they are called barrier islands and they extend from Long Island to Key West and all around the Gulf Coast. In their natural state they are mobile and do a pretty good job of protecting the coastline of the continent -- or they did until we started messing with them.
Problem is, we've let people build houses and businesses on them . . . and building groins, piers, jetties, and seawalls, all of which disturb the natural and seasonal process of beach erosion and rebuilding.