The Atlantic Plain is one of eight distinct United States physiographic regions. This major division consists of the Continental Shelf and Coastal Plain physiographic provinces. It is the flattest of the U.S. physiographic divisions and stretches over 2,200 miles (3,500 km) in length from Cape Cod to the Mexican border and southward an additional 1,000 miles (1,600 km) to the Yucatán Peninsula. The central and southern Atlantic Coast is characterized by barrier and drowned valley coasts. The coastal Atlantic plain features nearly continuous barriers interrupted by inlets, large embayments with drowned river valleys, and extensive wetlands and marshes. The Atlantic 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. The relief at the land-sea interface is so low that the boundary between them is often blurry and indistinct, especially along stretches of the Louisiana bayous and the Florida Everglades.
This province consists of the following physiographic sections: the Embayed, Sea Island, Floridian, East Gulf Coastal Plain, Mississippi Alluvial Plain, and the West Gulf Coastal Plain. The "rocks" consist for the most part of layers of sand and clay which are not yet hardened into sandstone and shale. The surface is generally flat.
The Atlantic Plain is generally gently dipping undeformed Mesozoic and Cenozoic sediments, with the sedimentary wedge thickening toward the sea, reaching a maximum thickness of about 3 kilometers (10,000 ft) in the vicinity of Cape Hatteras, North Carolina.
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