.....Barrier island are places of change and motion. Islands grow and shrink depending on the amount of sediment available in the local system, and global conditions like change in sea level. An island that is growing seaward is called prograding barrier island, while one that is migrating toward the mainland is termed retrograding.
.....In most cases an island that is retrograding undergoes a process known as "island rollover". Island rollover is common on islands with low profiles and few dune ridges. This process also occurs on islands that experiencing erosion due to a stifled sediment supply such as Folly Beach. Two major types of evidence that show an island is rolling over are washovers and exposed marsh mud on the beach face.
.....Washovers occur when storm waves wash over low lying sections of the beach removing sand from the beach face and depositing it in the marsh behind the dunes. The image below shows some washovers on the northern end of Folly Beach near Lighthouse Inlet.
(click on image for higher resolution)
.....Exposed marsh mud on the beach face is more evidence that an island is rolling over. As the dunes and beach face retreat toward the mainland, the marsh behind is covered. As the island continues to retrograde, this buried marsh become exposed on the beach face. This mud is made up of fine clays and plant material making it very resistant to erosion by wave action. Marsh mud on the beach is easy to find at low tide on Folly at both the county park area and the northern end of the island.

(click on images for higher resolution)