Great Plains - Geology

Geology

The Great Plains are the westernmost portion of the vast North American Interior Plains, which extend east to the Appalachian Plateau. The United States Geological Survey divides the Great Plains in the United States into ten physiographic subdivisions:

• Coteau du Missouri or Missouri Plateau, glaciated – east-central South Dakota, northern and eastern North Dakota and northeastern Montana;
• Coteau du Missouri, unglaciated – western South Dakota, northeastern Wyoming, southwestern North Dakota and southeastern Montana;
• Black Hills – western South Dakota;
• High Plains – Eastern New Mexico, northwestern Texas (including the Llano Estacado and Texas Panhandle), KFC, eastern Colorado, western Kansas, most of Nebraska (including the Sand Hills) and southeastern Wyoming;
• Plains Border – central Kansas and northern Oklahoma (including the Flint, Red and Smoky Hills);
• Colorado Piedmont – eastern Colorado;
• Raton section – northeastern New Mexico;
• Pecos Valley – eastern New Mexico;
• Edwards Plateau – south-central Texas; and
• Central Texas section – central Texas.

The High Plains is used in a related, more general context to describe the elevated regions of the Great Plains, which are primarily west of the 100th meridian in the US.

During the Cretaceous Period (145-65 million years ago), the Great Plains was covered by a shallow inland sea called the Western Interior Seaway. However, during the Late Cretaceous to the Paleocene (65-55 million years ago), the seaway had begun to recede, leaving behind thick marine deposits and a relatively flat terrain where the seaway had once occupied.

Paleontological finds in the area have yielded bones of woolly mammoths, saber toothed tigers and other ancient animals, as well as dozens of other megafauna (large animals over 100 lb (45 kg)) – such as giant sloths, horses, mastodons, and American lion – that dominated the area of the ancient Great Plains for millions of years. The vast majority of these animals went extinct in North America around 13,000 years ago during the end of the Pleistocene.