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The Epic Interactive Encyclopedia 1998
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Epic Interactive Encyclopedia, The - 1998 Edition (1998)(Epic Marketing).iso
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Alluvial_deposit
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1992-09-02
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A layer of broken rocky matter, or sediment,
formed from material that has been carried in
suspension by a river or stream and dropped
as the velocity of the current changes. River
plains and deltas are made entirely of
alluvial deposits, but smaller pockets can be
found in the beds of upland torrents.
Alluvial deposits can consist of a whole
range of particle sizes, from boulders down
through cobbles, pebbles, gravel, sand, silt,
and clay. The raw materials are the rocks and
soils of upland areas that are loosened by
erosion and washed away by mountain streams.
Much of the world's richest farmland lies on
alluvial deposits. These deposits can also
provide an economic source of minerals. River
currents produce a sorting action, with
particles of heavy material deposited first
while lighter materials are washed
downstream. Hence heavy minerals such as gold
and tin, present in the original rocks in
small amounts, can be concentrated and
deposited on stream beds in commercial
quantities. Such deposits are called `placer
ores'.