Multimedia Mania/abacus-multimedia-mania.iso/dp/0099/00998.txt

Text File | 1993-07-27 | 19KB | 328 lines

$Unique_ID{bob00998}
$Pretitle{}
$Title{Glacier Bay
Tidewater Glaciers}
$Subtitle{}
$Author{Kirk, Ruth}
$Affiliation{National Park Service;U.S. Department Of The Interior}
$Subject{ice
glacier
muir
bay
feet
meters
glaciers
pacific
time
water
see
pictures
see
figures
}
$Date{1983}
$Log{See Muir Glacier*0099801.scf
}
Title:       Glacier Bay
Book:        Part II: Of Time And Ice
Author:      Kirk, Ruth
Affiliation: National Park Service;U.S. Department Of The Interior
Date:        1983

Tidewater Glaciers

     In a small way, I once touched time.  It was July and my husband, Louis,
and I were camped in Reid Inlet, an exquisite fjord fingering off Glacier
Bay's main, upper waterway.  Our tent was pitched near a 1940's gold miner's
shack, which that summer was serving as headquarters for park ranger Ole Wik
and his wife, Manya.  Rock peaks and ridges walled our horizon.  At the
inlet's head a glacier tongue calved icebergs directly into saltwater.  From
basketball to Detroit limousine size, these ice chunks rode the currents and
stranded ashore on each outgoing tide, making the beach a sculpture garden.
Manya lugged small stranded bergs home in pails hung from a shoulder yoke. The
ice turned a pit dug in the coarse upper beach gravel into an icebox.

     One evening, the Wiks and Louis and I decided to make ice cream in an old
hand-crank freezer.  Out from the pit came fresh eggs, which Manya mixed with
powdered milk dissolved in creek water and sweetened with honey.  Ole and
Louis chipped salvaged bergs and packed the ice fragments into the freezer. We
turned the crank till it would turn no more and then spooned out the ice
cream.

     Icebergs floated on the tide just offshore.  We sat reveling in the 11
p.m. sunset and feasting on the ice cream.  A cormorant, its sleek body and
upright neck a dark silhouette against the water's pink tint, rode one berg.
New bergs sporadically broke from the glacier, their birthing thunder a
syncopation for the evening's hush.  Ole mused aloud on our having used the
iceberg's fossilized cold to freeze the ice cream.  For the ice it concluded
unknown decades of an existence begun as fluffy snow and then compressed to
ice, owing to the sheer weight of snow accumulating above it.

By the Grand Pacific Glacier

     Reid Inlet ice cream comes to mind now, four Julys later, as I cook
breakfast aboard R.V. Growler, a U.S. Geological Survey ice research boat.
Oatmeal bubbles on the galley's oil range as I set out the corn muffins I've
baked.  The big galley table is at once workbench, library desk and center for
food preparation, eating, and socializing.

     Five of us are aboard.  In charge is glaciologist Austin Post - tall,
strong, with a grizzled beard that hangs to his chest and gentle eyes that
laugh.  His assistants, college-age, capable, enthusiastic, are David Janka,
Emily Chase, and Austin's son Charles Post.  My role is as observer and
photographer.

     For two hours we've been taking depth readings in front of Tarr Inlet's
Grand Pacific Glacier.  Its ice, along with that of the Margerie Glacier,
blocks the extreme upper end of Glacier Bay.  Data recorded by Growler's
electronic sounder will make it possible to chart the bottom contours here.
The contours will help in understanding tidewater ice, which responds to
various factors aside from climate.  Why, for example, is the Grand Pacific
Glacier advancing, while just to the east, the Muir Glacier has been rapidly
retreating for a century?  What accounts for such diverse behavior in the same
area?

     Glacier ice today whitens a tenth of the world's land surface, as much as
is now farmed.  A few thousand years ago glaciers covered triple this area, as
they someday surely will again.  Boston's Bunker Hill is a drumlin left behind
by glacier ice.  Erratic boulders dot Manhattan's Central Park, transported
from Canada by glacier ice.  Duck hunters in Minnesota set decoys on pothole
lakes formed by melting ice remnants.  Plains farmers grow wheat in loess,
windblown glacial sediment.  French vineyards are also in loess. Drink French
wine and you toast the Ice Age.  Yet despite the magnitude and recurrence of
glaciers, knowledge of them is little more than well begun. Glacier Bay is one
of the widely recognized field laboratories for glaciology.

