east of Broadway Hill.
Stair Hole
UK Dorset Coast Rock Limestone Jurassic Strata Fold Erosion
On the South coast the Jurassic rocks have been affected by strong Alpine earth movements. This can be seen at Stair Hole near Lulworth in Dorset. The picture shows steeply dipping and contorted limestone strata. To the right, waves have broken through a weak point in the lower layers of resistant limestone, allowing erosion of the more broken upper beds. Notice how load release has encouraged gravity collapse of the limestone on exposed bedding planes. Lulworth Cove itself, a little further to the east of Stair Hole, was formed by enlargement of a similar hole. This hole was created when the sea broke through a weak part of the steeply inclined and resistant Jurassic limestone strata, enabling it to attack the much softer sandstone and clays beyond.
Natural "Cotswold" Stone Buildings
UK Cotswold Rock Limestone Jurassic Freestone Building
Jurassic limestone, in particular the oolitic "freestone", has been used extensively as a building stone within three or four miles of its outcrop. Cotswold villages like Stanway have a charm of their own. "Freestone" is not a description of the rock's relative abundance but refers to the way in which it reacts to the mason's chisel. The oolitic texture allows it to be carved very precisely without a tendency to split in any particular direction. Oolitic is a word derived from the Greek word "oolos", meaning egg. The rock is composed of tiny spheres of lime with a texture like fish roe. Where the cost of transport could be justified, Jurassic limestone has also been used in different parts of Britain for constructing large buildings such as offices, churches and cathedrals. Many large blocks of building stone found in London, including those used on some of the Thames bridges, are Portland Stone.
Stone Buildings in Oxford
UK Oxford Rock Limestone Jurassic Building
Almost all the building materials seen in this picture of part of the old University town of Oxford have been obtained from Jurassic limestone rocks. Although the city stands in the centre of a broad clay vale, there were substantial supplies of the buff-coloured building stone about 12 miles to the north-west in the Cotswolds. Note that the window frames, doorways and roof tiles are stone. Even the mortar between the stone blocks will have come from a cement works using local Jurassic limestone and clay as raw materials.
White Cliffs
UK Dorset Coast Rock Chalk Cliff Vertical Flint Beach Shingle Stack
Towering white cliffs, sometimes over 100 metres high, are frequently found where chalk outcrops on the UK coast. The picture shows part of the Dorset coast. Wave attack at the base of the cliff has caused undercutting and gravity collapse of the overlying chalk. The near vertical cliff face is maintained by rapid removal of fallen rock and renewed undercutting. Note the stack, a relatively resistant mass of chalk that has somehow managed to survive the rigours of erosion. The soft chalk soon crumbles, leaving only hard flints behind on the beach. Flint is a siliceous material that is found in joints and along bedding planes in chalk. It is a secondary mineral formed from concentration of the skeletal remains of minute marine organisms that lived in Cretaceous times.
Red Chalk
UK Norfolk Coast Rock Chalk Cliff Red Flint
The picture shows an interesting outcrop of almost horizontally bedded white chalk resting on red chalk. It was taken near Hunstanton in Norfolk. The red colour is due to the presence of iron oxides which accumulated by some freak event at the time the stratum of chalk was being formed. Note the bands of flint in the chalk. Flint is a secondary mineral that only occurs in chalk. It originates from siliceous skeletons of minute creatures which lived in Cretaceous seas, becoming concentrated along bedding planes and joints during the later stages of chalk lithification. Because of its nature, chalk always forms near vertical cliffs. However, the rock is relatively soft when broken by wave action and cliff falls are common. Some particularly dramatic landslips have occurred on the Isle of Wight near Ventnor. The chalk cliffs have collapsed as the underlying Gault Clay has been eroded away by the sea.
The South Downs
UK Sussex Downs Rock Chalk Scarp Dip Cuesta
Chalk is well known for steep scarps along its upward tilted outcrops. In this view of the South Downs north of Brighton, the scarp and dip slopes of the cuesta can be easily seen. The crest of the chalk scarplands usually rises to just over 200 metres, although in the Berkshire Downs chalk summits reach over 300 metres. Chalk is a pervious rock, permitting rainfall and surface water to percolate underground. Later, when the rock becomes saturated or when underground movement is hampered by impermeable rocks above or below the chalk stratum, the water emerges as springs. The spring line at the base of the scarp can be seen in this view. It is further emphasised by a line of scarp-foot spring line settlements. Land use on the steep scarp is usually rough grazing.
The Nature of the South Downs Scarp
UK Sussex Downs Rock Chalk Scarp Slope Nivation Solifluction
Nivation processes on north-facing slopes during the Ice Age probably helped to maintain the steepness of the South Downs scarp. The original line of the scarp was probably cut by marine erosion at an even earlier date when sea-level was higher than today. Solifluction (soil creep) has helped to create the small terracettes on a slope of about 12 degrees from horizontal (although it first appears to be much steeper than this). The pattern of terracettes is disturbed by an ancient track-way on the South Downs north of Worthing. The Downs were comparatively drier and more open than the adjacent clay lowlands and frequently show evidence of early settlement and communication.
The Dip Slope of the South Downs
UK Sussex Rock Chalk Dip Slope Coombe Valley Dry Farming Arable
The picture shows a classic chalk dip slope landscape north of Shoreham in Sussex. Note the broad, deep dry valley. There are several theories relating to the origins of dry valleys and any situation probably results from a combination of theories. Land use in the area shown is almost exclusively arable farmland with few trees or hedgerows. Only the steeper valley sides are unfarmed. The latter is due to thin flinty soils. These are very acidic and are caused by the high rate of mineral downwash. The valley bottoms are infilled with fertile coombe deposits originating from mass movement under periglacial conditions during the Ice Age.
The Dip Slope of the Chilterns
UK Chilterns Rock Chalk Slope Dip Valley Dry Settlement Linear
The picture shows part of the Chiltern dip slope to the north of High Wycombe in Buckinghamshire. The underlying rock is chalk which dips gently towards the viewpoint from the horizon. To the top left you can just see a telecommunications tower some ten miles away on the crest of the Chiltern cuesta. The main feature in the picture is a broad dry valley (coombe or combe). There are several theories relating to the formation of dry valleys. Some involve a lowering of the water table due to physical changes in the environment. Others relate to erosion during the Ice Age when the sub-soil would have been frozen, thus allowing streams to flow across the surface. Note the rich farmland in the valley, the more extensive woodland cover on the clay, and the flinty soil on the Chiltern summits. The village of West Wycombe shows a characteristic linear development along the main road that flanks the valley before ascending to a distant ridge top.
The East Anglian Heights
UK East Anglia Rock Chalk Heights Glacial Loam Farming Arable
In East Anglia glaciation smoothed off the upland summits of the chalk cuesta and deposited on top a thin veneer of boulder clay and sand. Mixing of the glacial material with weathered chalk has resulted in probably the most fertile loam soils to be found in Britain. Arable farming dominates the landscape.
A Map Locating the Younger Limestones in Britain
UK Limestone Young Jurassic Cretaceous Chalk Outcrop
The map shows the outcrops of the younger limestones in Britain. At a glance one can see that these are mostly to the east of a line running between the mouths of the rivers Tees and Exe. The key to the map is as follows:a - Jurassic limestoneb - Chalk (Cretaceous)c - local thinningd - major anticlinee - major synclineThe Jurassic limestones are for the most part oolitic in character. The series is affected by three important axes to which the succession thins. These were caused by local uplift at the time the rocks were being laid down. "A" is the Mendip axis, "B" the Moreton-in-the-Marsh axis and "C" the Market Weighton axis. The latter was also active during deposition of Chalk in Yorkshire. Both the Jurassic limestone and Chalk usually occur as gently inclined strata dipping mainly to the south east. They form prominent cuestas (escarpments) with steep north west facing scarps. More complex Alpine folds in southern England have caused the anticlinal axis of the Weald in Kent and Sussex. This created the inward facing scarps of the North and South Downs. Further west along the south coast the younger limestones were subject to strong folding and, in places, they form vertically dipping outcrops.
A Map Locating the Older Limestones in Britain
UK Limestone Outcrop Silurian Carboniferous Magnesian Palaeozoic
The map shows the main outcrops of older limestones in Britain. With the exception of the Magnesian Limestone "a", which forms a narrow band stretching from Yorkshire to Nottingham, they are located to the north west of a line between the mouths of the rivers Tees and Exe. Carboniferous Limestone "b" is prominent in the northern Pennines, Peak District, South Wales and the Mendips. Silurian limestones "c" are found in Shropshire and there are several other places where localised outcrops of older limestones "d" occur.
A Model Carboniferous Limestone Landscape
UK Limestone Carboniferous Landscape Landform Karst Diagram
Solution processes acting on the surface of limestone outcrops and underground along joints and bedding planes produce some unusual landforms. These are well represented in the diagram of a model Carboniferous Limestone landscape. "A" is a swallow hole or sink hole where a stream disappears below the surface. "B" is a plunge pool scoured out by waters cascading into a cavern system "C". Stalactites "D" grow down from the cavern roof fed by lime-rich waters percolating from above. Where this water drips from the end of each stalactite, a stalagmite "E" grows up from the cavern floor. The stalagmite is usually flatter and broader than the corresponding stalactite. Sometimes the two join to form a pillar. Cavern walls will also become encrusted with lime deposits resembling organ flutes. Sometimes cavern floors will be flooded due to heavy rainfall above and evidence of erosion by running water is common. Higher level caverns "F" often provide the best features. Today these are mainly dry but they were occupied by powerful Ice Age streams and they once opened out high up on limestone cliffs. Today these ancient cavern mouths "G" are often obscurred by weathered debris. Stream occupied caverns do not always form a good outlet for natural water flow. Siphons "H" may occur where the stream has to by-pass obstructions. Where air flows strongly through a cavern it sometimes affects the nature of stalactite growth. Stalactites which curve in the direction of air flow are not uncommon. High above the caverns where solution processes are strong there will often be a natural pavement of clints and grykes "J".
The Formation of Dry Valleys
UK Chalk limestone Valley Dry Formation Diagram
Chalk cuestas are well known for their dry valleys. The diagram examines five different theories related to the formation of such features. In the examples shown all the valleys were cut at a time when the water table was at a higher level than today. The former position of the water table "a-a" can be compared with its present position "b-b". The shaded zone indicates the areas of possible dry valley formation. Can you work out what has happened in each situation?
33
1.02
Geography
UK Igneous and Metamorphic Rocks
2605
The pictures and diagrams in this module are concerned with igneous and metamorphic rocks. The examples are from the UK and they provide good illustrations of the features and scenery associated igneous and metamorphic rocks. They also provide an insight into the processes which led to landform development. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere" (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.Igneous Rocks are formed by crystallisation from a cooling melt. The melt, often called magma, is due to extremely high temperatures associated with tectonic activity. The latter is caused by changes in the Earth's crustal stability. For instance where the margins of two crustal plates collide. Friction between the colliding plates raises temperatures so high that the crustal rocks, and perhaps part of the the Earth's upper mantle, become molten. In this state they behave like a fluid. Sometimes the molten rock cools slowly deep below the surface to form "plutonic" igneous rocks. Being fluid and very mobile the molten rock can also flow upwards to the surface, making its way along weaker zones of the crust. The cooling of magma in such a position produces creates "intrusive" igneous rocks. There are also times when magma reaches the surface where it spreads out and cools quickly to form "Extrusive" igneous rocks. The term "volcanic" is sometimes used to describe rocks in this third class.Large scale igneous activity can be illustrated by the massive intrusions of magma which occurred in what is now Devon and Cornwall during Hercynian times. Huge masses of magma intruded into the country rocks which themselves became assimilated into the molten mass. The magma eventually cooled and solidified into an enormous subterranean batholith which had dome-shaped up-wellings in places where the crust was weak. Subsequent erosion of the tops of the granite batholith and the remaining country rocks formed the granite masses of Land's End, Carnmellis, Hensbarrow Down, Bodmin Moor and Dartmoor. There was mineralisation around the Carnmellis intrusion where magmatic solutions penetrated fault lines depositing important ores of tin and copper. Mineralisation is also found in the zone of baking of the country rocks, the metamorphic aureole, in contact with the intrusion.Metamorphic means "changed in form". Metamorphic rocks are those that have been subjected to conditions of temperature and pressure very different from those in which they originated. This has led to the formation of new rock textures and sometimes the creation of new minerals. Where the forces that caused metamorphism were very powerful, completely new rocks have been formed with no trace of the original parent rock remaining. All types of rock can be affected by metamorphism whether they are sedimentary, igneous or even metamorphic rocks that have been previously metamorphosed. Metamorphism takes place because of change in the rock environment. Because of a rise in temperature, because of increasing pressure and due to chemical changes caused by migrating fluids. These changes are associated with major tectonic disturbances like the convergence of crustal plate margins. It follows then that, in the UK, areas where metamorphic rocks occur are close to major igneous intrusions and where ancient mountain ranges were uplifted.
Two Varieties of Granite
UK Rock Igneous Metamorphic Granite Crystal
There is some debate as to whether granite rocks are igneous or metamorphic. Traditionally they have always been classified as igneous. However, there is increasing support for them to be labelled as metamorphic since many granites cool from a melt produced by extremely strong heating of country rocks. The picture shows two types of granite from the South West Peninsula. In both cases the crystals are fairly large indicating slow cooling from the parent melt. Both are composed essentially of three minerals: quartz, feldspar and mica. Variations in colour are due to chemical differences in the feldspar content. Note the large lath-like crystals of feldspar in the Dartmoor Granite (the larger of the two rocks). This is an indication that the feldspar crystallised from the melt quite early during the rock-forming process.
Typical Granite Scenery
UK Rock Igneous Scenery Granite Plutonic Tor Dartmoor
This picture of part of Dartmoor shows typical granite scenery. Granite intrusions were deep-seated (plutonic) movements of molten rock associated with major tectonic upheavals. Since it was covered by a thick layer of crust, the melt cooled slowly to form the granite. Granite is a hard rock made up of easily visible crystals. Usually the granite forms an upland plateau where millions of years of denudation have removed the less resistant covering. These granite moors receive high precipitation and have little value except for recreation use and grazing of hardy livestock. The feature in the middle distance is a tor. Tors are described in more detail elsewhere in this module.
Rough Tor, Bodmin Moor
UK Rock Igneous Scenery Granite Tor Plutonic Denudation
The picture is a close-up of residual granite blocks on a summit on Bodmin Moor. The residual blocks represent a small tor. They originate from parts of an exposed granite intrusion that was less well-jointed than the overlying and surrounding rock. The agents of weathering and erosion have, therefore, been less effective in denuding the land surface, leaving large granite blocks littering the surface. Land use of these moors is usually rough grazing but such areas are under increasing pressure from people in pursuit of leisure activities. There is more information on tors elsewhere in this module.
An Exposed Granite Tor
UK Rock Igneous Scenery Granite Weathering Tor Sheeting
The large tor in this picture is composed of Dartmoor granite. It shows a layered nature due to weathering along joints. These joints are partly due to contraction of the granite mass as it solidified from its parent melt and partly an effect of the physical processes of decompression and sheeting action on the exposed granite. The latter followed removal of overlying rocks by denudation. The scale of the landform can be easily judged by the people on the rock.
A China Clay Quarry
UK Rock Igneous Scenery Granite Clay Stoping Quarry
The picture shows a massive clay pit on Hensbarrow Down near St Austell. China clay is weathered from the granite by a process called "stoping". This is the washing out of the clay using high pressure water jets. Stoping is an artificial means of speeding up the natural weathering process in which the feldspar content of the granite breaks down into clay minerals. The white sand remaining has little commercial value. It is removed on a conveyor belt before being piled up on peculiar man-made tips known locally as the 'Cornish Alps'.
Unstratified Igneous Rock
UK Rock Igneous Scenery Granophyre Unstratified Quarry
One of the important structural features of igneous rocks is that, in general, lavas apart, they are unstratified. This picture shows a quarry in unstratified granophyre north of the Wrekin in Shropshire. This material was quarried for roadstone. The rock face has been considerably weathered since quarrying operations finished.
An Igneous Landscape on Skye
UK Rock Igneous Scenery Gabbro Intrusion Dyke Weathering Skye
The picture illustrates the effect of weathering and erosion on a hill in the Cullin Range on the Isle of Skye. Igneous rocks, dating from tectonic activity during the Tertiary period, are a dominant feature of the landscape. The main part of the hill is the exposed top of a massive plutonic intrusion of gabbro. Gabbro is the basic equivalent to granite, having crystallised from a melt that was lacking in silica and rich in iron/magnesium minerals. Running up the hill is a characteristic dyke-gully. The dyke resulted from a later intrusion of dolerite. The dolerite was softer than the gabbro and therefore less able to withstand weathering and erosion; hence the gully.
Dolerite
UK Rock Igneous Scenery Intrusive Dolerite Weathering Spheroidal
Igneous rocks commonly associated with dykes and sills are basalt and dolerite. This view shows an outcrop of dolerite to the west of the Stiperstones in Shropshire. In this case the igneous rock is more resistant than the sedimentary rocks into which it is intruded. It therefore forms higher ground capped with residual boulders. Note the spheroidal, block-like weathering similar to that of the granite tors but on a much smaller scale.
A Dyke Exposed on the Coast
UK Rock Igneous Metamorphic Scenery Dyke Intrusive Erosion
The picture shows a good example of an igneous intrusion, in this case a dyke, crossing the shore in eastern Scotland. Intrusion of the hot melt along the dyke baked the surrounding rocks close to the point of contact. These hardened country rocks remained resistant whilst marine processes eroded the softer dyke-rock. The end result was the deep and narrow cleft across the shore platform.
Dykes Cutting Cambrian Strata
UK Rock Igneous Scenery Intrusive Dyke Vein Dolerite Hydrothermal
During the Ordovician period there was a considerable amount of igneous activity which can be traced in the rocks of North Wales. Basalt lava reached the sea floor of the Welsh basin along deep fractures, penetrating through the crust from the underlying mantle. After the volcanic event terminated, basic (iron/magnesium silicate-rich) lava cooled in the fractures to form a complex of dolerite dykes. These are seen in the picture cutting across gently inclined Cambrian strata. Notice also veins of quartz that crystallised out from hydrothermal solutions during the final phase of the igneous event.
The Field Occurrence of a Dyke
UK Rock Igneous Metamorphic Scenery Intrusion Dyke Contact
The picture shows the line of contact between stratified rocks (in the foreground) and a dyke. The unstratified rock of the dyke rises beyond where the person is sitting. This particular example was found in Ordovician rocks in Shropshire. That area is well known for a variety of scenery associated with a close juxtaposition of igneous and sedimentary rocks.
Volcanic Features at Edinburgh
UK Rock Igneous Scenery Volcano Neck Sill Edinburgh Extrusive
Salisbury Crags and Arthur's Seat in Edinburgh are part of a complex resulting from igneous activity during Carboniferous times. Tectonic activity at that time caused both intrusion and extrusion of molten lava and the eruption of volcanic ash on a large scale. Most of the extrusive lavas and ash deposits have long since been removed by denudation. However, the intrusive igneous rocks that cooled in dykes, sills and in volcanic necks were more resistant and can be traced in the present day landscape. Arthur's Seat is a complex of two vents (dormant and not active as might be suggested by the smoke in the picture). Salisbury Crags is an outcrop of hard, dark igneous rock, the remnant of a sill which has even protected the softer country rocks beneath it from erosion.
Whin Sill
UK Rock Igneous Scenery Intrusive Sill Wall Hadrian
Whin Sill, shown here, near Crag Lough in Northumberland was recognised by the Romans as a natural obstacle. They reinforced this by building a defensive wall, Hadrian's Wall, atop the steep north-facing scarp of the sill. The wall itself was built of small regular sized blocks cut from the igneous rocks (dolerite and basalt) of the intrusion. The sill dates from igneous activity during Carboniferous times and is one of the best examples of its kind in the UK.
Cader Idris
UK Rock Igneous Scenery Intrusion Sill Dolerite
The picture shows the Cader Idris escarpment. It is typical of the higher and more rugged mountains of North Wales built up largely of igneous rocks. The north-facing slope, steepened by nivation processes, is the outcrop of a huge sill of granophyre. The latter is partly covered with scree and is interbedded with mudstones and volcanic ashes. The whole feature is capped with a smaller but very resistant sill of dark-coloured dolerite.
A Volcanic Plug
UK Rock Igneous Metamorphic Scenery Volcano Plug Vent Extrusive
The picture shows the remnants of volcanic plugs now exposed along the coast of eastern Scotland. In the latter stages of volcanic activity the lava often fails to erupt from a volcano and merely solidifies in the vents leading up to the surface. In effect this chokes (plugs) the eruption and the lava may seek an outlet through secondary vents. The lavas are often much more resistant than the surrounding ash beds. The result is that the volcanic plugs remain prominent as isolated and very often steep-sided, conical masses. There is one small plug in the foreground, another one slightly offshore and, on the horizon, the much larger Traprain Law. These appear to occur along a zone of weakness as shown by the yellow line. The volcanoes fed by these vents were active during Carboniferous times.
The Stoer, Isle of Skye
UK Rock Igneous Metamorphic Scenery Extrusive Lava Basalt Plateau
Basalt is the most common rock in the northern half of the Isle of Skye. The lava welled out of great fissures during Tertiary times. It was a basic lava rich in iron/magnesium silicates, and was therefore very fluid in character, spreading out evenly over a large area. It now forms a plateau that rises to about 700 metres. In places the plateau is deeply dissected. On the eastern side the plateau ends in an abrupt escarpment and sea cliffs. In numerous places, where it lies on top of Jurassic sediments, the basalt has slumped in massive landslips. This has produced a confused landscape with some bizarre-looking pinnacles near the escarpment.
Pillow Lava
UK Rock Igneous Metamorphic Scenery Lava Volcano Pillow
Pillow lavas have a distinctive structure that results from the rapid chilling of the outer surface of the lava flow when it comes into contact with sea water. The lava shown in this picture is located in the Cader Idris area of North Wales. It was formed by volcanic activity during the Ordovician. At that time there was no Atlantic Ocean and the collision zone between the North American crustal plate and the Eurasian crustal plate ran south west to north east across Wales, the north-west of England and Scotland.
Volcanic Agglomerate
UK Rock Igneous Metamorphic Scenery Agglomerate Ash Volcano Bomb
The picture shows a close-up of agglomerate, a rock formed by the consolidation of deposits of ash and tufa thrown out by a massive volcanic eruption. The smooth hollow in the agglomerate marks the position where a volcanic bomb landed and became embedded in the softer ash. The camera lens-cap gives an idea of scale. Ash, tufa and volcanic bombs are usually associated with the eruption of acid magma which is cooler (850 deg C), more viscous and explosive. The rock shown is of Ordovician age and is found in Shropshire. It was extruded some distance to the east of a zone where two crustal plates were colliding, which probably explains the acidity of the magma from which it was derived.
