are prevalent beach materials in the Pacific Northwest, Alaska, and the Great Lakes area. Sand, the most
common shoreline material, is found in virtually all coastal areas. Silts and clays generally occur on
bluff shorelines or marshes, such as along the Great Lakes and various bays.
Littoral materials are derived from the deterioration and erosion of coastal bluffs and cliffs; the
weathering of rock materials found inland and transported to the shore by rivers and streams; the
disintegration of shells, coral or algae to form carbonate materials; and the production of organic material
(generally peat) by coastal marshes and wetlands.
Failure or erosion of a bluff causes material to be deposited at the base. Waves sort this material
and carry the fine-grained silts and clays offshore where they settle to the bottom. The original deposit is
eventually reduced to sand and gravel fractions, which form a beach. Eventually, if no other littoral
material is carried to the site by waves, even the sand and fine gravel will disappear down the coast or
offshore, leaving only coarse gravels behind. However, a new supply of material may be deposited on
the beach by a fresh failure of the bluff, and the process begins again. In many cases, therefore, the
littoral materials comprising beaches are often derived from erosion of the shoreline itself.
Rivers and streams that carry sediments eroded from the inland land mass are a second source of
littoral material, particularly during floods. Material from this source is predominantly smaller than
sand, particularly for large rivers. These silts and clays are largely deposited far offshore. Smaller rivers
that flow through sandy drainage areas may carry significant quantities of sand during floods. However,
the total contribution of sand by rivers and streams is probably considerably less than from erosion of the
shores themselves.
Coral reefs, shells, and other plant or animal matter are a third material source. They gradually
break and weather into carbonate sands, which are, for instance, the primary components of beaches
south of Palm Beach, Florida. Swamps, marshes, and coastal wetlands produce peats and other organic
matter, also a source of littoral material. Too light to remain in place under continued wave action, they
are ultimately washed offshore unless stabilized.
Littoral materials are transported along the shore by wave action. Approaching from deeper water,
the shoreward portion of a wave moves in progressively more shallow water than the section farther of f
shore. This portion begins to slow, which causes the wave to bend (refract) until breaking at an angle to
the beach. This creates considerable turbulence that temporarily suspends the bottom sediments and
carries them up the foreshore (beach face) in the general direction of wave advance. The motion stops a
short distance up the beach, and then reverses direction back down the slope. However, the downrush
does not retrace the same path, but rather, moves directly down the foreshore in response to gravity. The
next wave repeats the process, moving the material downdrift along the beach.
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