Geoscience Reference
In-Depth Information
ory of barrier island formation seems to apply to most barrier islands along the southeast Atlantic and Gulf of
Mexico coasts, including the Outer Banks of North Carolina.
Along the northeast coast, from New York northward in the region directly affected by the glaciers, barrier
islands seem to be most commonly formed when spits created by longshore drift are later breached. First, bar-
rier beaches are formed by the erosion of a sea cliff or associated beach. Longshore currents then transport the
sand parallel to the shore to form a spit. This elongated, fingerlike ridge is attached at one end to the mainland
but curves into open water. At some point, the spit is breached to create an island. The barrier island system
along the south shore of Long Island evolved in this way, as did Monomoy Island of Cape Cod.
Barrier islands, like these on the north shore of Prince Edward Island, have been formed from the sediments washed
down from the ancient Appalachian Mountains and form an almost continuous chain from Long Island to Florida.
Barrier islands can also develop from submarine bars. As waves transport material landward, an offshore bar
is created that eventually becomes an emergent island when it builds above sea level. It seems likely that some
small barrier islands along the Gulf Coast originated in this way.
However they are formed, barrier islands are dynamic entities that migrate and change shape almost like liv-
ing things. Storms strip away sand from the upper beach and dunes and carry it toward the sea, where it is
stored temporarily offshore as a storm bar. Then, in the following weeks, this sand borrowed from the beach is
gradually returned and again “welded” to the beach. This system of sand-sharing reaches a state of dynamic
equilibrium, in which the sands are subtracted from and then added back to the beach. However, severe storms
sometimes breach the dune or push through low areas in the dune ridges. When they lose energy during this
landward surge, they deposit the sand as an overwash fan. This overwash process reshapes barrier islands over
time, and eventually the whole island migrates landward. As the island moves, however, the sequence of habit-
ats re-establishes itself. A wave-swept sandy beach faces the sea, sand dunes rear up in the middle of the is-
land, and a lagoon fringed with salt marshes forms between the island and the mainland.
The creation of barrier islands is a striking example of geology in action, of the dynamic processes that con-
stantly reshape the coastline, just as the mostly hidden process of plate tectonics is moving continents and in
the process making mountains and opening oceans.
