When you walk on the beach and find oyster shells and clam shells, do you wonder how those shells got there? Those are shells found in a sound or the back side of an island. I always assumed it was because the tide washed them out to see and then redeposited them on the shoreline. Wrong! This is the evidence used to proof that islands migrate. Radiocarbon dating have shown that some of the shells found exposed on the beach can actually be 7,000 to 9,000 years old. But how do islands migrate?
In his book, Living with the South Carolina Shore, William Neal wrote, “The forces acting on the islands today are the same ones that originally created them…South Carolina has barrier islands because of the interaction of rising sea level with a coastal plain…You might as why, if sea levels continue to rise, the newly formed islands were not themselves covered by the sea. The answer is -when the level of the sea is rising, barrier islands refuse to stay in one place.” (Neal, W.J., 1984) Yes they migrate and here is how.
So first in order to migrate, the beach side of the island must erode, then the back side of the island must grow. Higher tides due to the rise in the sea level is the main cause for the erosion. The beach sand is taken away and redeposited in the inlet, which is a channel of water between adjacent islands. As more sand is deposited the inlet begins to migrate as well, leaving sandbars (flood-tidal delta) that become the new back side of the island. Once enough sand is deposited, salt-marsh grasses begin to grow and trap more sediment causing the creation of new land. This happens over time but I should point out that the faster the sea rises, the faster the island migration.
Narrow island migrate even faster due to overwash of water from the ocean side, particularly during storms. Overwash waves actually transports sand from the beach side and deposits the sand on the back side of the island. The sediment deposited usually buries the pre-existing marsh. Today there are some islands that do not have marsh on the back-side. In this cause, both sides would suffer erosion. It is believed this happens to large islands that are actually thinning down in response to the rising sea level in order to prepare for migration.
It is important to understand that is an integrated system between barrier islands. As sandbars (ebb-tidal delta) form on the ocean side of the inlet, it changes wave direction for the island below it. So the wave moves from north to south on the island to the north of the inlet. The wave changes direction below the inlet from south to north carrying more sand to the inlet further adding to the growth of the northerly island and assisting the migration process. This is also why large barrier islands tend to be narrow on the north side of the island and wider on the south side.
Neal, W. J., Blakeney Jr, W. C., Pilkey Jr, O. H., & Pilkey Sr, O. H. (1984). Living with the South Carolina shore. (O. H. Pilkey Jr, & W. J. Neal, Eds.) Durham, North Carolina, United States: Duke University Press.