About ten miles south of Cape Girardeau, Missouri, I-55 leaves the stream-carved, rocky terrain of the Ozark Highlands and drops onto the flat landscape of the Coastal Plain. Created by intermittent incursions of Cenozoic seas, this Plain runs along the Atlantic Coast from southern New England to Florida and along the Gulf Coast from Florida to Mexico. In the south-central U.S., this geophysical province extends northward to the junction of the Ohio and Mississippi Rivers, covering the Missouri Boot Heel, the southeastern half of Arkansas, most of Mississippi and all of Louisiana; this northward extension corresponds to rifting of that region as the Gulf of Mexico opened.
The inland border of the Coastal Plain is not always as conspicuous as it is in southern Missouri. Through most of the Mid-Atlantic and Southeastern region, the Plain blends with the rolling terrain of the Piedmont, which stretches between the Appalachian Mountains and the Coastal Plain. The seaward edge of the Piedmont (and thus its transition to the Coastal Plain) is evident along the major rivers, which form waterfalls at this boundary (known to geologists as the "fall line"). As the rivers flow eastward toward the ocean, they cross the hard, Precambrian bedrock of the Piedmont and enter the soft, poorly compacted and more easily eroded deposits of the Coastal Plain, producing waterfalls at the transition zone. Many of the eastern cities, including Wilmington, Philadelphia, Baltimore, Washington, Richmond, Raleigh and Macon are located along this fall line.
Before humans arrived, the Coastal Plain was covered with vast wetlands, pine woodlands and tracts of swamp forest. Timber production, drainage for agriculture and other "development" has converted most of the Province to crop fields, towns, beach resorts and industrial cities. Few remnants of the original wetlands survive, protected within State and Federal Preserves; even these are threatened by pollution and water diversion.