     Growler is in Glacier Bay as part of a continuing study of tidewater ice.
Austin Post can visualize these ice tongues and how they behave.  For 20 years
he has been making aerial photographs of glaciers from the Andes to the
Aleutians and painstakingly mapping their changes.  Austin does not merely
rejoice to know that the world is not only blue and green but also white.  He
prefers the white.  When I came aboard Growler, he asked me about the weather
in Seattle.  "Sunny and hot," I said.  "That's awfully hard on the ice,"
Austin muttered in reply.

     At the wheel Austin is now maneuvering Growler through floating icebergs.
They aren't packed solid this morning and we can work to within one-third
kilometer (1,000 feet) of the Grand Pacific's ice face.  We won't go closer
because of danger from falling ice, but we'll send a small, radio-controlled
skiff to bump against the glacier snout and read the water depth there.  The
glacier front is not floating.  It rests on a rubble ridge of its own making.
Emily, Dave, and Chuck are with Austin in the wheelhouse, correlating
Growler's precise position with the depth-sounder record and with Polaroid
pictures of the radar scope.  I can be spared to cook, but everyone else is
needed for the readings.  It's 0800 now.  We've been underway since 0630 and
will soon cut the engine to drift with the pack ice and eat breakfast.

     A moment ago we were swept off course by a melt torrent draining from
under the glacier.  No depth reading registered until we worked free of its
flow, because the stream disgorges so much suspended mineral material that the
signal from our depth sounder dispersed instead of striking bottom and
bouncing back.  Even the water surface is gray with glacial flour, bedrock
ground to powder by the pressure of moving ice.  Away from the ice front the
gray becomes turquoise as the silt mutes but no longer dominates the clear,
deep blue of open water.  Often distinct color bands persist, their moire
pattern maintained by the water's different temperatures and salinities.  Such
banding may reach all the way to Icy Strait, 70 kilometers (43 miles) from the
nearest tidewater glaciers.  We terrestrials think of seawater as homogeneous.
It's not.

     Nor is a glacier just a mass of frozen water.  It is ice, plus flowing
water, plus a vast amount of rock debris scoured, rasped, and plucked from the
mountains where glaciers are born and from the valley walls and bottoms they
inch across en route downslope.  In the color banding of a fjord's water
surface you witness the sedimentation process that in time fills in enormous
submarine troughs and turns waterways into valleys with freshwater streams and
wildflowers.

     Out Growler's porthole the far side of the Margerie Glacier is so
blackened with rubble that it looks like rock.  Only a melt sheen identifies
it from this distance as glacier ice.  Directly ahead of us, moraines of rock
debris streak the length of the Grand Pacific Glacier like ribbons.  At the
sides of the ice face they show as tilted layers dipping into the water.
Moraines form as rock tumbles and slides from steepened slopes onto the
glacier surface, there to ride the ice and eventually break free as part of an
iceberg, or to be released by melt.  Moraines are among the legacies of
glaciation.  They form abrupt ridges of loose rock, gravel, and sand often
several-score meters high and extending for long distances.  Though in time
moraines may become upholstered with plants, their origin remains easily
recognizable.

     When mineral debris is dropped directly beneath a glacier it may form
hills and short ridges known as kames and drumlins, or, if deposited by a
subglacial stream, as eskers.  Last evening we anchored Growler a half-hour's
run south of the Margerie ice front, then hiked up a side drainage and sat at
dusk watching a loon paddle across a small lake impounded by an esker. Slices
of time seemed separated from eternity's flow and laid before us.  We had
walked through a carpet of dryas plants shaggy with seedheads.  Dryas can
pioneer poor soil and so can quickly form a green aftermath of glacier ice. We
had hoped to find glacio-marine - clay lumps dropped, usually, from floating
icebergs - remnants from millennia ago when seawater covered where we sat
watching the loon.

     Floating bergs, the white peaks, and the processes of mineral transport
and deposit, have all repeatedly characterized the Glacier Bay scene.  Until
recently, geologists believed that the last million years had brought four
major ice ages.  Now they see these as composite glacier advances, retreats,
and re-advances.  The number of such pulses was closer to 40 than to four,
with one series often hard to discern from another.

     Expanding glaciers clear virtually everything movable from their paths,
so nothing more than traces of early glaciations are likely to remain.  One
such trace lies along the outer coast of Glacier Bay National Park.  Marine
tillite, glacial debris deposited in seawater, is there interbedded with
layers of siltstone and sandstone for a total thickness of nearly 2,000 meters
(6,500 feet).  The ancient tillite formed by the same mineral dumping process
I've been watching this morning.  It comes complete with the rafted lumps such
as we sought without success last evening - the sort that ride the icebergs I
see out the porthole now.

