Answer:
Longshore currents wash up sand and sediments, creating the base of a barrier island and eventually forming dunes.
Step-by-step explanation:
Tidal action, stirring up sediment from the sea floor, was mainly responsible for the formation of barrier islands. De Beaumont's theory states that as waves push up to the shore, sand is carried along with them. The waves crash, expending their energy and depositing the sand into a growing sand bar. As the sand bar grows, it eventually rises above the surface and a barrier island is formed.