The Xenolithic Margin of a Sill
UK Rock Igneous Scenery Intrusion Sill Xenolith Country Granophyre
The picture shows a piece of rock taken from the margins of a massive granophyre sill on Cader Idris in North Wales. The sill was formed by igneous activity during the Ordovician. As the magma was emplaced, blocks of the surrounding country rock were eroded and then became assimilated into the rapidly cooling melt. This produced the distinctive xenoliths which are clearly visible in the rock.
Volcanic Rocks in Cumbria
UK Rock Igneous Metamorphic Scenery Volcanic Rock Ordovician Cumbria
The picture shows typical volcanic rock scenery in Cumbria. The Langdale Pikes are formed of Borrowdale Volcanic rocks. These date from the Ordovician. Although the rocks are generally resistant, differences in competency have been worked on by the agents of weathering and erosion to produce a very rugged landscape. Glaciation during the Pleistocene played an important part in shaping the present day landscape. It over-steepened slopes and created wide, flat valley bottoms.
Gneiss
UK Rock Metamorphic Scenery Gneiss Augen Pre-Cambrian Regional
Gneiss is a coarse-grained partly banded rock composed of light and dark minerals. The light minerals are mainly quartz and feldspar, the dark ones hornblende and mica. Gneiss is typical of rocks that have been metamorphosed by extreme heat and pressure deep within the Earth's crust (regional metamorphism). Most of the re-crystallisation has taken place in situ whilst the affected rock was still largely solid. However, some gneisses often grade into granites, indicating more or less complete melting. The "augen gneiss" shown in the picture shows lens-like structures of light material from the original rock that may not have been completely melted. Gneisses are associated with the Pre-Cambrian metamorphic complexes to the north-west of the Great Glen in Scotland. They were probably formed by tectonic forces associated with the collision of two major crustal plates early in the Earth's history.
Hornfels in a Metamorphic Aureole
UK Rock Metamorphic Scenery Thermal Contact Aureole Hornfels
Heat from an igneous intrusion frequently affects the surrounding country rocks. The zone where the country rocks are changed is called the metamorphic aureole. The thickness of the aureole depends on the size and temperature of the intrusion and the ability of the country rocks to withstand heating. The picture shows a metamorphic rock called hornfels. It is a hard flinty rock that has been converted from soft mudstones. Some traces of the original bedding remain and the rock contains the sedimentary mica-type minerals chlorite and sericite derived from the original mudstones. Metamorphism of the Cambrian sediments occurred during the Ordovician.
Pre-Cambrian Schists
UK Rock Igneous Metamorphic Scenery Schist Regional Mica
Schist is a rock which has been formed by medium-grade, regional metamorphism involving extreme heating and dynamic pressure. Fine-grained sedimentary rocks re-crystallise in situ to form mica-schists with thin alternating layers, some rich in the mineral mica and perhaps with chlorite and garnet. Fine grained igneous rocks form darker coloured hornblende-schists. Most schists tend to have been heated sufficiently for all traces of the original (parent) rock to be obliterated and as the rock became more plastic characteristic foliated structures began to develop. Schists are associated with very old fold mountain complexes that once existed in the British area. They are common in north-west Scotland and in smaller outcrops that penetrate the cover of younger rocks as shown here in the Charnwood Forest region of Leicestershire.
A Granite Quarry
UK Rock Igneous Metamorphic Scenery Quarry Roadstone Aggregate
The generally hard nature of igneous rocks makes them suitable for a wide range of uses. This picture shows a quarry in ancient granitic rocks that outcrop in Leicestershire. These rocks are particularly important due to a general absence of their kind in the midlands and south-east of England. The quarried stone is used for roadstone, railway ballast and aggregate for concrete mixes. The picture shows part of Cliffe Hill quarry near Markfield in Leicestershire.
A Dolerite Quarry
UK Rock Igneous Metamorphic Scenery Dolerite Quarry Aggregate
Igneous rocks generally provide good construction materials and are quarried in several locations throughout the UK. This picture shows a road-stone quarry for the extraction of dolerite which occurs as a small igneous intrusion. It is located in mid-Wales. The stone is crushed to form a fine aggregate which is then spread on roads with tar to provide a durable surface.
A Slate Quarry
UK Rock Metamorphic Scenery Dynamic Compression Slate Cleavage
Slate is a rock which has been formed by low-grade regional metamorphism. This involved considerable pressures rather than high temperatures. The pressures were most likely those associated with lateral collision of adjacent crustal plates. Some dynamically altered rocks merely show interlocking or fused grains due to compression. Others show complete realignment and distortion of original components and the formation of new minerals such as chlorite and mica. Slate is a very fine grained rock that cleaves (fractures) along fine parallel planes perpendicular to the direction of compression. Because it is possible to split slate into thin, durable slabs, it was quarried extensively for use as a roofing material in the 19th century. Today, slate is relatively expensive and out of fashion. Many slate quarries have closed down but huge amounts of slate waste continue to litter the landscape in places where the rock is found. The picture shows part of a large, disused slate quarry in North Wales.
An Old Tin Mine
UK Rock Igneous Metamorphic Scenery Mineral Mine Tin Copper Vein
The picture shows a long-abandoned tin mine near St Agnes in Cornwall. It is typical of hundreds to be seen all over Cornwall associated with the metamorphic aureole surrounding the massive granite intrusions at Lands End, Redruth and Bodmin. Copper and tin were the chief minerals extracted from ores mined here. The ores occur in veins which mark the final phases of crystallisation from the melt that fed the granite intrusions. The building housed the steam engine which pumped water out of the mine levels. Spoil heaps containing mine waste can be seen in the foreground. These still contain a rich gangue of minerals and provide valuable pickings for modern age prospectors, albeit on a small scale. Most of the Cornish mines closed following the disastrous mining slump during the 1870's.
A Gold Mine in Wales
UK Rock Igneous Metamorphic Scenery Mineral Gold Ore Vein Wales Mine
The picture shows a scene in the Caegwerog gold mine near Dollgellau in North Wales. It shows a typical working, from which the ore was extracted, known by miners as a stope. Tiny amounts of gold were found here in quartz veins that were formed in association with Cambrian and Ordovician igneous activity. The timbers originally supported platforms on which miners stood to drill shot holes for the explosives. Work in these mines was unpleasant and full of danger. Gold is still mined in the area today but on a small scale.
An Old Lead Mine
UK Rock Igneous Metamorphic Scenery Lead Ore Mine Mineral Vein
This old lead mine at Cwm Ystwyth in mid-Wales reached the peak of its production around 1900 and at that time was reputed to be the biggest lead mine in the world. During the 19th century lead was in great demand throughout the industrial world for plumbing and as a roofing material. The mine was abandoned early in the 20th century as lead prices fell and more fashionable materials became available. The derelict building housed machinery to crush and concentrate the ore before it was transported to South Wales for smelting. Veins of the ore (galena) sometimes reached 10 metres in thickness, and were mined underground using a complex of adits, levels and stopes. Drainage proved to be quite a problem in most of these mines. The energy for pumping was provided by water wheels.
Igneous Rock at Stonehenge
UK Rock Igneous Metamorphic Scenery Granite Stone Monument
The world famous ancient monument at Stonehenge on Salisbury Plain is situated in an area completely dominated by sedimentary strata. It is hard to appreciate that it has any connection whatsoever with igneous rocks. However, the stones themselves are granitic and originate from the Prescelly Mountains in Pembrokeshire, south-west Wales. Obviously the ancient people knew where to find stone that was of the quality necessary to build a durable structure. Furthermore they possessed the know-how to transport it many miles over land and sea. The exact purpose of the monument is uncertain, but it is believed to have been some sort of enclosure where, after cremation, Secondary-Neolithic folk buried the ashes of their dead. Today, The Department of the Environment manages this historic site and public access is restricted. Stonehenge is just one example of the use of igneous rock for important structures or buildings.
The Origin of Tors
UK Rock Igneous Scenery Granite Tor Weathering
Here is a sequence of diagrams suggesting the origin of tors. "A" shows the normal process of weathering producing a fairly even cover of weathered detritus overlying bedrock. "B" shows an acceleration of the weathering process brought about by erosion and removal of weathered detritus from hill summits. Increased rotting of bedrock on the slopes of the hill where closely jointed granite occurs tends to steepen the flanks of the summit. In "C", probably under very severe periglacial conditions on exposed summits, weathering and erosion are attacking the joints within the bedrock. "D" shows the eventual form of the tor, a landform of residual boulders in a temperate landscape affected recently by periglacial activity. The size of the residual boulders, many of which moved considerably from the parent outcrop under the influence of gravity, coupled with the durability of the rock, make granite a very useful building stone.
Intrusive Igneous Rock Structures
UK Rock Igneous Scenery Sill Dyke Aureole Batholith Laccolith
Apart from their formation in deep-seated intrusions, igneous rocks were also formed in smaller intrusions near the crustal surface. This picture illustrates some of these intrusive structures. "B" is the batholith, the reservoir from which molten material escaped upwards along weaknesses in the crustal country rocks, "C". These country rocks were baked near the contact with the intrusion forming the metamorphic aureole, "M". The vertical intrusions are called dykes, "D". Large numbers of these are associated with Tertiary igneous activity in South West Scotland. Sills, "S", are more or less conformable to the stratification of the country rocks. They are usually intruded along bedding planes. Sometimes they are discordant where they move from one stratigraphic horizon to another. A classic example of a sill is the Whin Sill in Northern England. Thickened sills produced by up-doming of the country rocks are called laccoliths, "L".
Outcrops of Igneous Rocks in the UK
UK Rock Igneous Outcrop Map
The map shows the general occurrence of igneous rock outcrops in the UK. A geological time scale should be consulted to identify the main horizons at which igneous rocks occur. The periods of tectonic activity which were responsible for these rocks are as follows: Pre-Cambrian igneous activity is recorded in rocks in the North West of Scotland and locally in parts of the English midlands. Caledonian intrusions are found in the Highlands and Southern Uplands of Scotland, in the English Lake District and North Wales. Hercynian activity produced the igneous rocks of Central Scotland, Devon, Cornwall, Pembrokeshire and parts of the Pennines. Alpine tectonics created igneous rocks and structures in Western Scotland and the Antrim Plateau of Ireland.
30
1.02
Geography
Sandstones & Clay Rocks in the UK
2606
The pictures and diagrams in this module are concerned with sandstone and clay rocks. The examples are from the UK and they provide good illustrations of the natrure of these rocks and the scenery associated with them.They also provide an insight into landform development. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere" (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.SandstonesSandstones are sedimentary rocks and they can be found in all regions of the UK. They are formed by lithification of deposited sand. To be classified as a sandstone the constituent grains must be rounded and essentially quartz. The matrix cementing the grains can be different and is often hydrated iron oxide which gives the rock a red or orange colour. Where the grains are angular the rock may be called a grit. A very coarse grained sandstone is called a conglomerate and a very coarse grained grit, a breccia. As sandy deposits were laid down in layers most sandstones show bedding planes. Some were laid down in shallow water and may show current or false bedding. Others laid down in deserts can show dune bedding. Sandstones can also be jointed due to lithification and tectonic movements. The variety of possible origins explains the variety of sandstones found in Britain. The best known of these are:. Torridonian Sandstone of Pre-Cambrian age.. Old Red Sandstone of Devonian age.. Millstone Grit of Carboniferous age.. New Red Sandstone of Triassic age.. Cretaceous and Tertiary sandstones.. Glacial sands and gravels.Sandstones are widely used by man and provide the raw materials for a range of different industries.Clay rocksClays are also sedimentary rocks formed by lithification of sediments. In the case of clays the constituent grains are very fine and essentially clay minerals. Because of their amorphous nature clay rocks rarely show bedding features. Clay is a soft rock and is easily removed by the agents of erosion. For this reason it is often associated with valleys. It frequently forms the lowland in typical scarp and vale topography. Clay is found in all regions of the UK and it often provides the raw material for local industry. Clay rocks also feature throughout the whole of the geological column. They are particularly important in rocks of Carboniferous, Jurassic and Cretaceous age.
Arenaceous Rocks
UK Sandstone Sedimentary Rock Rock Conglomerate Arenaceous
Sandstones are arenaceous sedimentary rocks. They are derived from lithification of sediments composed essentially of quartz grains. The cementing medium may also be quartz or hydrated iron oxide. The latter accounts for the variety of red and orange-yellow coloured sandstones. Sandstones vary considerably in appearance due to the size and shape of the quartz grains, the nature of the cementing material and the degree of compaction of the granular material. The picture shows a collection of different varieties ranging from a course conglomerate to a sandstone where the grains are so fine that they are difficult to distinguish with the naked eye.
Torridonian Sandstone Cliffs
UK Sandstone Sedimentary Rock Scenery
Torridonian sandstone is probably the oldest non-metamorphosed sandstone to be found in the UK. It is of Pre-Cambrian age and outcrops in the north-west highlands of Scotland. It was originally laid down as a sediment resting on a much older basement of Lewisian gneisses. Torridonian sandstone scenery is particularly dramatic along the coast of Sutherland where the hard rock is resisting the rigours of Atlantic gales and seas. Minor lines of weakness such as deep vertical joints have been exploited to give this section of coast its distinctive character. The cliffs are sheer and there are spectacular pinnacles like the Old Man of Stoer.
Breccia
UK Sandstone Sedimentary Rock Scenery Arenaceous Breccia
Breccias are arenaceous rocks consisting of coarse angular blocks cemented in a finer matrix. Their nature suggests that the original sediments from which they formed became buried and lithified relatively quickly. This happened before the processes of weathering and denudation could break them down and round off angular corners. There are situations today where this can happen. An example is where scree collects at the foot of slopes over-steepened by glacial action and where landslips and cliff collapse occurs. However, there are in the UK examples of breccia which formed under circumstances foreign to present-day Britain. For example; those which began as deposits infilling hollows created by a major tectonic event, those which originated in desert basins and those that were deposited in deep submarine trenches. Gwna melange is an example of the latter type. It consists of large blocks (some up to a metre in length) of sandstone, limestone and other rocks. The picture shows a clast of pink dolomite. This unusual formation is thought to have formed by the sliding of sediments down a submarine slope, probably the edge of a deep ocean trench. These rocks can be seen in Bardsey Sound, N. Wales.
A Waterfall over Hard Grit
UK Sandstone Sedimentary Rock Scenery Waterfall Grit Arenaceous
Grits are arenaceous rocks that are composed of small, angular or sub-angular grains cemented in a finer matrix. Most grits are formed from water-borne sediments. The sediment will have been transported sufficiently far from its source for some sorting of the sandy grains into graded sizes, but not far enough for the grains to be worn completely smooth and rounded. Most grits have a terrestrial or shallow-water marine origin. Some, however, were formed by turbidity current action on the edge of the continental shelf or on the sides of ocean trenches. The resistant Cefn Coch Grit was formed by this process. It has led to the development of a spectacular waterfall, Pistyll Cain, where it outcrops amongst softer shales in the Coed y Brenin forest.
Rhinog grits
UK Sandstone Sedimentary Rock Scenery Grit Turbidity Arenaceous
The lower part of the Cambrian succession in the Harlech Dome of N. Wales is dominated by the massive grits of the Rhinog formation. These beds were deposited by turbidity currents carrying sediment into a deep ocean basin from a nearby landmass. The outcrop shows that coarse grit was deposited as the turbidity current first reached the basin floor. This was then followed upwards by laminated sands laid down in the wake of the turbidity current. The upper part of the outcrop indicates the arrival of a second turbidity current, with a return to deposition of coarse grit.
The Longmynd Plateau
UK Sandstone Sedimentary Rock Scenery Arenaceous Plateau Upland
Ancient grits and shales are usually extremely resistant to erosion. In many places they form upland plateaux and are sparsely populated. One example is the Longmynd in Shropshire. Here the arenaceous rocks, Pre-Cambrian grits and shales, have broken down to form a thin acidic soil. This supports only bracken, heather, bilberry and tough, hardy grasses like molinia and nardus. Apart from sheep grazing and limited recreational use, the land has little value.
Old Red Sandstone Scenery
UK Sandstone Sedimentary Rock Scenery Arenaceous Red Devonian
Whilst many arenaceous rocks are associated with poor quality acidic soils, some sandstones give rise to easily managed and reasonably fertile farmland. This picture shows a landscape developed on Old Red Sandstone. The location is in the Welsh borders to the west of Worcester. Mixed farming predominates. Note the reddish hue of the exposed soils in the foreground. This is due to hydrated iron oxide which is the cementing material in the rock. Much of the sediment that forms the Old Red Sandstone series was deposited in freshwater lakes during the Devonian period.
Millstone Grit Scenery
UK Sandstone Sedimentary Rock Scenery Millstone Grit Carboniferous
The Millstone Grit series of rocks were laid down during the Carboniferous period. They consist essentially of massive sandstones interbedded with shales. At their maximum development in the Peak District, they reach about 1,300 metres in thickness. They were probably deposited in a huge delta that extended southwards from a barren, mountainous landmass. Since their formation the rocks have been uplifted and now form the surface rocks over large areas of the central and southern Pennines. On the flanks of the high plateaux weathering and erosion have etched out differences between the hard sandstones and the softer shales, creating distinctive cuestas (escarpments). The sandstones are durable, rough-grained and somewhat porous. More detail of these is provided in the next picture in this module. In general the Millstone Grit soils are of restricted value. They are poor and acidic and support only sheep pastures, grouse moors and coniferous plantations. However, these areas are important gathering grounds for water supplies and are recreational areas for people living in industrial towns that flank the Pennines. Three National Parks, The Peak District, Yorkshire Dales and Northumberland are dominated by Millstone Grit. The picture shows part of the Peak District National Park to the south-west of Sheffield.
Stanage Edge
UK Sandstone Sedimentary Rock Scenery Grit Millstone Carboniferous
Arenaceous rocks often contribute to very distinctive scenery. This picture shows a close-up view of Stanage Edge to the south-west of Sheffield in the Peak District. A more general view of the area is provided in the previous picture in this module. The rock is Millstone Grit, a coarse-grained, durable type of sandstone. The rock probably formed under deltaic and estuarine conditions during Carboniferous times. The bedding planes are easy to distinguish, dipping gently back from the exposed scarp. The scarp is characterised by large vertical joints which resulted partly from shrinkage of sediment when the rock was lithified and partly from earth movements. The joints, in particular, have been opened up by weathering processes, and fallen blocks litter the pediment slope in the foreground. The durability of Millstone Grit accounts for its former use as a building material and for grinding stones in flour mills and cutlery factories. Today, "Gritstone edges" are an extremely popular attraction to rock climbers.
Fossilised Ripples in Sandstone
UK Sandstone Sedimentary Rock Arenaceous Carboniferous Ripple
Coal Measure rocks of Upper Carboniferous age are found widespread throughout parts of central Scotland, the midlands and north of England. They also occur in north and south Wales. For the most part the Coal Measures are dominated by great thicknesses of arenaceous sandstones and shales. Coal, the dark organic deposit from which the name of the series is derived, makes up only a tiny component of the succession. Coal Measure rocks were formed from sediments which collected in deltaic swamp conditions. The environment of that time was subject to periodic change. At one time there was shallow brackish water; this was filled in with sand as huge deltas spread from surrounding land masses. Luxuriant vegetation flourished on top of the exposed deltas, only for it to be inundated by the sea. The deltas spread out again and the whole process was repeated, in whole or in part, many times over. The picture shows an example of fossilised ripples in Coal Measure sandstone. This is evidence of a shallow water environment prior to lithification of the sediment from which the rock was formed.
Graded Bedding in Sandstone
UK Sandstone Sedimentary Rock Arenaceous Bedding Graded Sediment
Closer examination of arenaceous rocks (sandstones) can reveal even more evidence of their sedimentary origin. This picture illustrates the phenomenon of graded bedding. Graded bedding provides evidence of some degree of mechanical sorting of the sediment from which the rock has formed, prior to lithification. This phenomenon occurs where a great mass of sediment was deposited extremely rapidly, probably under the sort of conditions that one associates with turbidity currents or flash floods. Heavier granules of sediment sank to the bottom quickly, whilst the finer fragments remained in suspension for some time before settling. Thus, within the bedding of the rock, one can see coarser grains at the bottom followed by finer material above. This feature can be repeated in overlying beds. It can be a useful indication to field geologists as to whether or not rocks are the right way up.
Brimham Rocks
UK Sandstone Sedimentary Rock Scenery Natural Sculpture Weathering
The picture shows an amazing natural sculpture found high above Nidderdale in the northern Pennines, namely Brimham Rocks. The feature is a result of wind and water penetrating along weaker bedding planes and joints. The near horizontal bedding planes are clearly visible. The dark grey colour of the weathered rock is a normal feature of Millstone Grit. Freshly exposed surfaces are normally bright orange-yellow in colour. However, the rocks have also been affected to some degree by atmospheric pollution and have been blackened by a moorland fire. Natural sculptures of this kind are frequently connected with folklore. Does the outline shape suggest anything to your imagination?
Dune Bedding in Sandstone
UK Sandstone Sedimentary Rock Scenery Arenaceous Bedding Dune
The picture shows an example of dune bedding in Triassic sandstone (New Red Sandstone) exposed in a road cutting at Kidderminster in Worcestershire. This kind of structure is evidence of the rock's origin. It was formed by lithification of sediment which accumulated from aeolian deposition. The distinctive red colour of the rock is due to hydrated iron oxide. This is the mineral which loosely cements the sand grains together. Its presence also points to conditions of aridity at the time the rock was formed. Studies of the dune bedding have determined that the prevailing wind that formed the dunes was from the east. This would fit in with theories that Britain had, at that time, a latitude similar to present day North Africa. Not all the Triassic rocks were dune sandstones though many are arenaceous in character. They include pebble beds deposited from flash floods, marls and salt deposits. It appears that most of the British landscape consisted of low desert plains and playa lakes. Intermittent torrents sometimes spread into the area from a barren mountainous landscape to the west. There is more information on Triassic sandstones in the next two pictures in this module.
A Sandstone Cuesta
UK Sandstone Sedimentary Rock Scenery Trias Arenaceous Pebble Strata
Triassic sandstones tend to be less durable than their older geological counterparts and so form deeper and richer soils. However, contained within the Triassic sandstone strata are distinctive pebble beds. These are probably related to an environment of deposition similar to the present day "reg" of the Sahara. Here flash floods fill water-eroded gullies (wadis) with coarse gravel. Because of their greater ability to withstand denudation, the pebble-rich strata often form distinctive escarpments. Because of the extreme acidic nature of soil developed on these pebble beds they often form heaths or are planted with coniferous woodlands. This picture shows a distinctive Triassic cuesta (escarpment) in north Cheshire. It has been formed by the outcrop of a gently inclined bed of resistant arenaceous rock. There is more information on Triassic sandstones in the next and previous pictures in this module.