     Shells date the layered outer-coast sediments to about 15 million B.P.
(before present).  A park research biologist once told me he found a fossil
beech leaf in the deposits.  It must have been blown or washed seaward to
settle in the ocean bottom ooze.  To have endured for 15 million years seems
extraordinary; to be a beech leaf even more so.  For that testifies to a scene
far different than today's.  Glacier ice was then juxtaposed with deciduous
forest.  Nowhere does such a situation exist today except in Chile where a
relative of beech thrives close to ice.

     For most of southeast Alaska, the signs of early glaciation are not
deposition but erosion.  Sharply sculpted high peaks are those plucked by ice.
Lower rounded contours were overridden.  You can see this craggy horn peaks to
rounded and polished bedrock and so pinpoint the level of a former glacier.
In lower Glacier Bay this line comes at about 1,300 meters (4,200 feet).

     The period from about 30,000 to 10,000 B.P. brought the most recent
worldwide glaciation, known in America as the Wisconsinan because the southern
edge or terminus of a vast ice sheet sculpted much of that state's current
topography.  The Glacier Bay elevation region - and practically all high
latitudes and elevations - surrendered to ice during this time.  Juneau lay
beneath a white shroud 1,500 meters (5,000 feet) thick.  At Cape Spencer on
the park's outer coast the ice was still at least 900 meters (3,000 feet)
thick, with its leading edge somewhere far beyond today's coastline.

     Oddly, however, parts of the shore were not veneered by this ice.  It may
be they escaped because a geologic fault at the western base of the
Fairweather mountains acted as a gutter and shunted off encroaching ice.  Such
faults are cracks in the Earth's surface.  This one marks where the crustal
plate comprising the Pacific Ocean floor, abuts the crustal plate bearing the
North American continent.  California's famous San Andreas Fault is somewhat
comparable.  The Fairweather rift splits off land from Icy Point to Russell
Fjord, north of Yakutat.  Bedrock is out of alignment along opposite sides of
the fault.  In fact, whole provinces have slid northward as the oceanic plate
collides with the continental plate and heaves up the mountains.  Certain rock
found at sea level south of Icy Point stands north of the Point at an
elevation of more than 3,000 meters (10,000 feet).  Less active faults
underlie both Glacier Bay and Lynn Canal.

Above Tarr Inlet

     It is afternoon and Dave and Emily and I have climbed onto the highest
terrace of the slope above the Grand Pacific snout.  Dave's altimeter shows
our elevation as 269 meters (882 feet).  A multi-decked cruise ship drifting
among icebergs near the glacier face looks from here like an inconsequential
dot.  People on deck to watch bergs calve off must see the glacier front as
immense.  At 60 to 80 meters (200 to 260 feet) high and 6 kilometers (3.75
miles) across, it is.

     From our vantage point you see how much more glacier there really is than
shows from the water.  Grand Pacific flows as an infinity of ice coming from
far back in the mountains.  Except for crevasses near the Snout, its surface
looks like a broad white highway, which is how several coastal Alaskan
glaciers served Indians and prospectors traveling to and from the interior.
Using Grand Pacific as a conduit, wolves and bears have extended their range
onto Glacier Bay lands recently melted free of ice and beginning to host life
again.

     From where we sit it's easy to imagine Tarr Inlet stripped of today's
ice, seeing instead either a continuation of today's waterway or a broad
terrestrial valley bottom.  As recently as the 1920's the Grand Pacific melted
its way out of the United States and 2 kilometers (1.2 miles) into Canada.
That gave Canada a potential site for her northwesternmost seaport.  But by
1948 ice again moved forward as far as the border.

     The Glacier Bay bedrock trough is tremendously deep, dropping to 550
meters (1,800 feet) near Gilbert Island.  Muir Inlet reaches about 375 meters
(1,200 feet), a considerable depth, yet the inlet completely filled with
gravel following withdrawal of the Wisconsinan ice!  This fill stood higher
than present sea level, developing soil and a spruce-and-hemlock forest. Side
valleys, dammed by the gravel, seem to have held lakes, because traces of
glacial outwash gravels cling 250 meters (820 feet) above today's saltwater
shores, and lakebed sediments are still in place.  Dr. Richard Goldthwait,
emeritus professor at Ohio State University, believes that the fill in Muir,
Wachusett, and Adams Inlets probably averaged 150 meters (500 feet) deep and 5
kilometers (3 miles) wide for a cumulative 80 kilometers (50 miles) of length.
Except for mere whispers, this stupendous volume of gravel fill is now gone.
Readvancing ice swept it into the Beartrack Cove area and on southward.  The
broad Gustavus flatlands are partly old Muir gravel fill.

