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Sediment ranges in size from large boulders to very fine mud. Sediments
coarser than two millimeters are called gravel. Sand is defined as sediment
with sizes between one-sixteenth of a millimeter and two millimeters.
All sediment finer than sand is called mud. The coarser part of the mud
is called silt, and the finer part is called clay.
Sediments are formed when rocks on the land surface are broken down by
rain, wind, and sun. Sediments consist of particles of minerals, and also
loose pieces of rock. Streams and rivers move the sediments downstream
toward the ocean. Some of the sediment is stored in large river valleys,
but most of it reaches the ocean. Some is deposited in shallow water near
the shore, and some is carried far out into the deep ocean. Most of the
sediment deposited near the shore is coarse, and it gets finer farther
away from the shore. Most of the sediment in the deep ocean is very fine
mud.
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©
Society for Sedimentary Geology
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Sand-filled channels in an exposed portion
of a gravelly braided stream bed. Flow is toward viewer. Kicking
Horse River, near Field, British Columbia.
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Sediments are also formed when calcium carbonate minerals are separated
from warm, shallow waters in the ocean. Much of this is used by marine
animals to make their skeletons. After the animals die, their skeletons
become sediment. Where currents are weak, this sediment stays where the
animals lived. Where currents are strong, the shells are moved along the
bottom and are worn into rounded particles.
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