Laboratory Manual for Introductory Geology 4e

17.3.5 Depositional Features
Prominent depositional features develop where there is an abundant supply of sand
along shorelines. These features range from continuous sandbars that extend for miles
along the coast to small isolated beaches. The Gulf of Mexico and Atlantic coastal
plains are underlain by easily eroded unconsolidated sediments, and many places with
familiar names display classic depositional features: Cape Cod, Cape Hatteras, the
Outer Banks of North Carolina, the eastern Louisiana coast, Padre Island.
Common shoreline depositional features (FIG. 17.16) include the following:
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Beaches, which are the most common depositional features, consist of sand (or
coarser sediment), coral and shell fragments, and so forth—whatever type of
sediment is available.
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Spits are elongate sandbars attached at one end to the mainland. Some are
straight, like those in Figure 17.16, but some, known as hooks or recurved spits,
are curved sharply. Hooks curve in the direction in which a longshore current
is flowing. (Hint: This fact will come in handy for Exercise 17.9.)
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Barrier islands are elongate sandbars that lie offshore and are not connected
to the mainland (e.g., the Outer Banks, Padre Island). Their name comes
from the fact that they were barriers to early explorers, who had to search for
inlets that would allow them to reach the mainland.
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Sediment eroded from the mainland is deposited in bays between the shore
and the barrier islands, forming marshy wetlands called salt marshes or tidal
marshes. These wetlands are covered with salt-tolerant vegetation that is fully
or partially submerged at high tide.
Salt marshes are important parts of the food chain, providing rich sources of
nutrients for a wide range of aquatic life, and serve as breeding areas for fish and
the birds that feed on them. They are also a valuable part of our natural storm-protection
system. If storm surge manages to overflow the barrier island, the wetlands
act as a sponge, soaking up the water and lessening damage to the more densely
inhabited mainland. Protecting and preserving coastal wetlands should therefore
be part of any strategy to decrease potential storm damage. Unfortunately, these
FIGURE 17.16 Satellite view of southwestern Long Island, New York, showing common shoreline depositional features.
White areas on the south shores of spits and barrier islands are beaches.
Salt marshes
Coney
Island
Inlets
Bays
Spits
(connected to shore)
Bays
Barrier islands
(not connected to shore)
17.3 SHORELINE EROSION AND DEPOSITION
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