Coastal Plain

General term for the coastal lowlands of the east and southern USA, bordering the Atlantic Ocean and the Gulf of Mexico. The Coastal Plain is conventionally divided into the Atlantic Coastal Plain and the Gulf Coastal Plain.

The Atlantic Coastal Plain extends from New England, where Cape Cod, Nantucket, and Martha's Vineyard are its northeastern extensions, to Florida. Nowhere more than about 320 km/200 mi wide, it rises gradually to the Fall Line in the west, which marks the beginning of the Piedmont zone (the foothills of the Appalachians). The many bays, sounds and islands that formed in the sunken river valleys of the Plain since the last Ice Age played an important part in the largely maritime early history of the Eastern colonies.

The Gulf Coastal Plain stretches west along the Florida Panhandle and southwest along the coast of Texas. Like the Atlantic plain, it rises gradually towards the southern Appalachians (in the east) and towards the Balcones Escarpment and the Hill Country of Texas (in the west). It also extends northwards as the alluvial plain of the Mississippi River, to near Cape Girardeau, Missouri, some 900 km/550 mi from the Gulf of Mexico.