Shifting Sands on Petit Bois Island

Barrier islands constantly move

To geologists, the barrier islands of Gulf Islands National Seashore and Dauphin Island (which divides the Florida islands from the Mississippi Islands) are young – only a few thousand years old.  They formed after glaciers retreated from most of North America, sea levels changed, and shoals appeared along the Gulf Coast. Once formed, the shoals helped trap sand and sediment brought in by currents, creating islands. Even today, currents pick up sand in the east and carry it westward where some of it drops out of the waves on the western edge of each island. As this continues over time, the islands change shape. Many migrate west and towards the mainland, as the eastern and seaward ends of the islands erode and the western ends grow from deposited sand. Storms such as hurricanes can also change the size and shape of the islands by cutting through narrow or low-lying areas and accelerating erosion.

In fact, French explorers in the 18th century found that Petit Bois Island was connected to Alabama’s Dauphin Island to its east back then. A French mapmaker named Jean Baptiste Bourguignon d’Anville showed this connection in his 1732 map of Louisiana.

But by 1816, other maps show that Petit Bois had become its own island, probably because of hurricanes and other storms breaching the sand that connected it to Dauphin.

Since then, the barrier islands have continued to change shape. Information compiled by scientists at the U.S. Geological Survey from maps, aerial photographs, GPS, and lidar technology, reveals how Petit Bois has changed from 1848 through 2005 as sand continually erodes from its eastern end.   Look at the layered map below and examine where Petit Bois has lost land. Has it gained any?

Figure 3. Morphological changes in Petit Bois Island between 1848 and 2005. Robert A. Morton, "Historical Changes in the Mississippi-Alabama Barrier Islands and the Roles of Extreme Storms, Sea Level, and Human Activities", USGS, p. 7.
Morphological changes in Petit Bois Island between 1848 and 2005. Robert A. Morton, “Historical Changes in the Mississippi-Alabama Barrier Islands and the Roles of Extreme Storms, Sea Level, and Human Activities“, USGS, 2007, p. 7.