Sandstone Cliffs at Torbay
UK Sandstone Sedimentary Rock Scenery Arenaceous Cliff Trias Strata
The geology of the coastal cliffs around Torbay is quite complex. There are distinctive crystalline limestones of Carboniferous age intermingled with arenaceous rocks dating from Permian and Triassic times. The arenaceous rocks are varied both in age and appearance, though a generally pinkish-red colour tends to dominate. Even those tinted green turn to red due to oxidation when exposed. These rocks originated as sediment or debris resulting from erosion of the newly formed Hercynian mountains to the west. The deposits were coarse conglomerates and breccias to start with. They were laid down in tectonic basins sheltered from rain-bearing winds. Later flash floods brought gravel and outwash sands. There were also times when wind-blown sand collected in sheltered areas and fine-grained mud accumulated in shallow, temporary lakes. The cliff section in the picture shows the distinctive red colour, narrow bedding planes and the occasional pebble bed. More information on Triassic sandstones is provided by the two previous pictures in this module.
Cretaceous Sandstone
UK Sandstone Sedimentary Rock Scenery Cretaceous Greensand Carstone
Arenaceous rocks of Cretaceous age include sandstones that were laid down in shallow, estuarine conditions. In this environment drainage was often impeded, resulting in the formation of iron pans. In this picture of an old quarry face, the Lower Greensand rocks are irregularly patterned with bands of carstone. The latter is an almost pure ironstone which was used as a raw material for the early iron industry in Britain. Important early smelting sites were located in parts of Surrey and Sussex. Today, Cretaceous sands are quarried for foundry sand and as a raw material for making bricks and tiles. The Greensand is dominated by friable, unconsolidated orange sands. The "green" prefix is attributed to the green mineral, glauconite, which is found in freshly exposed examples of the rock. On exposure, however, glauconite (an iron silicate) quickly weathers to orange-brown iron oxide. The presence of glauconite provides evidence of the rock's marine origin.
Sandstone and Heath
UK Sandstone Sedimentary Rock Scenery Tertiary Heath
Tertiary sandstones are of fairly recent origin and outcrop over a wide area of southern England. Perhaps the best known locations are in the New Forest and the London Basin where they lie above London Clay. These sandstones are relatively soft and poorly compacted. They exist in a wide range of colours, as can be seen from the sands at Alum Bay on the Isle of Wight. The dominant scenery associated with Tertiary sandstones is open heath like that seen on Chobham Common near Bagshot in Surrey. The plant communities betray an acidic soil developed on underlying soft sands and gravels. Tertiary sandstones can be of shallow-marine, estuarine, freshwater lagoon or river origin. (PictureBase module COM2656 "Heathland Habitats", provides more information on places where Tertiary sandstones occur).
Covehithe Cliffs Suffolk
UK Sandstone Sedimentary Rock Scenery Coast Cliff Erosion
Where younger sandstone rocks outcrop along the coast, they frequently form cliffs that show signs of rapid retreat. In the cliff section shown in the picture the sea has met with partial resistance from a slightly harder, perhaps better cemented, stratum. The latter forms a low ledge beneath the more unconsolidated sandstone formation resting on top. Although what you see is entirely natural, almost horizontal layers of rock have been acted upon by marine and subaerial processes of denudation to produce a landform that looks as if it could have been "man-made". In parts of south-east England where sandstones of Cretaceous age or younger meet the sea, land is being lost at the rate of several metres a year. The only way of preventing this loss is to strengthen sea defences. Unfortunately this can be very expensive.
Moraine
UK Sandstone Sedimentary Rock Scenery Moraine Ice Deposit
Moraine is the collective name given to sediments that are deposited from ice sheets and glaciers. In the UK there are good examples of moraine in several places where, during the Pleistocene, ice emerged from mountain valleys and spread across lowland plains. Most examples are found north of the English Midlands. The picture shows a road cutting through moraine in Scotland. It shows a mass of unsorted fragments of rock ranging from coarse, arenaceous blocks (boulders) to essentially argillaceous, finely-ground rock flour. There is only vague stratification to the sediment as it has probably been re-worked several times. The agents of denudation have even worked on the moraine since it was exposed by road building. Moraine consists of a variety of deposits at former ice margins. Some formed by accumulation where the ice paused for some time as it melted. Others were bulldozed into ridges by temporary re-advances of an ice front.
Sand Dunes
UK Sandstone Sedimentary Deposit Scenery Sand Dune Coast Arenaceous
Sand dunes occur in many places around the coast of the UK. These arenaceous sediments are accumulations of wind-blown sand. The source of the sand is often other loosely compacted deposits of sand on beaches and offshore bars. Dry north-westerly winds following in the lee of a depression are responsible for many sand dunes in coastal areas. Sand dune deposits are unconsolidated and subject to constant change. Almost as quickly as new material is piled up, old dunes are worn away. Frequently, where erosion of dunes has occurred, sedimentary structures can be seen. This picture shows both bedding laminae and ripples. (PictureBase module COM2665 "Sand Dune Habitats", provides detailed information on sand dunes, their structure and ecology).
Shingle
UK Sandstone Sedimentary Rock Scenery Coast Shingle Arenaceous
Shingle is a common arenaceous deposit found in abundance around the coast of the UK. It is made up of a loose, unconsolidated mass of pebbles which are usually well worn and polished. The pebbles are often the remnants of cliff erosion elsewhere along the coast. They can also originate from erosion processes that operated in earlier times, for example glacial transport and deposition. The shingle beach itself is, however, a product of action by waves. Under normal circumstances constructive waves wash fresh pebbles on to a beach. Destructive waves, operating under storm conditions, are responsible for removing material from shingle beaches. Long-shore drift processes can also result in the movement of shingle along the coast. The shingle is, therefore, a feature that is constantly mobile and subject to rapid change. A shingle beach can almost be described as "a river of pebbles". Wide shingle beaches are usually an important natural defence against the sea and need to be conserved. The picture shows part of a shingle beach in Sussex. The pebbles are mostly of flint.
Hard Rocks, Soft Rocks
UK Sandstone Sedimentary Rock Scenery Arenaceous Argillaceous Fault
Sedimentary structures are usually well shown in places where arenaceous (sandstone) rocks alternate with argillaceous (clay) rocks. In this picture of part of the Welsh coast, the easily identified bedding planes have been highlighted by differential erosion of hard Silurian grits and softer shales and mudstones. The sandstones and grits form pronounced ledges. Where the underlying shale has been eroded, blocks of the more resistant rock fall onto the shore platform and are acted upon by wave processes. In this location, differential erosion of the rocks on the cliff face enables close examination of folded strata and a geological fault.
Micro Fold Structures in Mudstones
UK Mudstone Sedimentary Rock Scenery Argillaceous Fold
Mudstones originate from fine grained argillaceous sediments that have undergone a high degree of sorting and transport. They probably accumulated in deep offshore basins. Bedding structures are usually well preserved as illustrated by the rock section in the picture. The camera lens cap provides some idea of scale. An interesting feature is the occurrence of tiny folds within the bedding planes. These are probably the result of disturbance of sediment before the rock was lithified. Disturbance could have been due to gravity collapse on the sea bed triggered off by earthquake activity. It appears that the sliding sediment has eroded the upper surface of more compact mud across which it moved.
The Lias Clay Vale
UK Argillaceous Clay Sedimentary Rock Scenery Vale Lias
Argillaceous (clay) rocks rarely occur naturally on the surface. Their soft nature enables them to weather quickly and become buried with a thick layer of soil. They are often associated with broad, open, lowland landscapes fringed by hills or escarpments of more competent rock. The picture shows a view of the Lias Clay vale near Cheltenham. This major relief feature can be followed from the Bristol Channel to the Tees Estuary. It lies beneath the lower part of the drainage systems of the rivers Avon, Trent and Ouse (Yorkshire). Other important clay vales are found in the Weald, Bedfordshire and Oxfordshire. The Thames follows clay rocks for most of its course, Jurassic clays along its upper reaches and London Clay east of Reading.
The Fens
UK Clay Silt Sedimentary Rock Scenery Argillaceous Fen
The Fens in England form a distinctive region closely connected to the occurrence of soft, argillaceous rocks. Underlying the whole of the Fenland region are Jurassic clays. These were easily eroded to a low level and formed a basin extending out into the Wash from the lower reaches of the rivers Great Ouse, Nene and Welland. The Jurassic clays are now buried beneath a complex of younger deposits which infill the slowly subsiding basin. Around the edges of the Fens are sands and gravels mostly of glacial origin. On the landward side of the Fens peat has accumulated whilst on the seaward side fine-grained marine silts are found. The latter probably resulted from the slow silting up of a vast area of inter-tidal sand much like the present day Wash. Artificial drainage of the Fens enabled good agricultural land to be reclaimed as early as Roman Times. Most of the dykes and canals in use today date from the 18th century. The Fenland region continues to subside and is further threatened by a rise in sea level due to melting of the polar ice caps (a result of global warming). It is, therefore, an area which will require very careful consideration in the 21st century.
Types of Clay
UK Clay Shale Sedimentary Rock Fossil Gault Plant
The picture shows two different examples of argillaceous rocks. On the left is a piece of marine clay. It is fine grained, light coloured and contains a fossil ammonite and bi-valves. It is Gault Clay of Cretaceous age. On the right is a piece of Coal Measure shale. This is dark coloured, splits easily along the bedding planes and contains fossil plant remains. It probably originated under estuarine conditions during Carboniferous times.
A Sand Quarry in Shropshire
UK Sand Sedimentary Rock Scenery Mineral Quarry Quartz
Sand is one of the most quarried minerals in the UK. Sand is the weathering product of quartz, the essential mineral of arenaceous rocks. Quartz is also an important constituent of acid igneous rocks. In sand the quartz grains will have been worn down to a size where their diameter is between 2mm for coarse sand and 0.02mm for fine sand. Sand accumulates in a wide variety of situations including estuaries, deltas, beaches, offshore bars, dunes, river flood-plains and terraces, or in soil above arenaceous rocks. Sand has a wide range of uses. It is used extensively in the building and construction industry. It is a raw material for the ceramics industry and is used as an abrasive. In fact, sand is such a useful commodity that man has exploited it commercially almost everywhere it is found. The picture shows a large sand pit in Shropshire. The sand was originally laid down as a deposit on the bed of an ice age lake.
A Clay Pit in Sussex
UK Clay Sedimentary Rock Scenery Quarry Argillaceous Mineral
Clay is the general name given to rocks or sedimentary deposits that are rich in hydrous aluminium silicates. These minerals are related to micas. They result from weathering of argillaceous rocks and the breakdown of feldspar and ferro-magnesian minerals in igneous rocks. The constituent grains of clays are very fine, usually measuring less than 0.002mm in diameter. Clay tends to be very sticky when wet but dries in hard friable clods. Clay is found in estuaries, in glacial till, in deep sea deposits and in rocks that originated in these places. It also occurs locally as a product of weathering of granite. Clay has many uses and the mineral has been widely exploited by man. It is the raw material for the pottery industry, and is used in the manufacture of cement. It is also used extensively as a filler in paper and cosmetics, and is used when drilling for oil. The picture shows a large pit in the Gault Clay in Sussex. This clay has important combustible properties that make it valuable in the making of cement. The clay is scraped up by earthmoving machines and then mixed with water to form a slurry in a tank. It is then pumped along a pipeline to a cement works about 8 kilometres away.
A Brickworks in Bedfordshire
UK Sedimentary Rock Scenery Clay Brick Bedfordshire Material Building
A brick is a handy rectangular block of clay material that has been dried in a kiln to produce strength and hardness. Although bricks were used as construction material during Roman times in the UK, they did not become fashionable until after the Great Fire of London in 1666. However, in the long phase of urban growth that began in the 18th century and extended into the first part of the 20th century, bricks were extensively used. Most were made from locally obtained clay materials but some areas like Bedfordshire, with massive reserves of excellent "Brick clay", became major suppliers. After mining the clays are dried, ground to an even texture and then screened to remove large lumps. Different clays are then blended as required. These are mixed with water, moulded to shape and finish, dried and finally fired in a kiln. Heating for the kilns was provided by coal furnaces and as a result hundreds of tall chimneys cluttered the landscape of brickmaking areas. Modern brickmaking is largely automated, less labour intensive and much more environmentally friendly than it was in earlier times.
A Map of Sandstones in the UK
UK Sandstone Sedimentary Rock Scenery Map Outcrop
This picture presents a map of Britain showing the location and geological age of the more important sandstone formations. The key to the map is as follows:A - Sandstones of Tertiary age or younger, B - Cretaceous Sandstones (Lower Greensand and Wealden Sandstones), C - Triassic or New Red Sandstones, D - Coal Measure Sandstones including the Culm measures of the South West Peninsula, E - Millstone Grit F - Devonian or Old Red Sandstone. Smaller, older outcrops of sandstones, grits and quartzites are not shown. The spaces between the sandstone outcrops in south-east England tend to be filled with outcrops of clay or chalk.
30
1.02
Geography
Rock Weathering in the UK
2608
The pictures and diagrams in this module are concerned with weathering processes which act upon rock surfaces exposed to agents of the atmosphere. The examples are from the UK and they provide good illustrations of the different ways in which rocks are weathered prior to their removal by agents of erosion. They also provide an insight into landform development. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere" (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.Weathering is the collective term used to describe a variety of processes, which act alone or together, to break down rocks thereby preparing them for removal by agents of erosion and transportation. A piece of rock may disintegrate into smaller and smaller units with a correspondingly dramatic increase in surface area which becomes available for attack by weathering processes. In nature sharp surfaces soon become rounded but the effect remains the same. Once formed, the smaller weathered fragments need to be moved away from weathered surfaces otherwise the debris collects and forms a layer insulating the rock from further attack.Weathering processes can be classified under three main headings although there will be some interaction between these. Mechanical processes are: crystal growth, sheeting and unloading, insolation, abrasion, mechanical collapse, swelling, slaking, cavitation and fire. Chemical processes are: solution, corrosion, hydrothermal contamination, carbonation, chelation, hydrolysis and hydration. Biotic processes are: mixing and transfer to other environments, burrowing, growing roots, absorption and respiration of plants, shade, fermentation, effects of plants on soil moisture. Water is important to just about all types of weathering process. Its presence tends to speed up the rate of rock decay and disintegration.
A Weathered Gravestone
UK Weathering Rock Process Rain Acid Capillary Flaking
Some of the best examples of rock weathering are to be seen in stone which has been used in buildings or worked by man. Fresh stone faces are exposed to atmospheric processes, causing the rock to crumble and disintegrate. A typical gravestone in a cemetery illustrates the point. The limestone shows signs of considerable deterioration since the date it was erected as a fresh and newly sculptured memorial. In the second part of the 20th century acid rain seems to have been an important factor causing a dramatic increase in the rate of weathering. Note how the lower part of the stone is affected by flaking. This is probably due to excessive wetting and drying when ground water is drawn upwards by capillary action. Are there gravestones like this in a churchyard near you?
A Weathered Sandstone Pebble
UK Weathering Rock Process Sandstone Chemical Oxidation Iron
A close-up of a fractured sandstone pebble shows that weathering does not attack the whole of a piece of rock straight away but affects the outer layers first. Note how discolouration, due to the oxidation of iron minerals, has penetrated only a few millimetres in from the outside of the pebble.
A Weathered Upland Surface
UK Snowdonia Weathering Rock Process Upland Severe Mechanical
The picture shows the typical appearance of a mountain landscape at a height of 500 metres or more. Here the surface is subject to severe mechanical weathering, particularly freeze-thaw action, and large angular blocks litter the ground. Biotic processes act very slowly and a skeletal soil remains only in places where it has not been washed downhill. What little vegetation there is helps to protect the surface from mechanical processes but, in turn, enhances activity of chemical and biotic processes.
Sheeting of an Exposed Rock Surface
UK Weathering Rock Process Mechanical Sheeting Unloading
The sheeting or unloading process is an example of mechanical weathering. It is shown in this picture acting on an outcrop of slatey rocks. Note how the top layers break up and fall away under gravity, thus relieving pressure on the underlying rock. The lower layers then begin to push outwards in response to stress release. Rain flows readily into the cracked surface and this, along with the freeze-thaw process which is very effective in winter, helps to complete the break up of the exposed rock.
Weathering Due to Load Release on a Coastal Cliff
UK Weathering Rock Process Mechanical Load Release Sheeting Cliff
Sub-aerial processes, including various kinds of mechanical weathering, are usually very active on the upper parts of exposed cliff faces around the coast of the UK. The picture shows an area where stratified rocks are being attacked. The rocks are dipping towards the viewpoint. Large slabs of the rock appear to be breaking away in response to stress release once the top layers have fallen away. Some of these can be seen in the foreground but most have probably been transported away by waves acting at the base of the cliff. As wave erosion eats away at the foot of the cliff, the exposed surface is steepened. Loose material, which would otherwise have protected the cliff, then falls away. The cliff in the picture is about 20 metres high.
Exfoliation
UK Weathering Rock Process Chemical Mechanical Exfoliation Basalt
"Exfoliation" refers to a flaking and peeling process. It can arise from a variety of causes which can act individually or together. The picture shows a natural ball of weathered Skye basalt. Chemical processes ("rotting") and changes due to heating and cooling of the outer surface of the rock have combined to produce a striking effect.
Cave Dale
UK Weathering Rock Process Chemical Solution Cavern Collapse Limestone
The effect of chemical weathering is amply illustrated by this steep sided, debris filled gorge. The gorge was formed by collapse of the roof of an underground cavern in the Peak District. Solution of limestone by acidic waters, percolating the cavern from above, played an important part in weakening the roof. The cavern itself was originally enlarged during the Ice Age by a sub-terranean stream. Both run-off and the height of the water table would have been higher at that time.
Solution Due to Sea Spray
UK Weathering Rock Process Chemical Solution Coast Limestone
The picture shows a case of extreme solution of coastal limestone in South Wales. Sea spray has been an important influence. However, to dissolve limestone the sea spray needs to be acidic. The most plausible explanation for the acidity is excess carbon dioxide produced by marine life in the inter-tidal zone. Ozone levels are known to be higher near the coast and it is thought that this may also contribute to increased acidity.
A Growing Tree Splitting a Rock
UK Weathering Rock Process Biotic Tree Crack Enlarge Lichen
In this picture a huge boulder is slowly being split by the pressure of a growing tree. Somehow a tree seed fell into a crack in the rock and found sufficient nutrients to germinate and grow. As the trunk thickened, the original crack was enlarged. This is a good example of biotic weathering. Note also the patchy colouring of the rock surface. This is due to lichens. These tiny organisms usually mark the earliest stage of vegetation development on a rock surface. They too assist with weathering. Nearer to home you may see walls and pavements that have been affected by growing plants.
Freeze-Thaw
UK Weathering Rock Process Mechanical Freeze Thaw Icicle
The availability and movement of water below and just above the surface is vital for weathering processes. In the UK there are periods when water becomes temporarily unavailable, as shown by this wintry scene in the English Lake District. The ground and sub-soil are completely frozen. However, under such conditions other weathering processes become dominant. Water freezes in cracks and pores and there is expansion which causes the rock and soil to break up.
Weathered Regolith
UK Weathering Rock Process Mechanical Chemical Biotic
In this picture weathered shales show disintegration and curvature, and have formed a distinctive regolith (soil and sub-soil). This has been affected by a variety of weathering processes; freeze-thaw, oxidation, hydration and biotic activity. Regolith of this kind is typical of upland Britain where rocks are generally hard and impervious, rainfall is high and biotic processes are impeded by low temperatures.
Screes
UK Weathering Rock Process Mechanical Freeze Thaw Scree Talus
Freeze-thaw processes acting on steep, frost-riven slopes of a north-east facing cirque are responsible for the formation of scree. The picture shows good examples of scree or talus slopes in Snowdonia. In places, vegetation is now growing on the more stable scree slopes. Elsewhere, absence of vegetation indicates that the freeze-thaw process and gravity movement are still active. In this view the cirque headwall is about 250 metres high and the screes extend down into the lake, partly filling it in.
Rough Tor, Bodmin Moor
UK Weathering Rock Process Mechanical Chemical Tor Joint Granite
Rough Tor on Bodmin Moor in Cornwall is a good illustration of a distinctive landform produced by weathering. Severe weathering processes acting under periglacial conditions during the Pleistocene Ice Age attacked jointed granite deep beneath the surface. Where the joints were close together the granite was more easily weathered. Where they were well spaced large blocks were relatively untouched. Post-glacial denudation has now removed much of the weathered material, exposing the residual blocks. These blocks appear as piles of large granite boulders. Weathering continues to attack the rocks today. The jointing of the granite is largely due to contraction of the rock mass when cooling from the molten state at the time of its original intrusion. Granite is made up of three main minerals; Quartz, which weathers to form sand, Feldspar, which breaks down to clay, and Mica.
Weathering Along a Fault Plane
UK Weathering Rock Process Mechanical Fault Weakness
Geological faults provide zones of weakness that are rapidly exploited by weathering. Some faults are merely belts of shattered rock between displaced blocks on either side. Such zones are readily penetrated and attacked by acidic waters. Active chemical processes are often identified by distinctively coloured water in the vicinity of the fault, especially if oxides of copper and iron are present. In the field faults can often be detected by close examination of surface features. Depressions and unusual drainage features often outline the fault. Occasionally, along the coast or in quarries, a fault plane is exposed. The picture of a section of a quarry in Shropshire shows how weathering processes are exploiting a newly exposed fault plane.
A Millstone Grit Edge
UK Weathering Rock Process Bedding Joint Weakness Millstone
Weathering processes attack both bedding planes and joints. This action is responsible for the distinctive appearance of Millstone Grit edges in the Peak District. The bedding planes dip gently to the left (in this case, east) and they originate from differences in sedimentation when the rock was formed. There are different sets of near-vertical joints which have helped break up the hard strata into blocks. Note the comparatively large blocks which have become detached from the face of the rock outcrop. This allows the processes to act on freshly exposed areas. At one time the blocks provided an ideal raw material from which millstones could be sculptured.
Differential Weathering
UK Weathering Rock Process Differential Mechanical Chemical Strata
The picture shows differential weathering of horizontally bedded limestones, marls and sandstones. The view is of part of the Cotswold scarp near Cheltenham. Acid rain will have enlarged surface fissures, first created due to load release, by solution. The more competent strata stand proud with the less competent rock sandwiched between the ledges. Notice how weathered material rests on some ledges, thus acting as a shield against further weathering.
