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The Interior Plains are highlighted in red.

The Interior Plains is a vast physiographic region that spreads across the Laurentian craton of North America. This area was originally formed when cratons collided and welded together 1.9-1.8 billion years ago in the Trans-Hudsonian orogeny during the Paleoproterozoic.

Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability.

The Great Plains region of the United States and Canada falls within this area. The interior Plains were often covered by shallow inland sea. Sediments from the shield and the Rocky Mountains were deposited in these seas over millions of years. Eventually the sediments were compressed by the weight of the layers above into sedimentary rock. Part of the sedimentary rock deposited in these areas consists of coral reefs that formed close to the surface of seas during the Paleozoic era.

Throughout the Paleozoic and Mesozoic eras, the mostly low-lying Interior Plains region remained relatively unaffected by the mountain-building tectonic collisions occurring on the western and eastern margins of the continent. During much of the Mesozoic Era, the North American continental interior were mostly well above sea level, with two major exceptions. During part of the Jurassic, rising seas flooded the low-lying areas of the continent, forming the Sundance Sea; in the Cretaceous, much of the Interior Plains region lay submerged beneath the Western Interior Seaway.

The Interior Plains continued to receive deposits from the eroding Rocky Mountains to the west and Appalachian and Ozark/Ouachita Mountains to the east and south throughout the era. The flatness of the Interior Plains is a reflection of the platform of mostly flat-lying marine and stream deposits laid down in the Mesozoic and Cenozoic eras.

There are rolling hills, and deep, wide valleys. There are slopes downward from west to east and some part is separated by a sharp rise. Cattle is raised in places where the climate is too hot for crops.

This article does not cite any references or sources. (November 2007) Please help improve this article by adding citations to reliable sources. Unverifiable material may be challenged and removed.

• USGS description of the U.S. Interior Plains region

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