     Wisconsinan glaciers melted back perhaps 11,000 years ago and probably
stayed back until about 3,500 years ago, when harsh climatic conditions again
favored glacier expansion.  Worldwide this Little Ice Age was not dramatic. At
Glacier Bay, however, snowfall produced ice more than 1,000 meters (3,200
feet) thick and pushed the glaciers far forward.  A tongue of ice once more
filled the entire Glacier Bay fjord and bulged out into Icy Strait.  This
dammed Muir Valley and backed up an enormous lake there that drowned the
forest.

     About a thousand years after the glaciers' encroachment into the main
Glacier Bay fjord, an ice tongue began to advance down Muir Valley.  At the
outlet it was blocked by the glacier already there.  Unable to thrust farther
forward, this new ice ponded and flowed back on itself, filling depressions
along lower Muir Inlet to such depths that remnants still haven't melted
today.  They are popular destinations for hikers and give geologists a look at
how melting glacier ice has produced much of today's northern latitudes
topography.

     The ebullient, voluble, knowledgeable, and renowned John Muir was an
early exponent of the continental glaciation theory in North America.  He
canoed north from Fort Wrangell in October 1879, coaxing his Tlingit Indian
paddlers onward against their judgment.  Fall struck them as a foolish season
for venturing among icebergs.  But for Muir ice was the reason for the
journey.  Aged 41, acclaimed champion of all nature, specifically fascinated
by glaciation, Muir became the first Glacier Bay sightseer to write
extensively and glowingly about the wonders of what now is the national park.
His first trip was brief, but in the summer of 1880 and again in 1890, Muir
returned.  By the time of his third trip, tourists were visiting Glacier Bay
aboard sidewheel excursion steamers.  Eliza Ruhamah Scidmore wrote in a
National Geographic article of "stopping, backing, and going at half speed to
avoid the floating ice all around [which] occasionally was ground and crunched
up by the paddle wheels with a most uncomfortable sound."

     The Muir Glacier - newly named for the famed naturalist - had scarcely
begun its retreat at that time, though the ice filling Glacier Bay proper had
drawn back 60 kilometers (37 miles) in the century since British Captain
George Vancouver had noted its presence in 1794.  This glacial unzipping is
the fastest known anywhere, anytime.

[See Muir Glacier: Mad wreckage of the retreating Muir Glacier ice front
chokes adjacent waters. The glacier retreated about 5 kilometers between 1972
and 1982. It has retreated more than 25 kilometers (15 miles) in this century,
at widely varying rates.]

     Fortunately for the understanding of Glacier Bay's chronology, Harry
Fielding Reid, a pioneering glaciologist, arrived here in 1890, about 30 years
after the Glacier Bay unzipping had exposed the Muir Glacier for the first
time.  Through the summer of 1890 and again two summers later, Reid explored
by rowboat and on foot, lugging a cumbersome theodolite and plane table for
mapping glacier positions.  "We once approached to within a quarter mile of
the ice cliffs of Muir Glacier, which towered impressively above us," Reid
wrote.  "Suddenly a large berg broke off, followed immediately by a second,
and then several arose from below.  Great breakers which must have been 30
feet high, rushed forward, but fortunately subsided into an even swell before
reaching us.  The fragments of ice spread out with great rapidity and in a few
minutes quite surrounded our boat."

     The Grand Pacific Glacier was then fused with the Margerie, their joint
terminus barely separated from the Johns Hopkins Glacier.  Reid wrote that the
continual calving of that great ice cliff, nearly 10 kilometers (6 miles)
long, was "keeping the inlet well covered with floating ice and the air
pulsating with the thunder of its fall."

     No wonder the Tlingits thought John Muir reckless, though he gloried in
the calving bergs' "awful roaring, tons of water streaming like hair down the
sides, while they heave and plunge again and again before they settle in poise
and sail away as blue-crystal islands, free at last . . . ."  John Muir, too,
had touched time.