Weathering of Folded Strata
UK Weathering Rock Process Mechanical Chemical Strata Fold
The picture shows some of the effects of weathering processes on folded strata. The Silurian mudstones and shales exposed on a cliff in West Wales have been folded into a small anticline and syncline. Note how the axes of the folds are more broken than the limbs. You can also see how the joints open upwards to the elements above the anticline, making it more susceptible to attack by weathering agents.
Weathering and Coastal Landforms
UK Weathering Rock Process Differential Competent Incompetent
The picture shows an attractive part of the Dorset coast at St Oswald's Bay. The spectacular scenery is closely related to the different rates at which the rocks in the area are weathered. Wave action, of course, is responsible for removal of the weathered debris, allowing further differentiation to take place. On the seaward side and running parallel to the coast is a narrow band of almost vertically dipping limestone. This is comparatively resistant to weathering and forms the end of the promontory in the foreground and the band of rocks offshore. On the landward side is a mass of fairly competent chalk that forms vertical cliffs where undercut by wave action. Between the two is a belt of incompetent sands and shales that have been easily excavated to form a bay.
A Natural Arch
UK Dorset Weathering Rock Process Coast Arch
Weathering and erosion processes have combined to create this natural arch. Distinctive landforms like this are only created where the weathered detritus is transported away from the site. In this case wave action has removed rock fragments which have fallen from the limestone cliff, thus helping to ensure continuous development of the landform. In time, the span of the arch will be weakened by weathering and will collapse. The picture is of Durdle Door near Lulworth in Dorset.
Old Man of Stoer, Skye
UK Scotland Weathering Rock Process Basalt Pinnacle Differential
The exotic pinnacles shown in this picture appear to have resulted from differential weathering of old basalt lava flows. The pattern of cracks and joints are of variable intensity. The spires represent the more sound, homogenous and therefore resistant remnants of the old Tertiary basalt flows. In the surrounding area, where the basalt was more intricately fissured, more weathering has occurred.
Periglacial Head Deposits
UK Weathering Rock Process Head Periglacial
During the Ice Age in southern Britain there was a severe climate and lack of vegetation, coupled with increased run-off due to summer thaw. These factors combined to produce a thick blanket of "head" deposits in many valleys. Head deposits are angular and poorly sorted, as shown in this cliff exposure at Clarach in Wales. As the "head" deposits are loosely compacted they are very prone to gravity movement on steep slopes. Vegetation appears to be growing only where the gritty soil stays around long enough for plants to become established.
A Cliff-Scree System
UK Weathering Rock Process Cliff Scree System Feedback Negative
The mantle of weathered material, if not removed, accumulates at the foot of the exposed surface being subjected to weathering. As the height of the scree increases and the height of the free face is reduced, the rate of weathering eventually slows down. In the cliff-scree system this is known as a negative feedback loop. This kind of phenomenon is quite common in natural systems.
The Rate of Cliff Weathering - 1
UK Weathering Rock Process Water Scree Head Change
If one is lucky enough to identify sites where weathering processes are likely to cause rapid change, then visiting that particular site in successive years can provide an idea of what processes are active. This picture was taken just a few years after a road widening scheme was completed. It is the first of two interrelated pictures in this module. The man-made cliff was cut through a section of steeply dipping mudstones and shales. Part of the section was covered by unconsolidated "head" deposits of periglacial origin. Water, streaming down from the hillside above the cliff, found an easy route through the head deposits and encouraged their downslope movement. Note the small cones of scree building up at the base of the cliff. To see how the situation developed, see "The Rate of Cliff Weathering - 2".
The Rate of Cliff Weathering - 2
UK Weathering Rock Process Water Scree Head Change
This is the second of two interrelated pictures in this module. It shows the same road cutting as in the previous picture just two years later on. One can readily appreciate the changes which are occurring. Weathering has excavated more deeply into the "head" deposits, exposing the base of the fencing. Also the cones of scree at the base of the cliff are much bigger. What do you think could be done to prevent further deterioration of this site? (One solution is discussed in "Drainage and Slope Stabilisation" in COM2609).
Man as an Agent of Weathering
UK Weathering Rock Process Agent Man Mining Spoil
There are many ways in which humans act as agents of weathering. Mining activity is perhaps the most important of these. In the search for minerals near the surface, or sometimes deep underground, powerful machines and explosives are used to break up the rocks. Much of the quarried or mined rock is then brought to points on the surface where it is further crushed or treated to extract the valuable minerals. The unwanted residue called "spoil" was often dumped nearby in unsightly heaps. The picture shows spoil heaps at the site of a former crushing mill alongside a 19th century lead mine in mid-Wales. The masonry structures mark the position of the waterwheels that drove the mine machinery.
Weathering of Man-Made Materials
UK Weathering Rock Process Wall Brick
The picture shows a weathered brick wall in Kent. Even ordinary bricks succumb to weathering. Once the mortar has weathered away, many of the bricks have become rounded. The wall was built in the 1920s - was this a bad batch of bricks? The wall is in a very exposed, open position. Aspect often has an important bearing on the rate or severity of weathering. Bricks at the base of any wall are often more extensively weathered than those above. Can you think why? There are, of course, ways of making bricks and walls more resistant to weathering. You could conduct a survey to try to discover more about this topic.
Weathering Due to Farming Activity
UK Weathering Rock Process Farming Man Rain-Wash
Weathering processes can easily be enhanced by the activities of farmers. In this view the slope is so steep the farmer is unable to cultivate his land by contour ploughing. Consequently there will be a high risk of rain-wash affecting exposed soil on the newly ploughed hillside. The chances are that naturally weathered soil will be moved downhill more rapidly than if the field had not been ploughed at all. The hillside will, of course, become more stable again once the newly seeded vegetation becomes established. Even in lowland Britain where slopes are less steep there can be problems related to newly ploughed land. Wind erosion of top soil, sometimes including seed and fertilizer, is quite common in parts of Lincolnshire and East Anglia. The agent of weathering isthe farmer. The field was ploughed and harrowed, but it was not the intention for it to be blown away in a dust storm.
A Rabbit Warren
UK Weathering Rock Process Biotic Animal Rabbit Warren Dunes Sand
Animals play their part in weathering natural systems. This picture shows part of a sand dune environment. In the foreground the flat surface of the dune slacks has been grazed very short by rabbits, making it more susceptible to attack by the agents of weathering. In the background in the drier fixed-dune area there is an extensive rabbit warren. The warren is a network of underground tunnels (burrows) used by the animals for a home. Rabbits live and breed in large numbers in places like this. In addition to their grazing habits, rabbits act as agents of weathering by digging burrows.
A Badger Set
UK Weathering Rock Process Biotic Animal Badger Set
Apart from some species of deer and wild ponies the badger is the largest mammal living wild in the UK. It is essentially a nocturnal creature, feeding at night and spending most of the day in its underground home. This picture shows a huge pile of sandy material thrown out from beneath the hedgerow when a badger family was creating its home (set). Badgers rarely do any harm to farmland as their sets are located mainly in woodland or thick hedgrows. Their burrowing habit, however, makes them an agent of weathering. Fortunately, unlike rabbits, they do not occur in large numbers.
These Boots Were Made for Weathering!
UK Weathering Rock Process Man Leisure Footpath Erosion Conservation
People's increasing enjoyment of remote and hilly landscapes for leisure pursuits has contributed to the need for awareness of over-use. Landscapes become threatened where soil is exposed by removal of vegetation. This scene is in a National Park. The sign, saying "To prevent erosion, follow main path", has been erected to inform hikers of the need to conserve the environment. In such areas it is important to take measures that prevent rapid weathering and subsequent erosion of the landscape. Can you think of other places where it is necessary to control the movement of people in order to protect the environment? Is there somewhere in your school grounds which is protected as a "no go" area?
34
1.01
Geography
Slopes and Mass Movement in the UK
2609
The pictures and diagrams in this module are concerned with the nature and development of slopes and mass movement of weathered material. The examples are from the UK and they provide good illustrations of these topics. They also provide an insight into the processes responsible for landform development. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere" (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.Slopes There are lots of words we use to describe slopes. Those in common use include steep, gentle, long, short, convex and concave. Less familiar terms are rectilinear, pediment and peneplain. We often look upon slopes as two dimensional features and forget that they have a third dimension. Furthermore we tend to think of a slope only as we see it today. We rarely see it as a developing feature, something that was different in the past, something that will change in the future. All slopes are graduating towards some state of equilibrium. Pictures in this module address these ideas in more detail.There are various forces which act on a slope and these can cause it to change its form in time. The parts of a slope which it is useful to be able to identify are as follows:1 The crest of the slope.2 The cliff or free face.3 The debris slope.4 The pediment.Two major types of force act on a slope, these are: i) Tectonic forces from within the Earth.ii) The water and solar energy components of the atmospheric envelope.The processes which are involved in slope development are:A Freeze-thaw action at high altitude on steep slopes.B Gravity force is high on steep slopes leading to rock fall.C Strength of rock type helps to withstand rock breakdown.D Cohesion of rock particles acts against down hill movement.E Transport by running water or snow avalanche on the surface.F Solifluction or soil creep. As soil layer expands and contracts due to normal heating, wetting and drying processes, particles move down hill.G Man's activities may steepen slopes or make them less severe.H Plant roots can be used to stabilise slopes.I Seepage of groundwater carries away minerals in solution. Increase in water load further encourages movement.J Streams erode the foot of slopes and transport debris in suspension and in solution.The distribution of slopes in a landscape can be far from uniform. However, one can usually see certain patterns. Low angle slopes appear to be located in the lower part of river basins and on the higher interfluves of a basin. The steepest slopes are to be found in the middle part of a basin where stream erosion is most effective. The distribution of slopes in a similar sized basin which has been affected by glaciation will show a higher frequency of steeper slopes.Some of the more important theories on slope development incorporate ideas of various geomorphologists. In the field it has been found that slopes do not always correspond to what would be expected if one considers particular hypotheses regarding slope development. It is necessary, therefore, to study individual slopes and test them with general theories. In reality, a slope can be the result of a complex interaction of processes and time. Four basic ideas are incorporated in the following classification:1 Slope development involving the formation of a peneplain surface.2 Parallel retreat of slopes leading to the formation of a pediment.3 The subdued slope development.4 Slope development by wasting and by accumulation.Mass movementMass movement is the term used to describe the falling, slumping, slipping, sliding, flow or creep of weathered material down a slope. The principle force that causes such movement is gravity. All movement is an attempt to reach a position of equilibrium. In nature there are many phenomena that disturb slopes and trigger off mass movement. The principle ones are:. Rainwash.. Water saturation.. Water lubrication.. Freeze-thaw action. . Nivation. . Wind.. Earthquakes.. Volcanic eruptions.. Vibrations from traffic.Rotation slips frequently occur along a coast. The slope is rendered unstable by marine erosion and transportation of debris from the foot of the cliff. Part of the cliff slips downwards along a gently curved slip plain. The latter attracts water which, in turn, makes the structure top heavy, intensifies the processes of rock decay, and ecourages further slipping. Temporary lakes may persist for a time at the top of the slipped block. Slumped material will gradually be moved away by marine processes.
An Uphill Struggle!
UK Slope Concept Climb Hill Steep
Almost all of us from time to time have experienced the sheer physical character of a slope. There are slopes to climb, slopes to run down, slopes to ski down and slopes we should take care not to fall down. But have you ever stopped to consider why there should be a slope there at all? Furthermore do you see the slope as being something developing, in time, towards some state of equilibrium? This picture was chosen to make you think. Other pictures in this module highlight some of the things you can think about concerning slopes.
Scarp Slopes and Dip Slopes
UK Pennine Rock Tilted Slope Erosion Weathering Rectilinear Scarp Dip
Most slopes originate from a complex interaction of both internal and external earth processes. These include mountain building, volcanic activity, denudation (weathering and erosion) and deposition. In any landscape, even on vast plains, you will see slopes. Slopes can be classified in various ways. The landscape of the Southern Pennines in North Derbyshire shows particularly good examples of rectilinear slopes. The steep scarps alternating with gentle dip slopes is typical of a cuesta landscape. It owes its origin to tilted rocks of varying hardness. The easterly dip is to the right of the view.
The South Downs Scarp with Convex and Concave Slopes
UK Slope Movement Mass Gravity Weathering Convex Concave Downs
Convex and concave slopes are well illustrated by the chalk scarp of the South Downs in Southern England. The base of the scarp has a distinctive concave nature and the top of the feature is markedly convex. Both shapes have resulted from weathering processes acting on the steep hillside underlain by chalk. There are various theories on how the steep slope originated. Weathering processes including nivation, and mass movement, in the form of soil creep, appear to have contributed to development of the present surface character.
Slope Components in a Landscape
UK Slope Movement Mass Gravity Weathering Component
The northern part of the Stiperstones Ridge in Shropshire shows a landscape in which there are convex, concave and rectilinear components. For each component it is possible to look critically at the slope, describe its extent and estimate its steepness. In reality few slopes are actually as steep as they appear. Remember also that a slope is a three-dimensional form. The slopes shown in the picture owe their origins to a variety of factors. The most important of these are rock type and its resistance to weathering, geological structure, historical events related to erosion processes and mass movement of weathered regolith under gravity.
A Near-Vertical Slope
UK Slope Glaciation Gravity Weathering Vertical Rock Climb
The picture shows an example of a near vertical slope. Most of us have seen vertical cliffs around the coast but, inland, slopes as steep as this are quite rare. They are usually the result of severe weathering and erosion. The picture shows part of a cirque headwall. Cirques are features that were excessively deepened by glacial processes during the Ice Age (see COM2603 "Past Glaciation and the UK" for details). The steep slope is maintained by freeze-thaw processes acting on the exposed rock aided by gravity. Note the absence of vegetation. Glaciated regions tend to have a much higher incidence of steep slopes when compared to other regions. The scale of the feature can be judged by the rock climber wearing a dark red jacket about half-way up the slope.
A Landscape Incised by Steep Slopes
UK River Slope Rejuvenation Capture Incised Steep Erosion
Where a landscape appears to have been cut into by steep sided, gorge-like incisions it is usually a sign of rejuvenation. Rejuvenation of a river basin can be caused by capture of a stream when a second stream has graded its valley to a lower level (see COM2601 "Rivers in the UK" for details). The picture shows a narrow, steep-sided gorge over 100 metres deep that resulted from river capture. The original surface corresponded to the gentle slopes above the gorge. Capture caused a dramatic increase in the erosive power of the river, enabling it to cut the gorge. Steep slopes like this are rare in inland situations and their occurrence is due to a very special set of circumstances.
A Landscape of Gentle, Lowland Slopes
UK Oxfordshire Slope Lowland Weathering Erosion Undulating
Over much of central and southern Britain the land surface is characterised by more gentle (subdued) slopes. The undulating landscape shown in this picture is typical. The slopes are the result of weathering and erosion processes which have combined and operated on a relatively stable landscape for a long time. On gentle lowland slopes drainage is good, the soil is usually well developed and farming is prosperous. The picture shows a scene in Oxfordshire.
A Gentle Upland Slope, a Peneplain
UK Slope Weathering Erosion Peneplain Plateau
An example of a gentle upland slope is the peneplain occurring at 650 metres OD in the Pennines. During Cretaceous times some 60 million years ago, sea-level was much higher than today and the plateau-like surface represents a peneplain cut to that level (an ancient "base level"). In more recent times rivers and, in some places, glaciers have cut deeply into peneplain surfaces. Zones of weakness such as incompetent rocks and certain geological structures controlled the pattern of denudation. Minor earth movements occurring since the peneplain was formed may have caused some warping of what was essentially an extensive, level surface.
Free Face and Pediment on Millstone Edge
UK Slope Movement Mass Gravity Weathering Cliff Rock Pediment
This is a view of the scarp of Millstone Edge in the Peak District. It illustrates the concept of parallel retreat of the free face and pediment development. Weathering processes acting on the free face of the exposed cliff have caused blocks of gritstone to become loosened. These then break away and fall to the foot of the free face. In turn these blocks break up into smaller pieces. Slowly a blanket of weathered rock builds up below the cliff as a pediment. As long as there is a supply of freshly weathered rock from above, the pediment will grow. Eventually it will grow to such an extent that most of the free face will be covered. The surface slope of the pediment will be determined by the nature of the weathered material and the angle at which it rests, with no further downhill movement. Between the free face and the pediment is the very mobile debris slope.
Slopes of Accumulation and Wastage
UK Slope Movement Mass Gravity Weathering Accumulation Wastage Scree
Slopes of accumulation and wastage can be seen in this view of part of Cader Idris in Snowdonia. They have resulted from extreme weathering of exposed rocks on the free face above. Lack of vegetation on the surfaces indicates that, for the most part, these slopes are mobile. A small stream (left) disappears beneath the surface as it encounters the loose scree on the debris slope. The presence of water on scree encourages movement not only by lubrication but also due to the extra weight that increases the chance of gravity collapse.
The Effects of Rainwash
UK Slope Movement Mass Gravity Weathering Rainwash
Evidence for mass movement of material down a slope is seen here by the exposure of tree roots and curvature of tree trunks. Trees do not normally grow with their roots in the air and it is natural for them to grow erect. Since the area in question is part of a recreation area, some of the dangerously poised trees have been cut down. Both rainwash and gravity have been effective in moving the soft sand and gravel. Movement has been further encouraged by a lack of vegetation beneath the woodland canopy. Extensive conservation measures are needed to protect this site.
Rocks That Can Skate
UK Slope Movement Mass Gravity Weathering Skate Rock
In cold climates movement of surface material can be much enhanced by a combination of freeze-thaw processes and skating of large blocks. The blocks skate as they move downhill slowly on a thin film of melted ice. Friction between the poised block and the frozen ground is sufficient to cause the melting. It must also be remembered that large stone blocks are capable of storing summer heat (natural storage heaters!). These too can melt the ice. During the Ice Ages skating movement was common in many parts of upland Britain, and under severe conditions it may still be going on today. The picture shows a mass of boulders of all shapes and sizes that have migrated down a slope in Snowdonia.
Stone Stripes and Polygons
UK Slope Movement Mass Gravity Weathering Stone Stripe Periglacial
Evidence of the mass movement of weathered material can be seen in stone stripes and polygons. These are a periglacial feature, formed where a surface has been subjected to very low temperatures, probably in the vicinity of a large ice mass. Such conditions existed for long periods during the Ice Age. The stone stripes shown in this picture are on the Stiperstones in Shropshire. Movement has been down a slope of between two and five degrees.
Solifluction Terracettes
UK Slope Movement Mass Gravity Weathering Solifluction Terracette
Soil creep processes (solifluction) are active on the hillside to the left of the picture. The result can be clearly seen in the shadows cast by parallel terracettes. The latter are an example of mass movement of weathered material. Surface soil tends to move downhill under gravity more quickly than lower layers, as it is more readily affected by normal heating, cooling, wetting and drying processes. Roots of the grass plants temporarily stop the movement and help form the terracettes. The location is on the Permian scarp in South Devon. The slope is probably about eight degrees.
Patterned Ground
UK Slope Movement Mass Gravity Weathering Ground Patterned
This picture of part of Plynlimon in Central Wales shows patterned ground or thufur. It is the result of smaller scale movement which usually coincides with a cold winter. A grass mat often covers the clay interior of the thufur. Thufurs are about 25-30cm high and 1-2 metres across. Although their formation is uncertain the lines in the hollows probably represent meltwater drainage zones. Patterned ground is common in places which are normally covered with vegetation but frequently subject to cold temperatures and heavy precipitation.
The Tal-y-Llyn Landslip
UK Slope Movement Mass Gravity Weathering Landslip Scree Cliff
The view is of the western end of Tal-y-llyn in North Wales. The landslip which occurred here is a large-scale example of mass movement on a steep slope. The shales of Tal-y-llyn were relatively easily weathered and a huge mass of them have slumped down into a glacially deepened trough. This has blocked the natural drainage of the valley and caused the lake in the foreground. A small stream overflows from the lake and has cut a channel through the landslip debris. The free face (cliff) exposed by the landslip formerly extended over the top two thirds of the hillside. Weathering processes acting on the free face, plus rock fall and debris slide, have created screes of considerable height. These are helping to protect the free face from further weathering. The angle of rest of the scree slopes is fairly constant, about 32 degrees from the horizontal.
Landslip and Slumping on Back Tor
UK Slope Movement Mass Gravity Weathering Landslip Solifluction
Good examples of slumping and slipping, along with minor features such as solifluction terraces, can be seen here. The picture is a view of Back Tor near Castleton in the Peak District. The slipped materials include masses of sandstone interbedded with shales. Excess water collected in porous sandstone layers sandwiched between impermeable shale horizons. This added weight to the structure and caused the lubricated shales to flow out under pressure, carrying isolated blocks of sandstone with them. The solifluction terracettes are small scale features indicating the down slope movement of soil near the surface.
A Landslip Scar and Debris Slope
UK Slope Movement Mass Gravity Weathering Landslip Scar
This view shows a relatively small landslip feature that has produced a distinctive scar on a hillside. The picture was taken near Edale in Derbyshire. The structure of the area is dominated by alternating layers of porous sandstone interbedded with impervious shales. Excess water collected in the sandstone and added weight to the structure. The shales were lubricated with water and flowed out under pressure carrying the sandstone with them. Screes can be seen on the free face (scar) indicating that the landslip occurred some time ago.
The Mam Tor Landslip - 1
UK Slope Movement Mass Gravity Weathering Landslip
The picture shows a classic example of landslipping on an unstable slope. The slope was probably over-steepened by nivation processes during the Pleistocene Ice Age. It is the Man Tor landslip near Castleton in the Peak District. This distant view of the face of the landslip gives some idea of its size. The debris slope extends well over a kilometre from the free face. The feature has developed in an area of sandstones and shales which were more easily affected by weathering processes and gravity collapse than the more resistant limestone area to the left of the view. The feature is very unstable and there are signs of recent movement. The picture is one of a series of two in this module. (The other picture, "The Mam Tor Landslip - 2", shows a view from the top of the landslip).
The Mam Tor Landslip - 2
UK Slope Movement Mass Gravity Weathering Landslip
The picture is a view from the top of Mam Tor showing part of the debris slope. There is ample evidence of recent movement provided by fresh scars and debris flow patterns extending out in to the Hope Valley. A major trans-Pennine road was built across part of the landslip but the local authority had considerable problems coping with road maintenance. The picture shows the road in a poor state of repair. Since then the route has been abandoned.
