Beaches are shoreline accumulations of loose sand, gravel or a mixture of the two, that are formed primarily by the action of waves. Beach sediment can be derived from a variety of sources including insular shelves, the adjacent land and upland sources, or other beach locations through alongshore movement of material. Beaches provide critical coastal habitat, such as nesting sites for sea turtles; they act as a buffer protecting adjacent land from storm wave attack; and they are an important cultural and recreational resource. Island beaches are the same as those on the continents, but island beach characteristics typically change over very short distances on account of rapid changes in coastline orientation, exposure to waves, and sediment source.