Geology

Georgia’s barrier islands formed between 40,000 to 4,000 years ago, with the oldest islands closest to the mainland.

Many do not know Georgia has two sets of barrier islands bordering its coast, formed in distinctly different time periods. It is hard to see, so a visit to the coast or a look at a map would not show you the age difference. The islands making up the western side of the chain were formed about 35,000 to 40,000 years ago, while the islands to the east are much more recent, dating back only 4,000 to 5,000 years.

The older islands are known as the Pleistocene islands, named for the time period in which they were formed. These islands were shaped before the last great continental ice sheet formed. At a time when the sea level was about 6 feet above the present sea level, the beaches of the older islands were formed. During the last freeze of the Pleistocene epoch, so much seawater was frozen that the sea level was lowered by 300 to 500 feet. At this time, the shoreline was nearly 80 miles offshore from the shoreline we have today.

About 18,000 years ago the ice sheets started to melt and the sea level began to rapidly rise. With the rise in sea level, newly formed barrier islands called the Holocene islands began migrating westward toward the established, older islands. Around 4,000 to 5,000 years ago the rate of the rising sea level began to slow to a rate of 4 to 6 inches each century.

The shape and size of the barrier islands constantly change. The wind, waves and tidal currents all cause erosion and redeposit sediments in new locations. In fact, the westward migration of the Holocene islands is still occurring today. The continued erosion of the eastern facing beaches causes the sediments to be carried and deposited in the marshes and lagoons behind the islands.

As the Holocene islands have migrated closer, some have become attached to the older islands. This is especially true for islands near smaller rivers because they have smaller sediment outputs when the river meets the ocean. Large rivers like the Savannah and the Altamaha Rivers have a high output of sediments, which blocks nearby islands from drifting at the same rate as other Holocene islands. The islands directly south of the large Savannah River, including Tybee, Wassaw and the north end of Ossabaw, and those south of the Altamaha River, like Little St. Simons and Sea Island, are more separated from their Pleistocene counterparts.

Sea level continues to rise in this area at an increased rate of 12 to 14 inches per century. One hypothesis for the increase is that the accumulation of carbon dioxide in the atmosphere has elevated the atmospheric temperature. The higher temperature causes ice caps to melt, which directly contributes to a growing rate of sea level.