The Gulf Coastal Plain is a vast, low-lying physiographic province dominating the southern edge of North America, curving around the Gulf of Mexico. It is one of the continent’s largest coastal plains, characterized by relatively flat topography. This region consists of a broad apron of young, unconsolidated sediment deposited by rivers and the sea over millions of years.
Defining the Geographical Extent
The southern boundary of the Gulf Coastal Plain is the shoreline of the Gulf of Mexico, stretching from the western Florida Panhandle to the Rio Grande in Texas. Inland, the plain’s extent is defined by a geological transition where younger coastal sediments meet the older, harder bedrock of the continent’s interior. This northern boundary is often identified as the Fall Line, which traces an arc against the Ouachita Highlands, the Interior Low Plateaus, and the southern Appalachian Mountains.
The Fall Line marks where rivers transition from resistant inland rock to softer coastal sediments, often resulting in rapids and waterfalls. The plain also extends westward, continuing into northeastern and eastern Mexico through states like Tamaulipas, Veracruz, and reaching the Yucatán Peninsula.
States Encompassing the Plain
The Gulf Coastal Plain covers all or most of the five U.S. states bordering the Gulf of Mexico: Texas, Louisiana, Mississippi, Alabama, and Florida. This includes the vast majority of Louisiana and Mississippi, the southern two-thirds of Alabama, eastern Texas, and the Florida Panhandle.
The inland reach of the plain is extended northward by the Mississippi Embayment, a trough-like extension following the Mississippi River. This embayment carries the coastal plain’s characteristics far inland, reaching parts of Arkansas, western Tennessee and Kentucky, the Missouri Bootheel, and the southern tip of Illinois. The total onshore area of the U.S. section covers approximately 230,000 square miles.
Physical Characteristics of the Landscape
The Gulf Coastal Plain is visually identified by its low elevation and gentle slope toward the sea. Elevations generally remain under 500 feet, and the surface is shaped by a thick wedge of unconsolidated sediments, including sand, silt, and clay. These materials were eroded from inland uplands and deposited over millions of years, reaching thicknesses up to 18,000 feet near the coast.
The region is defined by distinct coastal landforms, such as extensive marshlands, barrier islands, and large river delta systems. The massive Mississippi River Delta is a prominent feature that continually reshapes the Louisiana coastline. Further inland, the terrain is flat to gently rolling, often broken up by numerous streams and low escarpments.