The Process of Aggradation
UK Slope Movement Mass Gravity Weathering Aggradation Peat
The abandoned river valley, stretching southwards from Devil's Bridge in West Wales, was originally part of the Teifi drainage basin. The whole of the upper Teifi basin was captured by the river Rheidol a few kilometres north of where the picture was taken. The tract of land shown was thus deprived of a river which would be capable of eroding and transporting weathered debris out of the area. Consequently, the area shows signs of aggradation and infilling with weathered material that has continued to creep down slope from the surrounding hills. During the Ice Age, when there would have been very little vegetation cover, infilling would have been at a much faster rate. Note the extensive occurrence of peat (partly burned in places). Rainfall in the area is high, plant decay is slow due to cold temperatures and the gentle slopes of the aggraded valley are poorly drained. All this has encouraged the development of extensive peat bogs.
Mass Movement on a Coastal Cliff
UK Slope Movement Mass Gravity Weathering Cliff Change
Mass movement on cliff faces supplies beach material to many parts of Britain's coast. Here masses of mudstone are falling from free faces exposed on the coast of mid-Wales. Note in this case how certain geological structures appear to be more susceptible to gravity collapse and attack by the agents of weathering. Debris slopes and pediment are merely temporary features in coastal situations as they are soon eroded and their materials transported away by wave action. Locations like this are subject to rapid change.
Black Ven, a Coastal Landslip
UK Dorset Coast Slope Movement Mass Gravity Weathering Landslip
The picture shows an extremely unstable section of the Dorset Coast near Lyme Regis. Inland there are layers of limestone and shale of Jurassic age, capped by soft Cretaceous sandstones and clays. Cliffs, rendered unstable by differential weathering, have been eroded by wave attack at their base. This, coupled with gravity collapse, has led to considerable mass movement. The result is the Black Ven landslip. This will continue to move at frequent intervals as long as the sea nibbles away at the foot of the debris slope.
Mobile Sand Dunes on a Coast
UK Coast Slope Movement Mass Dune Sand Erosion Deposition
Mobile sand dunes are a feature of mass movement along exposed sections of the coast. Small grains of sand are eroded from beaches and offshore sand bars, particularly by drying north westerly winds. Where the wind strength weakens due to friction as it crosses the rougher surfaces on shore, sand is deposited in huge mounds called dunes. Dunes are generally steeper on the side facing downwind and it is there, in the lee, that most deposition takes place. Marram grass has been planted in the dunes shown in the picture in an attempt to help stabilise them. Sand dunes form an extremely delicate environment, and special conservation measures need to be taken to conserve and protect them. Along some sections of coast, dunes are the only defence against attack by storms.
A Beach and Mass Movement
UK Slope Movement Mass Weathering Beach Shingle Change
Beach material in the intertidal zone is continuously on the move. Incoming waves will either wash debris further up the beach or loosen it so that it is sucked out to sea by a strong backwash. Waves that break obliquely to the shore set in motion a process which moves beach material considerable distances along the coast. (See COM2602 "The Variety of the UK Coastline" for more details). Beaches can be loosely referred to as rivers of sand or shingle. The picture shows part of Chesil Beach in Dorset. The whole structure has been constructed by mass movement of shingle from places along the coast or offshore. Shingle is a product resulting from the weathering, erosion and transportation of rocks.
Attempts to Prevent Mass Movement Along a Coast
UK Coast Slope Movement Mass Weathering Sub-aerial Change
This picture illustrates several points in connection with slopes and mass movement. Firstly there is the near vertical slope of the chalk cliff in the distance. This resulted from wave attack at the base of the cliff, causing the rock above to collapse. Removal of collapsed debris from the foot of the cliff and further wave attack helps to maintain the steep slope. Sub-aerial processes of weathering have little time to affect a steadily retreating cliff face. Where the sea wall and promenade have been built in front of the cliff (in the foreground), sub-aerial weathering processes have begun to modify the cliff profile. There is also evidence of mass movement along the coast. Wooden groynes have been constructed to lessen the effects of long shore drift of beach material. The picture is of St Mary's Bay in Kent.
Stabilisation of Cliffs
UK Slope Movement Mass Gravity Weathering Sub-aerial Erosion Protect
Soft Tertiary sandstone cliffs at Boscombe would be particularly prone to erosion were it not for protection measures. There is a low sea wall and wide promenade which now protects the base of the cliff from wave attack. The cliff to the right of the view has only recently been protected in this way and remains steep in its lower part. As the rocks are soft and loosely consolidated, sub-aerial weathering processes soon reduce the top of the cliff to a more subdued profile. Although vegetation helps to stabilise the sandy slope it is clear that gravity collapse still occurs. Other measures need to be taken to stabilise the steep slopes. Improved drainage to take water away from vulnerable areas and the use of wire retaining nets on the upper slopes of the cliff are two possible alternative strategies. Can you think of others?
Mass Movement on Man-Made Slopes
UK Slope Movement Mass Gravity Weathering Slumping
This picture shows the effects of prolonged heavy rain on a man-made embankment. The latter was built out of clay and was used to support a ramp leading to a road bridge. The embankment had been stable during the eighteen years since it was constructed but following a period of heavy winter rain it began to collapse. The collapse was probably triggered by heavy traffic using the road. Note the freshly exposed scar (free face) and the debris slope that appears to be slumping across the pathway. On the day before this picture was taken the embankment was intact and there was no obvious sign that collapse was about to take place.
Preventing Mass Movement on Man-Made Slopes
UK Slope Movement Mass Gravity Weathering Construction Man Prevent
People are much more aware of the likelihood of mass movement these days. Ideas which reduce the risk of landslides are deliberately incorporated into large construction projects. The road cutting shown in this picture has been stepped to improve drainage and reduce subsidence risks on the rock face which immediately overlooks it. There is also a good chance that, when loosened by weathering processes, the more broken rock at the top of the cliff will not fall directly on to the road. Will the measures taken here be sufficient? What do you think?
Drainage and Slope Stabilisation
UK Slope Movement Mass Gravity Weathering
In this picture of a road cutting there are obvious signs of rapid weathering and slope modification. Several parts of the free face (cliff) have been more extensively weathered and loose material has been washed down to form distinctive talus cones (scree). A simple low-cost way of attempting to solve the problem has been to dig a trench across the top of the cliff and line it with impermeable polythene sheeting. Surface water flowing down from the hill is now diverted into the trench instead of passing down the unstable cliff face. The question is, how well will it work? Only time will tell! (This picture is related to "The Rate of Cliff Weathering 1 and 2", two titles in COM2608 "Rock Weathering in the UK").
Forces Which Act on a Slope
UK Slope Movement Mass Gravity Force Diagram
The diagram examines how various forces act on a slope and cause it to change its form.1 shows the crest of the slope, 2 the cliff or free face, 3 the debris slope and 4 the pediment. The major forces acting on the slope are tectonic forces from within the earth "T" and the water "W" and solar energy "S" components of the atmospheric envelope. Processes which are involved in slope development are:A - Freeze-thaw action at high altitude on steep slopes.B - Gravity movement particularly on steep slopes.C - The ability of strong rocks to withstand breakdown better than soft rocks.D - Cohesion of rock particles acting against down hill movement.E - Transport by running water or snow avalanche on the surface.F - Solifluction or soil creep. Soil layer expands "x" and contracts "y" due to normal heating-cooling, wetting-drying processes. Soil particles gradually move down hill. Top layers move more quickly.G - Human activity which makes slopes steeper or more gentle.H - Plant roots helping to stabilise slopes.I - Seepage of groundwater carrying away minerals in solution.J - Streams eroding the foot of the slope and transporting debris away from the site making the slope more unstable.
The Distribution of Slopes in a River Basin
UK Slope River Basin Slope Distribution Diagram Map
The map of the Rheidol basin in mid-Wales shows that the distribution of slopes in a landscape can be far from uniform. However, one can see certain patterns. Low angle slopes tend to be located in the lower parts of the river basin and on the high interfluves. The steepest slopes are found in the middle part of the basin where stream downcutting is most effective. Figure 2 is a histogram showing the frequency of slopes of particular steepness within the Rheidol basin. Figure 3 shows the frequency of different slopes in a similar sized area which has been affected by glacial erosion. The higher frequency of steep slopes is clearly shown.
Theories on Slope Development
UK Slope Movement Mass Gravity Development Theory Diagram
The diagram illustrates some of the more important theories on slope development. These incorporate the ideas of various geomorphologists. Scientific field studies have found that slopes do not always correspond to what would be expected if one considers a particular hypothesis relating to their development. It is necessary, therefore, to study individual slopes and test them with general theories. In reality a slope can be the result of a complex interaction of process and time. Four basic ideas are incorporated in the diagram with suggestions as to how they may be combined. 1 - Illustrates slope development involving the formation of a peneplain surface "PN".2 - Shows parallel retreat of slopes leading to the formation of a pediment "PD".3 - Suggests subdued slope development roughly conforming to the pre-existing surface profile.4 - Emphasises slope development by wasting "W" and accumulation "A".The diagram also includes models showing how slopes might develop in landscapes where either downcutting or lateral erosion is dominant.
A Diagram of a Rotation Slip
UK Slope Movement Mass Gravity Rotation Land Slip Coast Diagram
The diagram shows the development of a rotation slip (landslide) along a coast. The slope has been made unstable by marine erosion and transportation of debris from the foot of the cliff. Part of the cliff has slipped downwards on a gently curved plane. The slip plane attracts water which intensifies the processes of rock weathering and increases instability. A temporary lake "L" may persist for a time at the top of the slipped block. Slumped material will eventually be removed by marine processes.
44
1.00
Geography
The Earths Internal Processes
2610
The pictures and diagrams in this module relate to the Earth's internal processes and their relationship to earth structure, plate tectonics, volcanoes, earthquakes and major landform development. They are designed to enable the user to be able to: a) Describe the internal structure and composition of the Earth.b) Describe the major subdivisions of the crust and outline the differences between continental crust and oceanic crust.c) Outline the processes taking place at plate margins.d) Understand that there are different kinds of volcanic cone.f) Describe the morphology of simple folds and faults and understand how they were formed.g) Know how earthquakes are detected and measured, and understand their effects.h) Understand the rock cycle and be aware of the importance of geological time.g) Understand that different landforms originate from different processes and under different time scales.
The Earth is a spherical body slightly flattened at the poles; a geode. Its diameter is approximately 12,734 kilometres. The distinctive layers are the Inner Core (solid iron and nickel, 1370km thick), the Outer Core (molten iron and nickel, 1500-2000km thick with a thin sulphide outer shell), the Mantle (dense plastic rock rich in iron, magnesium and silicon, 2900km thick), the Crust (light continental rocks and some heavy oceanic rocks, 10-65km thick) and the Atmosphere (99% nitrogen and oxygen gases). The Earth is not a perfect sphere. Its equatorial circumference is 40,077km. and the polar circumference is 40,009km.
The Earth's Crustal Structure
Earth Structure Crust Sial Sima Discontinuity Moho Density Isostacy
The Earth's crust is made up of two distinct layers. The continental crust is made up of less dense rocks rich in silicon and aluminium (sial). The oceanic crust is composed of heavier silicon and magnesium rich rocks (sima). The rafts of lighter crustal rocks appear to "float" on the heavier material of the mantle. These rafts also respond to transfer of crustal material from one part of the surface to another in a predictable way. This rebalancing of the crust is called isostacy. There is an important discontinuity at base of the crust called the MOHO where there is a sudden increase in rock density passing down into the mantle. Moho is short for Mohorovicic, the Serbian geomorphologist after whom the discontinuity is named.
The upper diagram shows the crust being drawn apart above a rising convection cell in the upper mantle. Rocks in the zone of tension start to melt. In the lower diagram part of the continental crust has subsided and molten material is rising and cooling to form new crust. The world's major rift valleys, the east African rift, the Jordan valley, the Rhine rift and even the central valley of Scotland, were formed in this way.
The East African Rift Valley
Earth Structure Crust Plate Tectonics Mantle Convection Rift Valley Africa
With the exception of Lake Victoria, most of the larger lakes of East Africa are in a huge rift valley system. This stretches over 7,000 kilometres from the Jordan Valley through the Red Sea to East Africa, where there are two branches, and on to Beira in Mozambique. This view, of part of Burundi, shows the northern tip of Lake Tanganyika with Zaire in the distance. Lake Tanganyika stretches for 680 kilometres and is Africa's longest lake but it is more remarkable for its extra-ordinary depth. The surface of the lake is 773 metres above sea level, but the bottom of the lake falls to 700 metres below sea level in places. To the north of the lake is a flat alluvial plain which was probably covered by the lake at some time in the past. The rift valley is one of the Earth's major landforms. It was formed by tremendous lateral forces causing tension and splitting of the African crust. Millions of years of faulting and earthquakes eventually led to subsidence of the central rift. In some places, for example Mount Kenya, volcanic extrusions mark the margins of the rift.
The first diagram shows oceanic crust being stretched and drawn apart by a rising convection cell in the upper mantle. Rocks in the zone of tension begin to melt. In the second diagram new igneous rocks are filling the tension gap as the continents move apart. A study of magnetic reversal patterns proves that the crust is spreading either side of the mid-ocean ridge. There is a good example of this kind of feature running from north to south down the middle of the Atlantic Ocean. Volcanic islands such as Iceland, the Ascension Islands and Tristan da Cunha rise from the mid-ocean ridge.
Submarine eruptions are common events along mid-ocean ridges where massive forces are causing adjacent crustal plates to move apart. Molten rock rises from the zone of tension in the lower crust and upper mantle creating new crustal rocks on the spreading ocean floor. Such eruptions are usually unnoticed but, where lava is erupted above the surface, new islands can appear. A good example of such a landform is Surtsey off the coast of Iceland. Surtsey suddenly appeared in November 1963. Where the volcano erupted ash, as seen here with the Son of Surtsey, the loosely compacted deposits were soon destroyed by the sea.
Geologically Iceland is a very young place. It is made of volcanic rocks which were formed mainly in the last 20 million years. Furthermore at least one tenth of Iceland is covered by lava deposited since the Ice Age. During the last thousand years more than 30 volcanoes have erupted there, some many times. As well as from volcanic cones, lava has poured out of fissures. The one shown here is near Lake Myvatn, it stretches for several kilometres. Fissures like this reflect tremendous tension along a spreading or constructive crustal plate margin.
In the first diagram a convection cell in the upper mantle is causing two continents to collide. One is being drawn down and subducted into the mantle, the other buckles and rides over the downward moving plate. This downwarp is called a geosyncline. Enormous pressures create heat and cause rock in critical zones melt. In the second diagram, which represents a possible later stage, part of a continent is thrust forwards over a lower one. Downwarped crust is rapidly filling in with sediment. Crustal imbalance lifts this up to form new fold mountains. Massive deep-seated igneous intrusions often occur but thickness of the crust prevents widespread surface extrusions. Deep focus earthquakes are associated with movement in the subduction zones. Major landforms created by this kind of plate interaction include the Alps, and Himalayas.
Fold mountain ranges like the Swiss Alps, shown here, are major landforms created by the collision of crustal plates. In this case the African plate has pushed northwards into the Eurasian plate. The collision zone is marked by uplift, complex folding and metamorphism of sediments which earlier filled a Mesozoic geosyncline. The mountain ranges, which are undergoing severe denudation, show an east to west trend.
The upper diagram shows oceanic crust being drawn down beneath the edge of a continent. This process is driven by a descending convection cell in the mantle. Rocks in the contact zones are changed by heat and pressure. In the lower diagram ocean crust has been subducted into the mantle rock system. Where subduction occurs the ocean floor is drawn down into a deep trench. Molten rock from the contact zone is low in density and migrates towards the surface via major fissures. Eventually it cools and forms new rock. The shelf sea may be uplifted to a fold mountain range if crustal imbalance continues. Sometimes only the tops of the mountains penetrate the ocean surface. Major deep seated earthquakes are associated with plate movements of this type. Deep ocean trenches and mountainous archipelagoes are found side by side around the margins of the Pacific Ocean. The Marianas Trench and the Japanese archipelago are major landforms associated with subduction of a crustal plate margin.
In this view of part of the Yosemite National Park in the USA the tops of massive intrusions of granite have been uplifted then exposed by denudation. The dome-like appearance of the granite outcrops is distinctive feature of weathering and erosion of this plutonic igneous rock. The granite cooled and crystallised slowly deep underground from a magma which was related to melting of low-density crustal rocks. These were drawn down into a subduction zone beneath a collision (destructive) crustal plate boundary. In this case the Pacific Ocean plate margin was being forced beneath the western edge of the North American plate.
The unusual markings shown here on an upturned slab of Silurian mudstone are called sole structures. They show how fine-grained sediment collecting in a deep sea trench was disturbed at the time of its deposition. Larger rocks and debris seem to have rolled across the surface of the fine mud making imprints in much the same way as when a person walks across a mud bank. The larger rocks will have been loosened higher up the continental slope by earthquake activity. The earthquake was the result of an oceanic crustal plate being subducted beneath the edge of a continental plate. The existence of these features helps us to reconstruct the geography of ancient geological times. These rocks are found in North Wales. 450 million years ago this area must have experienced conditions similar to that of the west coast of America today.
A World Map of Crustal Plates
Earth Structure Crust Plate Tectonics Margin Map
The map shows the distribution and names of the world's major crustal plates. There are four distinctive types of plate margin: 1. A spreading ridge or rift. 2. A subduction zone.3. A Collision zone.4. A Tear zone where two plates are sliding along their edges.Major landforms created by interaction at plate margins are described and explained elsewhere in this module.
There is some evidence to suggest that in the past the Earth was smaller and that there was just one large continent, Pangea. The top left diagram in the sequence shows Pangea as it possibly was 300 million years ago. Note the position of the equator and a South Polar ice cap. Encouraged by an expanding outer surface and by convection cells in the upper mantle, Pangea began to break up. The second diagram in the sequence shows the situation as it may have been 150 million years ago. The African plate has moved slowly northwards and the southern continents have broken loose. About 80 million years agoIndia had collided with Asia and Australia had drifted east and rotated. The last diagram in the sequence shows the Earth as it is today. The Atlantic ocean is growing in the gap between the American plates and Europe/ Africa. The ancient equatorial forests of Pangea are now preserved in the coal-bearing rocks of the northern hemisphere.
An Erupting Volcano - 1
Earth Structure Crust Energy Volcano Eruption Lava Molten Ash Gas Heckla
Our image of a volcano is usually a huge conical mound of ash and lava, black and smokey, smelly, noisy, spectacular and awesome in eruption. In fact, there are few natural events more spectacular or frightening than an erupting volcano. Large eruptions not only completely transform the character of their immediate area but also release fine volcanic dust. This can be carried by global winds in the stratosphere, shutting out heat and light from the Sun and affecting worldwide climates for many years. Heckla, shown here, is Iceland's most famous volcano. It frequently throws incandescent ash, gases and smoke to great heights. Erupting volcanoes are best seen at night when glowing lava streams can be seen making their way down slope.
An Erupting Volcano - 2
Earth Structure Crust Energy Volcano Eruption Lava Molten Ash Gas Stromboli
Where the prevailing wind is reliable as on Stromboli in southern Italy, it is possible to approach within a few metres of an erupting crater rim. However, one should not underestimate the irregularity of eruptions which can render such expeditions extremely dangerous. Here small fragments of molten rock, some weighing several kilos, are being thrown into the air from the crater. Most of these projectiles fall back into the crater with a dull thud. As they cool they add to the height of the volcanic cone. Serious problems can arise when a vent becomes blocked with agglomerate from earlier eruptions. The new eruption at first needs to clear the blockage. This often happens with explosive force.
The Aftermath of a Volcanic Eruption
Earth Structure Crust Energy Volcano Eruption Lava Molten Ash
Following the eruption of Vesuvius in 79 AD, the prosperous city of Pompeii was completely destroyed. Incandescent pumice, volcanic bombs, cinders and ash rained on the city for a whole day. The whole settlement was completely buried. Rain accompanied the eruption and the volcanic deposits later solidified to form a crust of tuff. In places burial was 7 metres deep. Most of the 25,000 inhabitants fled but some locked themselves in cellars. Over 2,000 died, mostly asphyxiated by the hot, noxious gases. It was not until 1748 that systematic excavations took place. The last major eruption of Vesuvius was in 1944. Vesuvius and other volcanoes in the Mediterranean region are a product of instability due to collision of the African and Eurasian crustal plates.
Types of Volcanic Cone
Earth Structure Crust Energy Volcano Lava Ash Cone Diagram
Shield volcanoes "A" and "B" occur where low viscosity magma rich in magnesium/iron silicates flows out to form wide but relatively shallow cones. Steep sided ash cones "C" and "D" are a product of eruption of acidic, silica-rich magma. This is highly viscous and contains volatile gases and lumps of solid rock resulting from explosions. These cones build up very quickly. The shape of the cone can sometimes be sympathetic to the prevailing wind "E". The central part of the cone can collapse when the underground forces weaken after an eruption "F". The vent of the volcano may become blocked with solid rock and newly erupting magma will have to find a new way to the surface via secondary vents "G". Sometimes the top of the cone will be completely blown off in a huge explosion which clears the vent. A new cone then begins to grow inside the remains of the old "H".
World Distribution of Volcanoes
Earth Structure Internal Crust Energy Volcano Map
Volcanoes mark the points at which molten rock rises to the Earth's surface. This melt, or magma, rises from deep in the crust or from the upper parts of the mantle. The interior of the Earth is very hot indeed and some of the heat spreads outwards and causes convection cells to operate within the rocks of the mantle system. Because they are so hot these rocks behave in a plastic manner, moving upwards to the surface, cooling as they flow beneath the crustal rocks, then descending once more into the depths of the mantle. The pressures and stresses associated with this movement plus the heat energy itself causes pockets of the crust and upper mantle to melt forming a hot fluid called magma. Where the magma reaches the surface a volcano appears. The map shows the location of the world's major volcanoes. Almost all of these occur close to plate boundaries where the crust is weak and the stresses and strains caused by mantle flow are greatest. A detailed description of the nature of plate boundaries is given elsewhere in this module.
A Geyser
Earth Internal Crust Energy Hydrothermal Eruption Geyser
Geysers, like the one shown here in the Yellowstone National Park of the USA, represent the late stages of volcanic activity. The fumarolic stage, as it is called, creates bizarre landscapes in which hydrothermal activity is dominant. In the case of a geyser, ground water seeps into an underground chamber and becomes heated. Heating is steady and eventually causes a pressure increase whereupon the hot water and steam is ejected upwards with some force. Most geyers are very predictable in their behaviour and eruptions can often be timed to the second.
