The Mississippi–Alabama barrier island chain sits 10 to 14 miles (16 to 23 km) offshore from the Mississippi Gulf Coast and runs parallel to the coast for 70 miles (113 km),[1] roughly from the Bay of St. Louis to Mobile Bay, separating the Mississippi Sound from the Gulf of Mexico.

The bar is thought to have begun forming after about 2500 BCE as the Gulf's mean sea level fell after the Holocene climatic optimum.

[4] Recurring major tropical cyclones have periodically altered the islands' shapes through wind, waves, and accompanying storm surges.

[2]: 10  Although severe storms often noticeably disrupt the morphologies and ecosystems of the islands on the timescale of days, vegetation and land area normally recover over a period of years following a major weather event.

[5] The gradual sea level rise in the Gulf during the past century has contributed to the decrease in land area of the island chain, but the effect is not thought to have been large compared with the influence of the removal of sediment from the system through the dredging of shipping channels.

These seagrass beds provide food and habitats for a variety of aquatic fauna, including penaeid shrimp, blue crab, spotted seatrout, and other shellfish and finfish.

This island has experienced major morphological changes over the past two centuries, with its eastern spit rapidly receding, and its narrow central segment shifting toward the mainland.

[3]: 8  Several smaller shoals grew together into one larger island around the beginning of the 20th century, appearing on a 1917 map from the United States Coast and Geodetic Survey,[2]: 7  around 3 miles (5 km) long.

[2]: 7 Petit Bois Island runs for 6 miles (10 km) along the eastern part of the Mississippi Sound, south of Pascagoula.

[2]: 6–7 Dauphin Island, which has over 1200 residents, runs for 15 miles (24 km) at the east end of the Mississippi Sound and marks the western edge of the mouth of Mobile Bay.