Great Plains Province (U.S. National Park Service)

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Geologically, are the Great Plains of the U.S.A. essentially a Valley? It seems to be a flat fill between the Rocky Mountains and the Appalachian Mountains. Is that not a definition of

Rocky Mountain System Provinces (U.S. National Park Service)

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Great Plains Province (U.S. National Park Service)

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Center for Great Plains Studies

Great Plains Province (U.S. National Park Service)