A Sulphur Spring
Earth Internal Crust Energy Hydrothermal Eruption Mineral Spring
Sulphur springs or Solfatara are commonly associated with the late stages of volcanic activity. Here they are seen in an almost dormant crater near to Pozzuoli, west of Naples. Fumeroles, hot or boiling mud, and boiling mineral-rich water are found in the floor of the crater. Gases from the sulphur springs can be quite unpleasant as they smell similar to rotten eggs.
Hydrothermal Mineral Terraces
Earth Internal Crust Energy Hydrothermal Terrace Mineral Spring
Hydrothermal solutions of volcanic origin can contain significant amounts of dissolved carbon dioxide. The water then readily dissolves limestone rocks and the lime is carried in solution until it is exposed to the air. This usually happens at springs where the carbon dioxide is released into the air and the lime is deposited as terraces of travertine. Travertine terraces can be seen at Mammoth Hot Springs in Wyoming, USA.
Hot Springs
Earth Internal Crust Energy Hydrothermal Mud Steam Spring
Pools of boiling water, or sometimes boiling mud, occur when rainwater has found its way down into part of the crust which has become heated by forces associated with plate interaction. At depth the water becomes superheated and rises quickly to the surface along fissures. Minerals released from the hydrothermal solutions often create a natural bowl-like (cauldron) feature lining the fissure as it breaks the surface. The hot water pools shown here are in the Rotorua volcanic region of New Zealand.
Geothermal Energy
Earth Internal Energy Geothermal Crust Heat Technology
The Wairakei power station near Taupo in the centre of North Island, New Zealand, uses geothermal energy to generate electricity. The whole of the region is volcanically active due to interaction between the Pacific and Indo-Australian crustal plates. Boreholes have been drilled into the hot rocks and these have been filled with water which is converted to steam. The escaping steam is then harnessed to drive the turbines which generate electricity. New Zealand produces a high proportion of its energy needs in this way and has become expert in the design and use of geothermal technology.
An Extinct Volcanic Crater
Earth Internal Process Volcano Crater Cone Extinct
This is a view inside the crater of a cinder cone in Lassen Volcanic Park, California, USA. The cone has risen some 225 metres above an ash-covered plain, but today the vent has been blocked by ash and volcanic agglomerate. Some of the material has fallen from the over steep, inward-facing sides of the crater. More gentle scree slopes are now well developed. In this case the volcanic activity resulted from interaction between the North American crustal plate and the Pacific plate subducting beneath it.
The Volcanic Cones of Mauna Kea
Earth Internal Process Volcano Crater Cone Extinct Shield Cinder Hawaii
Mauna Kea in Hawaii is a classic example of a shield volcano. A shield volcano has a distinctive form which is a direct reflection of the nature of the lava it erupts. The lava cones tend to be broad and low-angled. This is because the magma extruding on to the surface is composed of iron-magnesium silicates similar in composition to rocks of the upper mantle. The molten lava cools to form basalt. Basalt lava is hot (1,000 degrees C), IT takes a long time to cool, has low viscosity and flows a long distance before cooling. Slopes of shield volcanoes are rarely more than 20 degrees. The slopes of the lava cone shown here are broken by cinder cones which occur around secondary vents.
An Ash Cone
Earth Internal Process Volcano Crater Cone Extinct Ash
Mount Ruapehu is the highest peak on the north island of New Zealand. It is a good example of a volcanic cone built from layer upon layer of ash and high viscosity lava. The latter was acidic in composition (rich in silica) and cooled quickly. The cone has steep sides in the order of 35 degrees. Despite the fact that the volcano rises above the level of the permanent snowline, there is a warm water lake inside the crater.
A Secondary Cone
Earth Internal Process Volcano Crater Cone Extinct
Volcanoes can be quite complex in structure. This view near the summit of Teide (3718 metres) in Tenerife shows both old and new secondary cones. The highest cone, in the right distance, is now extinct. It was probably due to blockage of the vent leading to the main cone that caused the secondary cones to begin to form. The most recent eruption has produced the darker coloured cinder cone and the blackish, basaltic lava flow. The older lavas appear lighter in colour due to longer exposure to atmospheric weathering processes such as oxidation.
Lava Flows Within a Caldera
Earth Internal Process Volcano Crater Cone Extinct
When a major active phase ends the central part of a volcanic complex can collapse to form a caldera. The caldera is bounded by steep inward-facing, fault-guided walls within which the volcano has subsided. Subsidence is due to release of the magmatic pressures which created the volcano. Here part of the caldera wall of Teidi, in Tenerife, is clearly visible. The central part of the caldera contains a dried lake bed. Drainage was hindered for a time by a major lava flow and the lake persisted until a waterway was cut through the blockage. There are also some residual pinnacles of hard crystalline lava which cooled in dykes and vents.
Deposits of Volcanic Ash
Earth Internal Process Volcano Crater Cone Extinct Ash Tunnel
Here is a road cutting which has been constructed to ease the movement of tourist traffic in the Teide National Park in Tenerife. It exposes layers of different coloured volcanic ash or tuff formed by successive eruptions. Dark coloured lavas are interspersed with the ash. The holes along the sides of the cutting mark tunnels where volcanic gases and water moved through the ash cone.
A Domed Cone
Earth Internal Process Volcano Crater Cone Extinct Tholoid Dome Puy
The extinct volcanic cone at Puy de Dome in the Auvergne, France, is composed of a silica-rich lava which would have been very viscous. Such lavas rarely flow for any distance and instead are squeezed upwards to form a dome which is sometimes called a cumulo-dome or tholoid. This example grew through pressure from within and its smooth summit slopes differ from the more pointed type of volcanic cone.
Wizard Island
Earth Internal Process Volcano Caldera Lake Island Cone Extinct
A great cavity or caldera usually remains when a catastrophic eruption removes the top of a former volcanic cone. The caldera of Crater Lake, Oregon, USA, is about ten kilometres across. It was formed by the explosive removal and subsequent collapse of what was once a lofty composite cone almost 4,000 metres high. A lake now occupies the caldera. At some point in time there was subsequent volcanic activity and the small cone known as Wizard Island rose from the lake. The discovery of obsidian artifacts beneath the pumice to the north of Crater Lake suggests that the Indians must have witnessed the destruction of the main cone. This probably happened about 6,000 years ago.
Rocher de l'Aiguile
Earth Internal Process Volcano Plug Extinct France Chapel Puy
In the Auvergne region of France several important buildings were built on top of steep-sided remnants of a former volcanic landscape. In this case, the Chapel of St. Michel d'Aiguile near Le Puy-en-Velay is strategically situated on top of an 80 metre high lava plug. The plug marks the position of a volcanic vent in which the cooling lava solidified during the final stages of volcanic activity. Softer ashes which originally surrounded the vent and plug have long since been removed by denudation.
The Devil's Tower
Earth Internal Process Volcano Plug Extinct Devil Tower
The Devil's Tower in Wyoming consists of igneous rocks formed by the cooling and crystallisation of once molten magmatic materials. Weathering and erosion have uncovered the more resistant igneous rock by wearing away the softer rocks that once enveloped it. This unique geological feature is 264 metres in height and 305 metres across at the bottom. It probably formed as molten rock pushed upwards and encountered a layer of hard rock which caused it to spread into a flat-topped plug.
Columnar Basalt
Earth Internal Process Volcano Lava Basalt Flow Columnar Extinct
Typical of basaltic lava which cooled and crystallised below the Earth's surface under immobile conditions is the classic columnar structure shown here. The hexagonal pattern of vertical joints is caused by stress resulting from contraction during cooling. The feature has since been uncovered by erosion. It is an entirely natural feature which can be found on the Antrim coast of Northern Ireland where it is known as The Giant's Causeway.
Earthquakes
Earth Internal Process Earthquake Seismic Shock Wave Epicentre Diagram
Earthquakes result from a release of energy when one part of the Earth's crust moves relative to another. It happens like this:First the crust is subjected to stress......then there is more stress......then something gives! Primary "P" and secondary "S" shock waves (seismic waves) travel out in all directions from the focus of the earthquake. These have short wavelength, move rapidly and are long lasting. long "L" waves only travel across the surface from the epicentre. They are powerful and often destructive in character but soon loose their strength.
Earthquake Evidence for Earth Structure
Earth Internal Process Earthquake Structure Evidence Seismic Diagram
The subject covered here is earthquake evidence for Earth structure.In figure 1 primary "P" and secondary "S" waves are refracted as they pass through different layers of the Earth. No "S" waves are detected beyond 103 degrees suggesting the outer core is molten. "P" waves continue through the liquid outer core and reappear beyond 145 degrees. Figure 2 shows how shock waves speed up as they pass into the denser rocks of the Earth's interior. Discontinuities occur near the base of the crust (Mohorovicic) and at the mantle/core boundary. Figure 3 shows the shadow zone where no "P" or "S" waves were received after an earthquake in Japan.
World Earthquake Zones
Earth Internal Process Earthquake Seismic Map
The world map shows the location of zones where earthquakes are frequent. Compare this map with crustal plate boundaries described elsewhere in this module. Earthquakes are common in Mexico, Alaska, Peru, Italy, The Middle East, Japan and Indonesia. Earthquakes are detected by instruments called seismometers but their intensity is described on verbal scales devised by Richter or Mercalli. The following is a simplification of the Mercalli earthquake intensity scale;I. Felt only by very sensitive devices.II. Felt by people at rest.III. Hanging objects disturbed.IV. Doors and windows rattle, vehicles shake.V. Felt outdoors, small "free" objects displaced.VI. Windows break, walls crack, church bells ring, people frightened.VII. Weak masonry falls, difficult to stand, small mudslides.VIII.Walls destroyed, tree branches fall, difficult to steer vehicles,cracks appear in wet ground.IX. People panic, heavy damage to property, underground pipes fracture.X. Much destruction, even well built property collapses, large landslides, damage to water courses. Many people injured.XI. Most infrastructure destroyed (Roads, railways, electicity, gas, water supplies, etc). Some people killed.XII. Catastrophic damage. Massive displacement of surfaces and rocks. Many people killed.
Folds and Faults
Earth Internal Process Tectonic Movement Fold Fault Diagram
Folding and faulting are a result of horizontal stresses applied to stratified rocks. The diagram illustrates some of the common types of folds. 1 is an anticline, 2 an assymetrical anticline, 3 a syncline, 4 an assymetrical syncline, 5 a monocline, 6 Nappe folds, 7 an anticlinorium and 8 a pericline (pitching anticline). Faulting involves a natural breaking and displacement of strata. Different examples of fault are shown. 9 is a normal slip fault, 10 is a reverse slip (Thrust) fault and 11 a tear fault. A combination of a tear fault with a normal or reverse fault is described as an oblique slip fault.
An Anticline
Earth Internal Process Tectonic Movement Fold Anticline Rock
An anticline is an upfold in stratified sedimentary rocks. This can be a large scale feature like the Pennine anticline of northern England or the Wealden anticline of south-east England. Alternatively it can be a small scale feature as shown here. Rocks outcropping today on the coast of Wales include interbedded resistant mudstones and softer shales. These were folded by earth movements during Silurian times. The crest of the fold has been broken by a small fault.
A Syncline
Earth Internal Process Tectonic Movement Fold Syncline Rock
A syncline is a downfold in stratified sedimentary rocks. This can be a large scale feature like Snowdonia in North Wales, the London Basin in southern England or the Paris Basin in northern France. Alternatively it can be a small scale feature as shown here. Rocks outcropping today on the coast of Wales include interbedded resistant mudstones and softer shales. These were folded by earth movements during Silurian times. The syncline has very steeply dipping limbs indicating huge lateral pressures were responsible for its formation. These conditions existed 400 million years ago due to collision of the North American and Eurasian crustal plates. At that time the Atlantic Ocean had not started to form. The trough of the fold has been broken by a small fault.
A Fault
Earth Internal Process Tectonic Movement Fold Fault Rock
A fault is a plane or shatter zone where stratified sedimentary rocks have been displaced. It results from stress and pressure differences acting within the crustal rocks. Movement along the fault allows the stress or pressure to be released. Faults can be large scale and continuous over large distances like the Pennine, Dent and Craven faults of northern England or the Great Glen fault of Scotland. Alternatively they can be small scale features as shown here. This fault has provided a line of weakness which has been readily excavated by sub-aerial weathering and marine processes acting at the base of a cliff.
The Rock Cycle
Earth Internal Process Cycle Change Rock Diagram
The rock cycle is fundamental to our understanding of processes which shape the landforms we see on the Earth's surface today. The cycle is driven both by solar energy and by the Earth's internal energy. Weathering, erosion, isostatic changes, plate movement, and igneous activity all play a part in the recycling of rocks and these processes operate on different time scales.
Scale and Process of Landform Development
Earth Internal External Process Tectonic Change Time Landform Diagram
What we see on the Earth's surface today is the complex result of a variety of processes operating over a long period of time. Factors that have operated to control the development of major landforms are very different to those that have operated to create smaller scale landforms. Small scale landforms that existed in earlier geological times have long since been destroyed when climate acted upon rocks and structures to create new landscapes. However, it takes a long time to destroy a major landform which explains why remnants of fold mountains, formed three or four hundred million years ago, can be seen today. On the diagram "A" represents continents and ocean basins, "B" ocean ridges/trenches, fold mountains and shields. "C" structural basins, rift valleys and mountain blocks, "D" anticlines, synclines, hills, vales and volcanoes, "E" cuestas, terraces, cirques, bays and headlands, "F" meanders and cliffs, and "G" soil.
36
1.01
Geography
Plants and Their Environments
2615
The pictures and diagrams in this module are concerned with plants and their environments. The examples are from the UK and they provide good illustrations of the nature of plants, plant communities and interactions therein. This includes classification and factors which influence plant development. They will be also be useful to studies of conservation matters. The module covers the relationships between plants and habitat in a general way. Detailed studies of plants in individual habitats are provided in other PictureBase modules. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They also provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3), "Environmental Geography" (Gg5) and "Life and Living Processes" (Sc2). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.Plants are living organisms made up of cellulose cell walls. They depend on carbon dioxide and water for their food supply. The action of chlorophyll in sunlight enables plants to convert their food into sugars and other complex materials. They are, therefore, primary producers and this energy can be passed on to other organisms which eat the plants.The study and description of the character and spatial distribution of living organisms, which includes both animal studies and plant studies, is called "biogeography". A study of just the plant elements is called "phytogeography". This series of pictures covers aspects of phytogeography in the British Isles. With a population in excess of 55 million, this comparatively small area of the world illustrates many concepts arising from man's interaction with his environment. It also illustrates quite simply man's dependence on plants for his very existence. Mankind feeds directly on plants, or on animals fed on plants, and plants also provide him with industrial raw materials, building materials, fuel and pure aesthetic pleasure. Although plants are an important part of the natural environment, they are also form an important component of man-made environments.
Deciduous Woodland, the Climatic Climax Vegetation
In theory, any plant succession which develops in the UK should converge towards the climatic climax. This is the point when the annual production of plant matter produced by the ecosystem is balanced by the recycling of decomposing remains. Complex interacting forces in the ecosystem are in equilibrium, with bacteria and animals playing an important part in maintaining this balance. The climax vegetation for Britain is deciduous woodland, but very little remains of the natural oak woodland which once covered the country. It has been lost by forest clearance and cultivation, and those areas that have been re-planted with forest contain different species to the slow-growing oak. Today's deciduous woods contain beech, lime, ash, birch and alder. Were it not for man's activities, much of lowland Britain would look like the scene shown in this picture. Deciduous woodland is the most complex habitat in the UK and, in the natural state, it contains the greatest diversity of living things.
The landscape shown here is an extreme contrast to the climax vegetation shown in the previous picture. Almost all the plants are species that have been introduced into the UK. All the vegetation is managed in some way or other. Grass is mown on a regular basis and trees and shrubs are carefully pruned. Also, organic and chemical fertilisers are added to the soil to promote growth. Herbicides, pesticides and insecticides are used to control plant development. Many of the plants originate from hybrid species grown in specialist plant nurseries. The scene is at Virginia Water in Surrey. Locations like this have considerable aesthetic appeal, not only because of the attractive appearance of the landscape but also on account of the variety of scents produced by different plant species. Introduced species seem to thrive when planted in the UK. This suggests that climate is less of an influence on what grows here than other factors like the ways in which plants are able to disperse their seeds and spread.
Plant Micro-Habitats
Habitat Environment Ecology Plant Habitat Association
This picture shows vegetation growing on a sheltered bank alongside a country lane in southern England. It illustrates the idea of plant micro-habitats. Different associations of plants can often be seen growing in close proximity to one another. The factors which control vegetation development are governed by plant responses to light, temperature, humidity, availability of water and soil fertility. These are in addition to the ability of the plant to reproduce by natural means. In any single location there will be variations in slope, aspect and underlying rock type. In some cases these variations will have been introduced by man's activities.
Natural Factors Which Hinder Plant Development
Habitat Environment Ecology Plant Development Arrest
In some places conditions exist which prevent plant associations from developing naturally towards the climax vegetation. Although the climax vegetation for the UK is oak woodland, there are times when, for some reason, the plant succession stops short. At this point the vegetation is said to be at the plagioclimax stage, i.e. the highest stage of development that can be achieved whilst the arresting factor continues to operate. In the location shown in the picture, exposure to strong winds and salt spray has prevented establishment of trees.
The succession of vegetation which develops on a bare rock or scree surface is called a "xerosere". First of all lichens grow on the bare rock. When the lichens die they decompose to form a nutrient layer that will support various kinds of moss. Dying moss produces an even richer layer of nutrients and eventually a soil is formed. Soon higher order plants such as rushes and grass invade the location. The seeds of these plants take root in the soil and, as more nutrients become available, they are replaced by herbaceous plants and shrubs. Eventually, if there are no natural arresting factors and if human activity does not intervene, deciduous woodland will cover the whole area. The picture shows many features typical of the early stages of a xerosere.
Some Plants Are Large!
Habitat Environment Ecology Plant Flora Tree Beech Tall
Some plants are the largest living things found in the UK. These are species of tree that grow in moist but well-drained soils in central and southern England. Although the oak is the dominant tree of climax forest, it is not the tallest deciduous tree which grows in the UK. Elm, beech and lime can be taller but their crown is not usually so broad and heavy with branches and leaves. The picture gives an idea of the massive form of a beech tree. A great variety of living things are directly supported by such trees. There are insects which feed on the leaves and there are birds that feed on the insects. Birds and squirrels eat the fruit, and tiny creatures feed on the things these animals leave behind. Each tree is almost a distinct ecosystem in its own right. In recent decades almost the whole mature elm population has been lost due to the ravages of Dutch Elm disease, a virus infection carried by a beetle which lives under the bark of the tree.
Some Plants are Small!
Habitat Environment Ecology Plant Flora Small Orchid Bee
The well known expression "small is beautiful" aptly describes many forms in the world of plants. This is well illustrated by the delicate Bee Orchid, Ophrys apifera, shown in the picture. It grows close to the ground and is rarely more than 15cm tall. The distinctive flower is barely 1cm across and is made up of three pale green sepals with tiny, pale green upper petals and prominent anthers. One of the petals is expanded into a lower lip, the colouring of which resembles a bumblebee. This variety of orchid is found in sheltered areas of short grass on lime-rich soils in southern England.
Conditions Which Favour Epiphytic Plants
Habitat Environment Ecology Plant Flora Epiphyte
Fog drip and spray from nearby waterfalls can produce locally a very damp micro-climate. Such conditions not only encourage the development of woodland but also favour the growth of epiphytic vegetation. Epiphytes are plants like ferns and mosses which are able to grow in a nutrient layer that accumulates on the limbs of the trees in damp places. The epiphytes do not take food from the tree itself but merely use it as living space.
Mistletoe, a Parasitic Plant
Habitat Environment Ecology Plant Parasite Mistletoe Custom Kiss
A parasitic plant is one which feeds directly from the host plant on which it lives. Its root system penetrates the host, and xylem and phloem of parasite and host are interconnected. Mistletoe, Viscum album, is a good example. It grows on a variety of trees including poplar, lime and apple in southern Britain. It is an evergreen and is more easily seen in winter when the deciduous host tree has shed its leaves. It usually forms branching clumps on the limbs of trees. The plant bears lime-green, narrow, leathery leaves in pairs. Tiny green flowers which appear at leaf joints in spring give way to conspicuous white berries in the autumn. Mistletoe is used as a Christmas decoration. It is customary for a girl standing underneath it to be kissed by the young man who finds her.
This picture provides a visual example of the effects of cultivation. It shows part of an agricultural research centre in southern England. Eighty years before the picture was taken, the whole of the area in the view was arable land under wheat. No more wheat was planted there, the field on the right was grazed regularly, the field in the foreground was cut once a year and the area in the background was left completely to nature. This resulted in the development of contrasting habitats. The grazed area is poor in plant species, the annually-cut area contains a wealth of flora and the area to the left has the appearance of climax woodland.
A Plant Succession Developing From a Water Surface
The succession of vegetation which develops from water is called a "hydrosere". Within the succession several distinctive stages can be recognised, each with its own particular association of plants. The picture shows a developing hydrosere at Crag Lough in Northumberland. Submerged plants will be colonising the bottom of the lake. These eventually die and their remains collect on the lake floor, providing nutrients for aquatic plants with floating leaves. Around the edges of the lake reed swamp occurs. Here plants are mostly above ground and need to be able to withstand periodic waterlogging. In the drier areas alder, birch and other plants of deciduous woodlands become dominant.
This picture shows part of Woodwalton Fen in Cambridgeshire where a natural fen habitat is the subject of conservation measures. Artificial draining of surrounding farmland has lowered the water table to critical levels. The natural fen vegetation will not survive unless water is pumped into the area to keep the soil saturated. What you see in the picture is not a drainage ditch but an irrigation channel. Water is pumped into this from external sources. Were it not for pumping water into the system, vegetation in the area would continue to develop towards oak woodland. The waterlogged conditions prevent this from happening and fen conditions prevail. The plants which live here provide the basis for a natural, fen ecosystem. It is very important that areas like this are conserved as they contain both rare and endangered plants and they support animals which have a unique lifestyle.
This picture provides a complete contrast to the previous one. It, too, shows a fenland landscape, but this time one which has been drained and cultivated by man. The view shows part of the New Bedford River in eastern England. With the exception of a few weeds, almost all the plants in this landscape were put there by man. The stretch of water on the left is an artificial river channel. Excess water is pumped into it from ditches draining surrounding farmland. The level of the farmland is well below that of the water in the dyke or drain. The dyke needs to be regularly cleared of vegetation and silt in order to reduce the risk of flooding.
