Geology Reference
In-Depth Information
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their seaward sides, they are smoothed by waves, but their
landward margins are irregular because storm waves carry
sediment over the island and deposit it in a lagoon where it
is little modifi ed by further wave activity. The component
parts of a barrier island include a beach, wind-blown sand
dunes, and a marshy area on their landward sides.
Everyone agrees that barrier islands form on gently
sloping continental shelves where abundant sand is available
and where both wave energy and the tidal range are low. In
fact, these are the reasons that so many are along the United
States' Atlantic and Gulf Coasts. But even though it is well
known where barrier islands form, the details of their origin
are still unresolved. According to one model, they formed
as spits that became detached from land, whereas another
model holds that they formed as beach ridges that subse-
quently subsided.
Most barrier islands are migrating landward as a result
of erosion on their seaward sides and deposition on their
landward sides. This is a natural part of barrier island
What Would You Do
While visiting a barrier island, you notice some fi ne-grained,
organic-rich deposits along the beach. In fact, there is so
much organic matter that the sediments are black and
have a foul odor. Their presence on the beach does not
seem to make sense because you know that sediments
like these were almost certainly deposited in a marsh on
the landward (opposite) side of the island. How can you
explain your observations?
currents. In some areas, jetties are constructed that extend
seaward (or lakeward) to interrupt the fl ow of longshore
currents and thus protect the opening to a bay.
Barrier Islands
Long, narrow islands of sand lying a short distance offshore
from the mainland are barrier islands (
Figure 16.14). On
Figure 16.11 Seasonal Changes in Beaches
Berm
crest
Mean high
watermark
Berm
Beach face
Mean low
watermark
Bar
Summer profile
Winter profile
Distance
Seasonal changes in beach profi les.
a
b
c
San Gregorio State Beach, California. These photographs were taken from nearly the same place, but
c was taken two years after b . Much of the change from b to c can be accounted for by beach
erosion during 1997-1998 winter storms.
 
 
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