United States: The Plains and Highlands of the Interior

Extending more than 1,000 mi (1,610 km) from the Appalachians to the Rocky Mts. and lying between Canada (into which they extend) in the north and the Gulf Coastal Plain in the south are the undulating Interior Plains. Once covered by a great inland sea, the Interior Plains are underlain by sedimentary rock. Almost all of the region is drained by one of the world's greatest river systems—the Mississippi-Missouri. The Interior Plains may be divided into two sections: the fertile central lowlands, the agricultural heartland of the United States; and the Great Plains, a treeless plateau that gently rises from the central lowlands to the foothills of the Rocky Mts. The Black Hills of South Dakota form the region's only upland area.

The Interior Highlands are located just W of the Mississippi River between the Interior Plains and the Gulf Coastal Plain. This region consists of the rolling Ozark Plateau (see Ozarks) to the north and the Ouachita Mountains, which are similar in structure to the ridge and valley section of the Appalachians, to the east.

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