A plant succession which develops on a sandy surface is called a "psammosere". Sandy environments occur in various parts of Britain. They are caused either by the accumulation of windblown sand accumulating around coasts or due to the character of the sub-soil. The picture shows a typical coastal dune environment. It illustrates the early stages in the development of a psammosere. Marram grass is dominant. It is a plant that can tolerate both saline and semi-arid conditions. As it grows it sends out lateral shoots which help to stabilise the loose soil. In this location several attempts have been made to stabilise the mobile dunes along a blow-out and thereby prevent further erosion. The view is part of the Ynys-Las dunes in the Dovey estuary in West Wales. There are distinctive plant associations at each stage in the development of a psammosere. The plants form the basis of unique ecosystems, each with its own collection of rare and endangered species.
Inland psammoseres can be seen on the heaths of Surrey and Hampshire in southern Britain. This picture shows part of Chobham Common where the vegetation succession has developed on Bagshot sands. These are essentially loosely compacted strata formed from deposits of estuarine sands of Tertiary age. Vegetation in this area was devastated as a result of its use as a military training ground in the 1940's. The area was not returned to agricultural use and vegetation developed more or less naturally from the sandy surface. Today the landscape is dominated by the occurrence of heather and bracken interspersed with hardy grasses like molinia. These are developing towards a birch sub-climax. Conifers have also become established from seeds blown in from plantations elsewhere in the area.
In the high, moorland environment characteristic of large areas of northern and western Britain there are several factors which prevent the existing vegetation from developing to climax oak woodland. Physical factors such as altitude, low temperatures, high rainfall, exposure to strong winds and a short growing season severely restrict plant growth. A point is reached in the vegetation succession where plants associated with a higher stage are unable to develop. This is called a "sub-climax". Sheep grazing and burning of heather and bracken to improve pastures are arresting factors introduced by man's activities. These lead to plagioclimax vegetation. This picture of part of the Scottish Highlands illustrates the concept of both sub-climax and plagioclimax vegetation.
The Effects of Sheep Grazing On Mountain Vegetation
This picture clearly illustrates the difference between a mountain environment which is continually grazed and one where sheep are excluded. The area in the foreground has been deliberately fenced off and the vegetation is being allowed to develop without interference (apart from clearing bracken). There is a much richer flora here including some alpine species. Insects and birds would soon return to mountain landscapes if more areas were allowed to develop in this way. This project is found at an altitude of about 300 metres on Cader Idris in the Snowdonia National Park.
Coniferous Woodlands
Habitat Environment Ecology Plant Tree Plantation Conifer
Coniferous trees are plants which are well suited to growth in poor quality, acidic soils. They are valuable both as an industrial raw material and in their ability to mature quickly. This has encouraged the establishment of huge plantations of conifers in some parts of the country. The picture shows a typical pine forest habitat. It consists of a dark, dense canopy with an extensive, almost sterile, litter of pine needles covering the ground. Ecosystems like the one shown are relatively sterile when compared to broad-leaved woodlands. They are now discouraged by the Forestry Commission in favour of mixed woodland.
This picture shows a moorland landscape of nardus, molinia and cotton grass on Plynlimon in Central Wales. Continuous grazing by sheep prevents the establishment of higher order plants. As long as sheep are allowed to graze such places the vegetation will remain at the plagioclimax stage. In the wetter hollows waterlogged soils, cold temperatures and a lack of oxygen severely restrict bacterial activity. This means that plants decay slowly and form peat. Natural recycling of nutrients is also slow, although the animals which graze the pastures are an important source of manure.
This picture shows an attempt at land improvement on a steep hillside in West Wales. It includes stripping the land of its existing vegetation and re-seeding. If the farmer is lucky he may be able to convert an unproductive hillside into valuable grazing land. The cost of projects like this can easily be outweighed by the returns from increased productivity. Sometimes farm subsidies help pay for some or all of the work. However, if the farmer is unlucky, the effect of stripping the hillside of its vegetation can herald disaster. Any gains will soon be wiped out due to the onset of soil erosion which can cause both financial and environmental losses. The plant environment may take a long time to recover from disturbances caused by this kind of activity.
The Fire Hazard
Habitat Environment Ecology Plant Forest Fire Hazard
The picture shows a forest fire on a hillside on the Lleyn Peninsula in North Wales during a hot summer. Over 25 hectares of pine forest were destroyed by the fire which was possibly caused by a careless hiker throwing away a cigarette or a glass bottle. Once alight, such fires are difficult to control. This is partly due to their inaccessibility by vehicles, and partly due to the problems of getting water to the spot. This particular fire took four days to burn itself out. Many plants were lost along with some animal life. Any animals that did survive the fire may have to move to other feeding and breeding places until the vegetation recovers. That may take many years in a location like this.
Plant Growth and Lowland Agriculture
Habitat Environment Ecology Plant Farming Science
Woodland clearance, grazing and a considerable amount of re-planting have combined to produce this typically English agricultural landscape. The picture shows part of a farm in east Berkshire. Many of the trees are species introduced for their decorative effect or shade-giving properties. They are, in fact, features of scientific farming. The grass is a hybrid Italian ryegrass noted for its high yield. Chemical fertilisers in the form of nitrates may have been added to increase grass production and herbicides sprayed on troublesome weeds. This kind of landscape is not unattractive and it looks neat. However, it is man-made and expensive to maintain. It also contains fewer plant and animal species than a corresponding natural system. The reason for this is that energy is concentrated on the production of grass and the animals which feed on the grass.
Farming activities are sometimes so specialised that a single crop may cover large areas. Widespread use of fertiliser, pesticides and herbicides ensures that seeded areas yield bumper crops. However, this kind of farming does little or nothing for the world of natural plants and animals. All species but the chosen one are excluded from the land. Monoculture, as it is called, provides an uninteresting and often temporary habitat. Repeating the same crop year after year exhausts the soil and often causes a breakdown in the natural recycling of nutrients. The environment can take a very long time to recover fully from a long period of monoculture. This picture shows a large field of immature field beans, presenting a rather dull spectacle. The plants will be allowed to grow on and will then be ploughed back into the soil. They are legumes and are grown for their ability to "fix" nitrogen in root nodules. When used in crop rotations they act like natural fertilisers, improving the soil for next year's crop of wheat or barley.
Wherever there is diversity of plants there is diversity of animals. The UK is fortunate in that the factors affecting plant growth change over relatively short distances, providing many different habitats. Each of these habitats contains unique associations of flora and fauna. Sometimes a species requires the resources from more than one habitat to ensure its survival. In other cases a species will be able to tolerate life only in one habitat, perhaps even only in one small niche of it. Thus, in the natural world there are many examples of close interrelationships between plants and animals. This picture illustrates one of these examples. The Marbled White butterfly, Melanargia galathea, is remarkably well camouflaged as it collects nectar from the Burnet Saxifrage, Pimpinella saxifraga. Both are found in dry, lime-rich grassland habitats of southern Britain.
One of the most important ecosystems in Britain is the urban one. This view of houses, streets, gardens and factories illustrates the kind of environments in which plants grow in towns and cities. Such man-made systems contain a great many plants which have been introduced not only from other parts of the home region but also from distant parts of the world. It will also contain species which have been bred and developed with a specific purpose in mind. Some of these require special environmental conditions and will be confined to glasshouses. The natural plants that invade urban habitats are regarded as weeds. Some of these produce attractive flowers and fruits, but most are boring or a nuisance and are removed by hand or by use of herbicides (weed-killers).
Researching Plant Growth
Habitat Environment Ecology Plant Research Trial Agriculture
This picture illustrates both man's influence over and dependence upon plants. It is a view of a plant-breeding station in Hertfordshire. This is scientific farming of an advanced kind. New and hybrid varieties of plant are produced in laboratories and eventually they are subjected to carefully controlled and monitored field-trials. After successful field trials, seeds will be issued to farmers and gardeners for more extensive evaluation. Some plant-breeding stations are operated by the government and some by private enterprise. Some are linked to agricultural colleges. They are located in all regions of the UK, each one specialising in some aspect of agriculture relevant to its local region.
Warning of Hazards to Plant Growth
Habitat Environment Ecology Plant Science Information Aphid Pest
In an attempt to increase productivity and thus maximise profit from farmland, the modern farmer needs to have advance warning of important factors which can influence his business. He requires up-to-date information, not only on things like the weather but also on the movements of insects like aphids (greenfly and blackfly). At several locations throughout the country, counts of such pests are made daily during the spring and summer. The information from all the centres is fed into a central computer which then prints maps of aphid distribution and density. This then provides a warning system so that the farmer knows exactly when to spray his plants in order to ensure maximum effectiveness. This picture shows the computer print-out.
A Changed Environment
Habitat Environment Ecology Plant Loss Development Change
Here we can see part of a huge land area and its plant life being drowned by the rising waters of the Empingham Reservoir. This reservoir is in the East Midlands and is now one of the largest in Europe. Increasing demands on land space for water storage schemes means that balanced ecosystems will continue to be destroyed.
Plants are primary producers. It requires a great many individual plants to support herbivores and carnivores at higher trophic levels within an ecosystem. These plants will not always be of the same species and they are also likely to be at different stages of maturity. In order to develop a more complete understanding of the relationships between plants and their environment, it is necessary to carry out a scientific count of plants which occur in a locality. Counting can be done in a variety of ways. One of the most simple techniques is illustrated in the picture. It involves recording numbers and details of plant species within a 25cm square quadrat. By collecting information from several randomly-sited quadrats, it is possible to obtain useful information describing the plants in any one area. The collected data may also be useful in an analysis of how plant populations change according to physical influences such as slope and aspect.
School Out - Plants In!
Habitat Environment Ecology Plant School Playground Xerosere
When left to their own devices plants eventually re-colonise areas of concrete. These areas, like the old school playground shown here, were formerly hostile to any kind of vegetation development. As the surface is no longer trampled by children's feet and remains unswept by the caretaker's broom, it provides the basis for development of a plant succession. It represents the early stages in a xerosere, a succession developed on a bare rock surface. Lichens give way to moss and, in turn, this is replaced by hardy grasses and weeds. It has probably taken about twenty years to develop to this stage. Development to higher stages will take considerably longer as the concrete provides little opportunity for establishing the roots of taller plants.
The diagram illustrates a simple classification of plants by growth-form, based on work by the Danish botanist Raunkiaer. The larger of the two circles shows the plant-form spectrum for the British Isles, a warm temperate region. This is compared to the spectrum for the world shown by the inner circle. The plant-forms shown read as follows: P - Phanerophytes, tall aerial plants, trees and shrubs; G - Geophytes - earth plants, bulbs, tubers and rhizomes; T - Therophytes - annuals; H - Hydrophytes, water plants; E - Epiphytes, growing on trees; HC - Hemicryptophytes, half earth plants, perennials generally protected by habitat; C - Chamaephytes, surface perennials, herbs and shrubs.
Factors Influencing Plant Development
Habitat Environment Ecology Plant Development Diagram Summary
The diagram shows the factors which influence plant development. Successful growth of the individual plant, "1", depends upon the production of fruits and seeds "2", and successful colonisation of a habitat, "3". Three important aspects, i.e. "a" evolutionary development, "b" genetic heritage and "c" geological history, determine why any plant-form is found in a particular place at a certain point in time. These factors are then controlled by environmental factors such as those of climate (temperature, precipitation, humidity, evaporation, light intensity, time of exposure, wind characteristics and seasonal effects). Biotic factors include the effects of man and animals such as irrigation, glasshouse cultivation, plant breeding, crop husbandry, grazing, browsing, control of pests and diseases, etc. There is also the sheer-physical competition between different plants. Physiographic factors are slope, aspect, altitude and drainage. The edaphic environment is controlled by chemical antagonism, the availability of nutrients and competition for root water.
The diagram illustrates interactions within a simple three-component ecosystem. Inputs to the system include climatic factors such as sun and rain, "R", and minerals from the weathering of rocks, "W". Outputs are losses due to surface run-off, "X", and losses due to leaching, "Y". The stages in the cycling of nutrients are (1) the creation of the biomass by the uptake of nutrients from the soil. This provides litter (2) from decaying tissues. The litter decomposes (3) to release new soil nutrients.
The diagram shows the main methods of seed dispersal. This is essential for plant reproduction and colonisation of new habitats by higher order plants. Wind and air currents disperse seeds in several ways: "1", winged seeds (pine and spruce); "2", winged fruits (birch and maple); "3", plumed seeds (willow herb and milkweed) and long-haired fruits and seeds (willows); "4", parachute fruits (dandelion, daisy, cotton grass); "5", jactitation (poppies); "6", spores (mushroom and fungi). Water distribution, "7", (duckweeds) results in robust and well-protected, buoyant seeds which are able to float and then re-establish in a suitable dry habitat. Animals carry pollen essential to reproduction, "8"; they also eat fruits, "9", such as raspberries, strawberries and plums. In this endozoic process the seed needs to be protected from digestion, but upon regurgitation or excretion it germinates well in a nutritious environment. Ectozoic processes of distribution use hooks and barbs, "10", or gummed substances which cling to animal bodies. Also some animals, such as the squirrel, carry off and store concentrations of seed as part of their winter larder. Human activity, "11", has considerable impact on the vegetation by planting seeds in unnatural environments and by preventing the spread of plants by anti-dispersal activities such as weeding and spraying with herbicides. Mechanical dispersal places the new plants only a short distance from the parent plant. The principle methods are rigorous growth of lateral roots or rhizomes, "12", and explosive discharge of spores of fungi and seeds of peas and beans, "13".
The diagram summarises the effects of plant cultivation and provides a basis for discussion of why it is important to be informed of these consequences.
The various stages of vegetation development leading towards a climax are called "seres". This diagram illustrates the various stages in the development of a hydrosere. At stage 1 small water-plants begin to grow in the silt, "S", which collects on the bed of lakes and ponds. Stage 2 shows colonisation by light-seeking, floating plants. At stage 3 bottom-growing plants reach upwards into the air. Dead and decaying vegetation is now collecting in shallow water contributing to a layer of peat, "P". Lack of oxygen and bacteria often prevents rapid decay. At stage 4, higher-order reeds increase the transpiration rate and encourage drying out of the habitat. At stage 5, water-loving shrubs and small trees like alder and willow establish themselves. Stage 6 is the climax forest with oak and birch.
38
1.02
Geography
Weather Conditions in the UK
2619
The pictures and diagrams in this module are concerned with everyday weather conditions. The examples are from the UK and they provide good illustrations of the variety of weather experienced in north-western Europe. They also help to explain why different weather conditions occur. The materials are flexible and can be used to increase knowledge and provide a better understanding of the subject at almost any level. They provide talking points for discussion and will act as a stimulus for further study. The module content has been compiled with the needs of the National Curriculum in mind. It will be particularly useful to those interested in "Physical Geography" (Gg3) and "The Earth and Atmosphere" (Sc3). Many of the pictures will also have a cross-curricular appeal and, as such, will be a valuable asset to libraries and resource centres.Weather conditions feature very strongly in the UK way of life. When did you last talk about the weather? One thing is certain, everyone will have talked about the weather fairly recently, but maybe we have different reasons for doing so. What is it that makes us all so interested in the weather? The reality is that it affects most things we do. The weather influences the clothes we wear, the things we eat, and the activities we take part in. Anyone who works out of doors or who has to travel outside of their home is affected by the weather. Did the weather affect you on your way to school today? For some people knowledge of the weather is vital e.g. farmers, sea fishermen and pilots of aircraft. For those who work in-doors in the artificial climates of homes and offices it appears less important, until there are transport delays, power cuts and food shortages resulting from some extreme mood of the weather outside. Evidence from historical records suggests that from the beginning an elementary knowledge of weather was clearly necessary for man's survival. Before the 17th Century a lack of proper measuring instruments limited meteorology to a few fairly accurate descriptions and explanations of weather phenomena. Aristotle wrote "Meteorologica" in the 4th Century. In general, there was little advance in scientific understanding until quite recently when improvements in technology revolutionised the way we studied what was happening in the atmosphere. The weatherman of today has, at his disposal, high technology for collecting measured information about the weather and rapid communications for spreading the collected data around. There are now efficient ways of storing this data and ingenious means of using it to model situations that help considerably with predicting (forcasting) short term and medium term changes in the weather. However, despite our increasing mastery over the environment, we are only just beginning to understand the weather and we are a long way from being able to control or significantly change it. What is weather? Weather describes the atmospheric conditions that are experienced at a particular place at a particular time. Weather and climate are not the same. Climate is a summary of weather conditions that prevail in a given region or zone over a number of years. Many different kinds of weather can be the components of a climate. This is certainly true of the UK's West European type of climate and it probably explains why we talk so much about the weather. It might not be the same if you were living in another part of the world. Can you imagine a Tuareg nomad in the Sahara saying "Do you think it will be a fine day tomorrow?", or an Eskimo commenting "Its freezing again today!"? Good weather creates a feeling of well being in humans, bad weather can be depressing, annoying, costly and sometimes hazardous. Who has not, at some time, been affected by bad weather? However, let us put things in true perspective - it is our feelings and moods that are affected by the weather. Sometimes we can enjoy staying indoors, perhaps to watch a good film on television, even though the weather outside might might be atrocious. We might also be highly excited at the possibility of being able to go out and play in snow despite the freezing temperatures. Britain's weather is characterised by rapid change and, as such, is said to be unreliable. In reality, although our weather is unreliable, the British climate is extremely reliable - we can depend on it being changeable! It should not come as a surprise, therefore, that our ancestors developed a culture which is extremely rich in weather lore (sayings about the weather).
"Red Sky at Night..."
Meteorology UK Weather Sky Red Lore Atmosphere Dust Pollution Sunlight
"Red sky at night, shepherd's delight! Red sky in the morning, shepherd's warning!" This saying is one of the most famous pieces of British weather lore. It suggests that if a red sky should appear at dusk, the next day will be fine. It is one of several sayings that are based on beliefs concerning the appearance of the sky. If a red sky appears at dawn the day will probably be wet. As it turns out, there is often a great deal of truth in weather lore but it should not be relied upon. There are many factors that contribute to the weather and the influence that any particular factor has can change within a very short space of time. Strongly-coloured skies are common at dawn and at sunset in the UK. Although dramatic at the time of their occurrence, they only last for a few minutes at a time. The colouring is due to a combination of factors. These include; the low height of the sun and refraction of its rays; reflection of sunlight from the base of cloud and vapour layers; the presence of foreign matter such as salt crystals, volcanic dust or pollutants in the atmosphere.
"Cows Lie Down Before the Rain"
Meteorology UK Weather Lore Animal Cow Behaviour Forecast
"Cows lie down before the rain" is an example of British weather lore that is based on the behaviour of animals. It is a statement based on the idea that other living things are more perceptive than man. In reality plants and animals cannot predict the future any more than man. They can only tell you about the past and present weather. However, some species do react very promptly and predictably to weather changes and it is this kind of behaviour that has been observed by man and translated into weather lore. A few sayings, like the one related to the cows, may or may not turn out to be true. This picture was taken during a long period of summer drought and there was no rain at all in the two weeks that followed. Another example of nature reacting to the weather is the Scarlet Pimpernel. This flower is sometimes called the poor man's weatherglass. "It closes its leaves against a rainy hour". This is probably a better indicator. Many plants react to small changes in temperature, humidity, atmospheric pressure and light which can herald the onset of new weather conditions. Such changes are not felt by humans.
The Pine Cone, Nature's Weather Forecaster?
Meteorology UK Weather Lore Forecast Cone Pine
The reaction of natural things to their environment can sometimes be a help to amateur weather forecasters. These people do not interpret natural behaviours to foretell future weather but rather base their ideas on what nature is telling them about the weather at that moment. Seaweed, pine cones and even people suffering from rheumatic pains are used in this respect. Pine cones, for instance, close up during wet weather and open out again when the air dries. This is the best time for the cones to release seeds which will then germinate on the newly moistened ground.
"The North Wind Doth Blow ..."
Meteorology UK Weather Lore Wind Snow North Landscape
Bits of British weather lore that refer to patterns associated with the direction of the wind are quoted by many people. One saying which relies on this relationship is "The north wind doth blow and we shall have snow". In reality, snow does not occur every time the wind blows from the north. Furthermore, snowfall is not just confined to times when the north wind blows. Extensive snow cover in Britain is quite rare but, when it does occur, it causes widespread disruption to traffic. However, the difficult conditions underfoot are not all bad. Winter landscapes such as this February view of the Kirkstone Pass in Cumbria possess a strange but fascinating beauty. There are weather sayings that feature winds blowing from each of the other main points of the compass. Can you think of these?
Gold at the Foot of a Rainbow
Meteorology UK Weather Lore Rainbow Prism Raindrop Shower
Folklore suggests that a crock of gold can be found buried at the foot of a rainbow. It would be a very rare event to discover a treasure of this kind under any circumstances. It would be a truly remarkable affair if one could be found every time a rainbow appeared in the sky. The reason for the saying probably has more to do with the elusive nature of these two things. Crocks of gold feature only in the dreams of most people. Rainbows may not be a subject of human fantasy but the way in which they suddenly appear, offer a bright and colourful spectacle, then fade away makes them an equally elusive phenomenon. A rainbow is a feature of showery weather. It results from the refraction of sunlight as it passes through a raindrop. The white light splits up into different coloured bands in the same way as does light passing through a glass prism. When you face a rainbow you can be sure that the sun is behind you. The outermost colour of the rainbow is red followed by orange, yellow, green, blue, indigo and, on the innermost, violet. Occasionally a secondary rainbow will appear outside the main one. The order of colours in the secondary rainbow will be reversed.
"Rain, Rain, Go Away..."
Meteorology UK Weather Rain Pattern Precipitation Condensation
Raindrops falling on a river make interesting patterns. These patterns seem to be the only interesting thing about a generally drab and depressing scene. Have you seen similar patterns on puddles or on window panes? Rain is quite common in the UK and is the main form of precipitation. Rarely do more than a few days pass by without it raining. When did you last have to wear special clothing in order to go out in the rain? When did rain stop you doing something you wanted to do? Does rain make people feel happy or fed-up? Rain is the product of condensation of water vapour contained in the atmosphere. Water vapour is invisible until the process of condensation begins. Condensation occurs as a result of cooling of moisture-laden air until it becomes saturated, i.e. it can no longer hold its water vapour content at that temperature. Cooling can happen in different ways. One of these is associated with uplift of the air due to movement over a relief barrier (the orographic effect). Another is associated with uplift of air due to excessive heating at surface level (convection). A third reason for uplift and subsequent cooling is associated with weather systems known as depressions (frontal uplift). Each of these items is described in more detail elsewhere in this module.
