High Plains (United States)
Geologically, the high plains are caused by the same process as caused the Rockies. The cause is the westward movement of North America away from Eurasia, which has been going on for over 100 million years. As the continent moved west, it subducted plates under the western side, and stretched out its eastern side. This explains why the western states all have some mountainous terrain, and the eastern states in the south are so low-lying.