Barrier islands typically have sand in the beach zone and dune field, and mud in the back-barrier. Overwash deposits sand in the back-barrier.
Barrier islands form in three ways. They can form from spits, from drowned dune ridges or from sand bars. Longshore drift is the movement of sand parallel to the shore caused by the angle of the waves breaking on the beach. ... When a storm such as a hurricane digs an inlet through the spit a barrier island is formed.
