TheAtlantic Plain is one of eight distinctphysiographic divisions of the contiguous United States. Using the USGS physiographic classification system, the Atlantic Plain division comprises two provinces and six sections. The Coastal Plain province is differentiated from the Continental Shelf province simply based on the portion of the land mass above and below sea level.[1]
The lands adjacent to the Atlantic coastline are made up of sandy beaches, marshlands, bays, and barrier islands. It is the flattest of the U.S. physiographic divisions and stretches over 2,200 miles (3,500 km) in length fromCape Cod to theMexican border and southward an additional 1,000 miles (1,600 km) to theYucatán Peninsula. The central and southernAtlantic Coast is characterized by barrier and drowned valley coasts. The coastal Atlantic plain features nearly continuous barriers interrupted byinlets, large embayments withdrowned river valleys, and extensivewetlands andmarshes.[2] The Atlantic plain slopes gently seaward from the inland highlands in a series of terraces. This gentle slope continues far into the Atlantic andGulf 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 theLouisiana bayous and theFlorida Everglades.
The Atlantic Ocean has a broad, flat continental shelf that reaches a depth of 100 meters.[3] The continental shelf off the Atlantic Ocean ranges in width from less than 1 km off Florida to more than 420 km off Maine. The average width is about 135 km.[4]
The Coastal Plain of theUnited States includes all or portions of thestates ofAlabama,Arkansas,Delaware,Florida,Georgia,Kentucky,Maryland,Massachusetts,Mississippi,Missouri,New Jersey,New York,North Carolina,Oklahoma,South Carolina,Rhode Island,[a]Tennessee,Texas andVirginia. From the northeast, it begins onCape Cod, Massachusetts and stretches to the southwest toSouth Padre Island, Texas. This is approximately 1,900 miles (3,100 km).
This province consists of the following physiographic sections: the Embayed,Sea Island,Floridian,East Gulf Coastal Plain,Mississippi Alluvial Plain, and theWest Gulf Coastal Plain.
The rocks in the province consist, for the most part, of layers of sand and clay which are not yet hardened into sandstone and shale. The Coastal Plain features nearly continuous barrier islands interrupted by inlets, large embayments with drowned river valleys, and extensive wetlands and marshes. The Coastal Plain slopes gently seaward from the inland highlands in a series of terraces.
The province's average elevation is less than 900 meters above sea level and extends some 50 to 100 kilometers inland from the ocean.[citation needed] The coastal plain is normally wet, including many rivers,marsh, andswampland.[citation needed] It is composed primarily ofsedimentary rock andunlithifiedsediments and is primarily used foragriculture.[5] The Atlantic Coastal Plain includes theCarolina Sandhills region[6] as well as the Embayed and Sea Island physiographic provinces. The Atlantic Coastal Plain is sometimes subdivided into northern and southern regions, specifically theMid-Atlantic andSouth Atlantic coastal plains.
The Atlantic Plain is generally gently dipping undeformedMesozoic andCenozoic sediments, with the sedimentary wedge thickening toward the sea, reaching a maximum thickness of about 3 kilometers (10,000 ft) in the vicinity ofCape Hatteras, North Carolina.[7]
Despite being previously overlooked in research, the Atlantic Coastal Plain is now identified as a global biodiversity hotspot, with over 1500 endemic plant species, and ~70% habitat loss. This endemism is particularly high in thelongleaf pine savannas and woodlands, along with the other herbaceous andfire dependent ecosystems of the ecoregion. Despite intermittent flooding, certain refugia have remained continuously terrestrial since at least the late Cretaceous (85-80mya), contributing to endemism. Generally speaking, despite the high diversity of soils, the soil is nutrient poor. This is primarily due to an abundance of well-drained soils, creating a primarilyxeric character to the floral makeup of the Atlantic Plain.[8][9] However, waterlogged soils are also notable, with wetlands andhammocks being important ecological features.[10]
TheEastern woodlands are the original, predominant ecosystem of the Atlantic coastal plain. TheAtlantic coastal plain upland longleaf pine woodland is an endemic plant community found in most of the Atlantic coastal plain, ranging from Virginia to northern Florida. These woodland savannas are reliant onsandy soils and are fire dependent, lest hardwoods start to dominate. Alongsidelongleaf pine, typically associated flora includesturkey oak andwiregrass. TheFlorida longleaf pine sandhill extends the longleaf pine forests into central Florida, withSouth Florida slash pine flatwoods,Florida sand pine scrub andFlorida dry prairie stretching into southern Florida.[11][12][13][14][15] Longleaf pine woodlands also stretch further west, to eastern Texas. These consist ofEast Gulf and West Gulf longleaf pineflatwoods, bisected by theMississippi Alluvial Plain.[12][13][16]
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To the north, the Atlantic Coastal Plain also broaches into the mesic hardwood forests of theMiddle Atlantic coastal forests, followed by the northernAtlantic coastal pine barrens.[17][18][19] The southernmost Atlantic Plain contains the onlyNeotropical ecoregion of the continental USA, being theEverglades andFlorida mangroves.[20]
The following species are largely endemic to the region. Amphibian diversity is especially notable in the Atlantic Plain.[9]
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Some of these species' ranges may extend into thelongleaf pine woodlands and savannas of the Gulf Plain.[16][21]
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