Dynamic Beach and Shoreline
Beaches are most dynamic where the waves wash up over the course of a daily tidal cycle, constantly moving sand up and down the beach and along the shoreline through the actions of the crashing waves and longshore transport.
Further up the beach, the berm, which is generally above the daily reach of the tides, will change under conditions where waves reach higher on the beach. Even storms that remain far offshore can increase the wave energy enough to erode the beach and move sand temporarily offshore.
Sand that is removed from the beach is stored offshore, usually in a structure called an offshore bar. At low tide, you may be able to see evidence of an offshore bar by the presence of a secondary line of breaking or choppy waves past where the waves are breaking and washing up on shore.
The size and shape of the beach is always changing. While sediment is constantly being moved more or less perpendicular to or from the shoreline by tidal and wave action, the net movement of sediment is along Fire Island's coast, parallel to the shore through the effects of longshore currents. The movement of sediment along the shoreline is called longshore sediment transport and resulted in extension of the western terminus of the island.