The Coastal Plain province, the flattest province, stretches over 3,540 km (2,200 mi) from Cape Cod to the Mexican border. The plain slopes gently seaward from the inland highlands in a series of terraces. This gentle slope continues far into the Atlantic and Gulf of Mexico, forming the continental shelf.
This region was formed during the breakup of the supercontinent Pangea in the early Mesozoic Era. From about 280-230 million years ago, (Late Paleozoic Era until the Late Triassic) the continent we now know as North America was connected with Africa, South America, and Europe.
As Pangea broke apart, rifting began as magma welled up through the weakness in the crust and created a volcanic rift zone. The rift between the spreading continents gradually grew to form a new ocean basin, the Atlantic. The rift zone known as the Mid-Atlantic Ridge continues to provide the raw volcanic materials for the still expanding ocean basin. As the edge of North America moved away from the hot rift zone, it began to cool and subside beneath the new Atlantic Ocean. This once-active divergent plate boundary became the passive, trailing edge of westward moving North America. In plate tectonic terms, the Coastal Plain is a classic example of a passive continental margin. - US NPS