The Great Plains are the broad expanse of prairie, steppe and grassland which lie west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S. states of Colorado, Kansas, Montana, Nebraska, New Mexico, North Dakota, Oklahoma, South Dakota, Texas and Wyoming, and the Canadian provinces of Alberta, Manitoba and Saskatchewan. Some geographers include some territory of Mexico in the Plains, but many stop at the Rio Grande.
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-Hudson 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.