More Than a Colourless Gas
Meteorology UK Weather Air Pollution Pollen Dust
The air that we breathe is much more than a colourless gas. It also contains tiny particles of solid matter from a variety of sources. These days our attention is often drawn to pollutants in the air resulting from industry, domestic heating and traffic. Heavy pollution can be serious and undoubtedly contributes to "acid rain". The latter is blamed for damage to buildings and the destruction of natural ecosystems. However, not all solid particles in air are a result of man's activities. Some are completely natural, such as wind-blown soil and salt crystals in air blown inland from the coast. Problems are also encountered with pollen grains released into the air by agricultural crops. These are the major cause of "hay fever". Some pollen releases can be quite prolific but we may not be aware of them until it rains and some of the pollen is washed out of the air. The photograph shows a heavy residue of pollen dust that settled from a brief shower only a few minutes after a vehicle had been cleaned. This happened during April in part of south-east England where coniferous (pine) trees were pollinating.
Water Shortage
Meteorology UK Weather Water Supply Shortage Drought Reservoir
Despite the fact that most of the UK receives ample rainfall, water shortages frequently occur. Part of the problem is because most of the population lives in areas in the south and east of the country where rainfall is less. Also, in these same areas, the population density is great and the demand for domestic water and industrial supplies is extremely high. Local supplies often become exhausted following even a short period of drought. It is not unusual for water supplies to be restricted, e.g. by a hosepipe ban, throughout most of the autumn following the drought. The city of Birmingham derives much of its water needs from the distant, but much wetter, Welsh mountains. This picture shows a reservoir with critically low water-level during a summer that immediately followed a relatively dry winter. The reservoir is part of the Elan Valley water storage scheme. Fortunately, the water level was restored to normal after three months of higher than average rainfall.
Signs of Drought
Meteorology UK Weather Drought Mud-Crack Clay
There are many parts of lowland Britain where clay or mud is the dominant surface material. Clay absorbs water quickly, swells and thereby closes up pores and cracks, so making the surface impermeable. Thus, in winter and following times of heavy rainfall, clay surfaces often appear waterlogged. During times of drought, however, the opposite occurs. The clay dries out, shrinks and a pattern of cracks develops at the surface. The ideal pattern, which would mean uniform stress as the clay shrinks, is hexagonal. There is some suggestion of this in the picture, but generally the pattern is irregular due to local conditions affecting the rate of drying. Should heavy rain occur, the cracks may fill in with mud before the swelling of the clay causes them to close again. If this happens, there is a good chance that the original mud-crack pattern will be preserved in fossilised state.
Fire Risk
Meteorology UK Weather Summer Drought Fire Damage Hazard
On hot summer days, when temperatures and evaporation rates are high, there is an increased risk from fire hazards. Fires can start for a variety of reasons, natural and man induced, when vegetation is dry and surface water is scarce. A combination of factors has resulted in a serious farm fire shown in this picture. Controlled stubble burning after a grain harvest appears to have got out of hand. The stubble fire is giving off whitish smoke. Black smoke indicates where the fire has spread to farm buildings. The fire required the efforts of the local fire brigade to douse the flames and prevent more extensive damage.
Hurricane Force
Meteorology UK Weather Wind Hurricane Force Storm Damage Hazard
On the 16th October 1987 much of south eastern England was affected by abnormally strong winds. Gusts of wind reached over 100 kph and considerable damage was caused to property. Fallen trees also blocked roads and railway lines, creating traffic chaos on a massive scale. Wind is the movement of air in response to a large pressure gradient. The air moves from high pressure zones to centres of low pressure. Low pressure at the surface is usually the result of strong local uplift. On the day of the storm portrayed here, a very deep low pressure system passed over southern England. The extreme low pressure happened because of a unique combination of both frontal and convectional uplift. Hurricane force winds of this nature are a comparatively rare hazard, but when they do blow they have a marked effect on people's lives and invariably make national media headlines.
Evaporation
Meteorology UK Weather Evaporation Water Vapour Condensation
Water in the atmosphere has to come from somewhere. Generally it comes from rivers, lakes and seas by direct evaporation. A proportion also comes from evapotranspiration by plants and animals. These processes form fundamental parts of the hydrological cycle whereby water is transferred between different parts of the land - sea - atmosphere system. One aspect of evaporation is illustrated in this view of a Welsh beach. Water vapour is rising from the beach under the influence of a warm, drying wind. Some vapour condenses around nuclei of salt crystals and remains visible for a time, appearing like steam rising from the sand. Eventually the vapour disperses (evaporates) into the atmosphere and it remains invisible until it condenses again when rising over inland mountains.
The Orographic Effect
Meteorology UK Weather Wind Condensation Orographic Relief
Here is a clear illustration of "the orographic effect". This describes the way in which moist winds cool and begin to condense as they climb a relief barrier. In this case the wind is blowing from right to left. The air has passed over a large area of sea, picking up moisture on the way. As it crosses the coast, the air begins to cool and clouds start to develop. Further inland, uplift is greater, cooling stronger and the clouds appear to be much more dense. Scenes like the one shown are common in western Britain when the weather is dominated by air from Atlantic source regions, tropical maritime (Tm) or polar maritime (Pm) air. A diagram explaining the process of orographic uplift occurs elsewhere in this module.
Heat Wave
Meteorology UK Weather Heat-Wave Sea Summer Resort Sunny
Clear, sunny skies and high temperatures are an occasional feature of British summers. They are often associated with a high pressure continental airmass spreading northwards from the Mediterranean and North Africa, the tropical continental (Tc) air mass. Temperatures in excess of 25░C encourage people to visit beaches and other resorts. Roads to such places may become heavily congested and the resorts themselves crowded. However, such times can leave happy memories. If this picture is anything to go by, people do like to be beside the seaside. In recent years a great deal of publicity has been given to potential health hazards when spending a day at the seaside. Due to breakdown of the ozone layer in the upper atmosphere, there is an increasing risk of a person developing skin cancer if they are not properly protected from the sun's rays.
Temperature Inversion
Meteorology UK Weather Fog Inversion Radiation Smog Condensation
Cold air often sinks to low levels during the night on account of its heavier density. This results in a surface level temperature inversion with associated fog on autumn and winter mornings. The fog is due to the condensation of water vapour trapped close to the surface beneath the cold layer. This picture shows an early morning scene in the Thames valley near London Airport. Radiation fogs due to inversion can greatly effect traffic movement both at airports and on roads. However, the fog usually disperses quite rapidly when the sun's radiation increases during the morning. When combined with industrial smoke and grime, the condensing water vapour droplets can produce a dense, unhealthy smog.
The Effect of Altitude on Temperatures
Meteorology UK Weather Condensation Cloud Banner Evaporation Snow
The importance of altitude and its effects on temperatures are well illustrated by this view of Keswick in Cumbria. It shows the mountain called Skiddaw in the background. The top of the mountain is enveloped in a dense, white banner of condensing water vapour. The latter has been caused by the chilling of slightly warmer moist-air as it passed over the cold snow-covered summit. As this air descends the mountain slopes to lower levels in the foreground, it warms up and the cloud evaporates.
An Approaching Warm Front - 1
Meteorology UK Weather Condensation Cloud Front Cirrus Stratus
This view shows a typical western sky with an approaching warm front. A warm front is a feature associated with weather systems called depressions (see elsewhere in this module for details of depressions). Behind the warm front is a wedge of warm moist-air that has originated from the tropical maritime (Tm) air mass. Ahead of it is an area of cooler dense-air originating from the polar maritime (Pm) air mass. At ground level the colder air is stable. However, at higher levels the intermixing of cold polar-air with the uprising warm front is already causing conditions of instability, condensation and cloud formation. At high levels wispy cirrus clouds can be seen. These are formed mostly of ice crystals. In the distance are stratus clouds with more dense cumulus clouds beyond. As the front approaches, the temperature will rise perhaps 2 or 3° C, the barometric pressure will fall and the wind will veer westerly. The next three pictures in this module illustrate more changes associated with the passage of a warm front.
An Approaching Warm Front - 2
Meteorology UK Weather Condensation Cloud Front Alto-Cumulus
Dense alto-cumulus clouds, commonly called "mackerel sky", are features associated with the approach of a warm front. They are usually a sign that wet weather is imminent. Cloud conditions like those shown do not last for long as change is rapid. Dense cloud at a lower level soon replaces the higher and more broken formation. The previous and next two pictures in this module illustrate a range of changes associated with the passage of a warm front.
An Approaching Warm Front - 3
Meteorology UK Weather Condensation Cloud Front Alto-Stratus Halo
We can now see the appearance of the sky at a later stage during the approach of a warm front. See the previous two pictures and the next picture in this module for more details about the passage of a warm front. The cloud base is still fairly high but completely obscuring the sky. The suns rays make a translucent halo where they partly burn through the alto-stratus. Low-level clouds are beginning to show a presence.
An Approaching Warm Front - 4
Meteorology UK Weather Condensation Cloud Front Cumulo-Nimbus
Grey skies with clouds developing at different levels are a very good sign of imminent rain. It will be time to fetch the washing in even if it has not already started to drizzle. The thickening, low-level cumulo-nimbus clouds signify that the front will probably pass overhead in an hour or so. With it will come a rise in temperature, steady rain and a change in the wind direction, usually from the west. The air will become moist and feel muggy. Air pressure will have been falling for some time now and will soon reach its lowest level. The previous three pictures in this module add further detail about the weather conditions associated with the passage of a warm front. A diagram representing a section through a typical warm front is provided elsewhere in this module.
Convection and Turbulence - 1
Meteorology UK Weather Condensation Cloud Cumulus Convection Thermal
The cloud shown here results from very strong thermal activity. The latter is possibly due to strong upward movement of air occurring as a result of convection. The thermal has broken through into the stable air above. Such clouds are a feature of instability and turbulence and are capable of producing heavy rain showers, often with hail or thunder. Rain or hail falling from the cloud can be drawn back into the cloud several times before falling to the ground. This increases the size of raindrops and hailstones. Note the distinct outline of the cloud and its cumiliform (cauliflower) shape.
Convection and Turbulence - 2
Meteorology UK Weather Cumulo-Nimbus Cloud Convection Anvil Storm
Here is a classic example of a cumulo-nimbus anvil resulting from strong convection on a hot summer's afternoon. Thunder storms are often associated with clouds of this type. The cloud walls are vertical or overturned and may be several hundred metres high. When viewed edge-on and with sunlight reflected from them, these clouds appear white. Beneath them or in shadow, they loom very dark and threatening. Rain or hail falling from the cloud can be very localised in its effect. At one end of a street there might be a downpour, while at the other no rain at all. Strong squally winds may accompany the passage of the storm cloud. The strong updraught of the convection cell rising through the middle of the cloud literally sucks air in from adjacent surface areas.
The Cold Sector of a Depression - 1
Meteorology UK Weather Front Cloud Condensation Depression Cumulus
Weather systems called depressions generally pass from west to east across the British Isles at frequent intervals. Depressions are features associated with the intermixing of warm, tropical maritime (Tm) air and cooler polar maritime (Pm) air. Detail of a typical depression is provided elsewhere in this module. The passage of a cold front eventually results in clearing skies dominated by cumulus clouds. However, at high levels cirrus and stratus clouds may persist for a while. The picture shows a view of the sky looking towards the east. It is about six hours after a period of rainfall associated with the passing of a cold front. The air temperature will have dropped by about 3░C and the wind will have veered to the north west. Humidity will have fallen and the air will feel "fresh". Barometric pressure will now be rising due to the dominance of colder, denser air. The next two pictures in this module illustrate more features associated with the passage of a cold front.
The Cold Sector of a Depression - 2
Meteorology UK Weather Condensation Cloud Cumulus Stability
This is the appearance of the sky on a summer day after the passage of a typical depression. Although this is within the cold sector following a frontal system, summer temperatures are warm and the sky is dominated by flat-based cumulus clouds. These pinpoint rising thermals of warm moist-air produced by local differences in surface temperatures. In this case, the flattish nature of the shallow clouds indicates fairly stable conditions. The small anvil represents some outward spreading of the thermal which does not penetrate the layer of warmer, stable air above. The condensation level is clearly marked by the base of the clouds. The previous and next pictures in this module provide further details of cloud patterns associated with the cold sector of a depression.
The Cold Sector of a Depression - 3
Meteorology UK Weather Condensation Cloud Cumulus Fair-Weather
Fair-weather cumulus clouds are often a feature of cold polar-air returning in the lee of a depression. Usually, as their name implies, they are a sign of better weather to come. The associated weather is often cool but fresh, the wind blows from the north-west and barometric pressure is rising. Eventually the wind speed will drop and stable sunny conditions will prevail until the arrival of a new frontal system from the west. In winter these conditions will often be followed by frost and fog associated with cold stable air of high barometric pressure (an anticyclone). The apparent fan pattern across the sky made by the clouds in the picture is simply due to perspective.
Aircraft Vapour Trails
Meteorology UK Weather Condensation Aircraft Vapour Trail
Air traffic has shown a dramatic increase in recent decades and cloud patterns are often complicated by aircraft vapour trails. Here you can see a vapour trail created by disturbance due to an aircraft passing high overhead. In the distance the white vapour trail is shown below the stratus layer. In the foreground the trail is above the natural cloud layer upon which it casts a shadow. Disturbance of stable air, heat from jet engines and tiny particles of spent fuel which act as condensation nuclei, all help to form a vapour trail. Eventually conditions return to normal and the trail will fade away. Distortion of vapour trails frequently provides an insight into upper air movements. The speed and direction of air movement at high level is often very different to that near the ground.
Frost
Meteorology UK Weather Condensation Frost Ice Dew
Frost is a common feature of Britain's weather during the winter months. It is normally associated with anticyclonic (high pressure) weather systems and, in particular, very cold air from the polar continental (Pc) source. Frost can, in fact, occur on almost any winter night when skies are clear and there is little wind. The lack of cloud cover means that most of the Earth's outgoing radiation escapes into space, resulting in a noticeable drop in temperature. As the temperature of moist air falls below its dew point, condensation occurs. Water vapour from the air condenses on the surfaces of plants. The condensation takes place on condensation nuclei on the leaves and stems. As the temperature drops even further, much of this condensation turns to ice. Here you can see the formation of ice crystals (frost) on a garden plant. Maps illustrating the occurrence of frost in the UK are provided elsewhere in this module.
The Effects of Towns and Industry
Meteorology UK Weather Urban Climate Heat Pollution Radiation Air
A complex of reflecting and absorbent surfaces, plus generation of heat by artificial means, gives urban areas a distinctive climate of their own. Surfaces like tarmac are good conductors of heat and absorb radiation during the day. At night they emit long wave radiation in the same way as a domestic storage heater. Sunlight, reflected back from buildings, changes to long wave radiation. This is more readily absorbed by dust and pollutants in the atmosphere. It is no wonder that people have identified towns and cities to be hotter and stuffier than the countryside. Towns can also be windier since surface air flows become channelled along narrow streets. When it rains over an urban area the run-off quickly disappears down drains and sewers. This means that less solar heating is required for processes like evaporation, thus providing more energy for heating the air. Town pollution adds to the availability of condensation nuclei. Urban areas are probably wetter as a result, despite the higher temperatures. Pollutants also shut out sunlight and reduce visibility. Cloudiness is greater and fogs are common. Despite this towns and cities seem to have little effect on large scale weather systems like depressions.
A Weather Station
Meteorology UK Weather Station Recording Stevenson Instrument
This is the layout of a typical British weather station. It is a view of part of a grassland research station in Wales. The weather station contains a variety of instruments for measuring and recording the weather. The main feature is the Stevenson screen. This houses a maximum and minimum thermometer recording the daily range of temperature, plus a wet and dry bulb thermometer for calculating humidity. The station also contains a wind vane for measuring wind direction, an anemometer for measuring wind speed and a sunshine recorder. There are also two different types of rain gauge (one automatic), thermometers for measuring both ground and soil temperatures, an evaporation tank and a device for recording evapotranspiration. There are stations like this located throughout the British Isles. They not only provide information which is important to the local community but also send records to national centres such as those run by the Meteorological Office.
The Meteorological Office
Meteorology UK Weather Met Office
Recordings from weather stations located at hundreds of points throughout the British Isles, as well as those reported from lightships, trans-atlantic aircraft and the like, are collected at the headquarters of the Meteorological Office at Bracknell in Berkshire. Here the information is fed into computers for speedy and largely accurate short-range forecasts. Long range forecasts are also made using more complex computer modelling and simulation techniques. The Met. Office is best known for its role in processing the data needed for preparation of forecasts to be broadcast on radio or television. However, it also provides a wide range of services to industry, commerce and scientific organisations, both at home and overseas. The function of the main Meteorological Office building is betrayed by satellite dishes, radar and transmitting equipment on its roof. Other parts of the organisation are housed in commercial premises elsewhere.
Mist Rolling in From the Sea
Meteorology UK Weather Condensation Coast Fog
The UK has a long coastline that is very much affected by the warming influence of the North Atlantic Drift. Frequently there is a marked contrast between the air over the land and the air over the sea. This can be noticed when conditions of stability prevail and the land rapidly looses heat at night. Such a situation is illustrated in this picture of part of the coast of north-western Scotland during October. Cold dense-air has moved out from the land, has mixed with warmer moist-air over the sea and has condensed forming a fog bank. As the land warms up during the day, the mist will often roll in from the sea before dispersing. Coastal fog banks are not just confined to winter and conditions of stability. They can occur at any time of the year when there is a tropical maritime (Tm) airstream. Generally the air is warmer than the sea over which it passes and condensation occurs. Sea fogs are often quite unpredictable in occurrence and extent. It is not unusual to find one side of a peninsula shrouded in fog and the opposite side clear.
Air Masses That Can Affect Britain's Weather
Meteorology UK Weather Air Mass Source Map
The diagram shows the principle sources of air which affect the weather of the UK. The situation is extremely complex. Britain is situated between ocean to the west and continent to the east. It also lies in the zone of intermixing between polar-air from the north and tropical-air from the south. A further extremely important factor is the presence of a warm ocean current, the North Atlantic Drift. (This is shown by the letters NAD on the diagram). This warm current sweeps the north-west European coast bringing winter warmth to latitudes that would otherwise be very cold. The names of the different air masses are listed in the key. The approximate frequency of their influence is related to the size of the arrow:Tm - Tropical maritime,warm, moist tropical-air.Tc - Tropical continental, hot, dry tropical-air.Pm - Polar maritime, coo], moist arctic-air.Pmr - Polar maritime returning, cool, moist arctic-air, returning in the lee of a depression.A - Arctic air, very cold, moist arctic-air.Pcl - Polar continental, very cold, moist arctic-air, which has travelled a long way across the North Sea.Polar maritime and tropical maritime air masses are responsible for variable weather conditions associated with the passage of depressions. Tropical continental air is associated with warm/dry spells including heat-wave conditions in summer. Polar continental air is associated with cold, dry spells particularly in winter. It often brings frost and freezing fog.
The Weather Machine
Meteorology UK Weather Hydrology Cycle Radiation Atmosphere Diagram
The diagram summarises the characteristics of the hydrological cycle as it might be applied to Britain or for that matter any other place on Earth. Heat energy radiated from the sun, balanced against radiation losses from the Earth and its atmosphere, provides the driving force for what is sometimes called "the weather machine". The various components of the hydrological cycle are shown on the diagram.
The Effect of Relief on Weather
Meteorology UK Weather Condensation Rain Orographic Diagram
In Britain the dominant direction of the wind (the prevailing wind) is from the west or south west. This causes moist Atlantic air to rise dramatically over the western hills and mountains. The resulting effect is shown in the diagram. The different condensation levels on either side of the relief barrier are due to the ability of dry air to increase its temperature more rapidly on descent when compared to the rate at which moist air cools when it rises. This "orographic effect" is a principle cause of rainfall in western Britain. In eastern Britain it contributes to a "rain shadow".
Weather and Frontal Systems
Meteorology UK Weather Front depression Diagram
The diagram shows a section through a typical depression. A depression is a weather system caused by intermixing of warm, tropical maritime (Tm) air and cool, polar maritime (Pm) air along the polar front. A warm front occurs along the leading edge of the warm sector. Here along the line of contact between warm, moist tropical-air and cold dense polar-air ahead of it there is considerable uplift of the lighter tropical-air. A cold front is found at the trailing edge of the warm sector where heavy polar-air is pushing down behind the lighter tropical-air. Depressions move fairly regularly from west to east across the British Isles. That is, unless a continental high pressure air mass is present in sufficient strength to "block" the movement. The lower part of the diagram represents part of a typical synoptic chart showing isobars and fronts. The upper part of the diagram shows, in three dimensions, the characteristics associated with the system.
Temperature and Sunshine
Meteorology UK Weather Temperature Map Sunshine
The maps show a comparison of January and July temperature conditions (reduced to sea level) and the inset map shows the average number of sunshine hours per day for different parts of the country. The key to the maps is as follows:January temperaturesA over 5░ CelsiusB 4░ to 5░C 3░ to 4░D 2░ to 3░E l░ to 2░F below l░July temperaturesG over 22░ CelsiusH 21░ to 22░I 20░ to 21░J 19░ to 20░K 18░ to 19░L 17░ to 18░M 16░ to 17░N below 16░SunshineO over 4.5 hours per dayP 4.0 - 4.5 hours per dayQ 3.5 - 4.0 hours per dayR 3.0 - 3.5 hours per dayS under 3.0 hours per dayThe main difference between the temperature maps is the general north to south alignment of the isotherms during January. This emphasises the much colder conditions on the eastern side of the country. In July the isotherms follow a more expected east to west trend, with higher temperatures in the south of Britain and cooler in the north. The sunshine map shows a greater amount of insolation in the southern part of the country, with the areas receiving the least sunshine being the highland areas of northern and western Britain.
Relief and Rainfall
Meteorology UK Weather Relief Rainfall Map
The maps provide a basis for comparison of relief and rainfall in Britain. A very close correlation between hills and high rainfall is easily identified.On the relief map the land over 200 metres is shaded brown. The key to the rainfall map is as follows:A over 3000mm B 180 - 3000mmC 100 - 1800mmD 75 - 1000mm E 60 - 750mm F less than 600mm.The inset map shows the frequency of rainfall in the UK. This is expressed in the number of days with more than 2.5mm daily rainfall. The key is as follows:G over 200 days, the wettest parts H 100 - 199 daysI 50 - 99 days J 10 - 49 daysK less than 1O days, the driest parts
First Frost, Last Frost
Meteorology UK Weather Frost Map
The occurrence of frost in Britain is critical to the growth of many crops and there is a pattern between farming activities and frost frequency. The maps date the occurrence of the first and last air frosts across the country. Early winter frost can affect harvests; very late spring frost can destroy fruit blossom.First Frost A after December 1st, very late winter B November 1 - November 30, late winter C October 14 - October 31 D October 1 - October 14 E September 1 - September 31, early winter F before August 31, very early winter Last FrostV before February 28th, early springW March 1 - March 31X April 1 - April 30Y May 1 - May 31, late springZ after June 1, very late spring
