The region is about 500 mi (800 km) east to west and 2,000 mi (3,200 km) north to south.
The northern section of the Great Plains, north of latitude 44°, including eastern Montana, north-eastern Wyoming, most of North and South Dakota, and the Canadian Prairies, is a moderately dissected peneplain. The strata here are Cretaceous or early Tertiary, lying nearly horizontal.
The surface is shown to be a plain of degradation by a gradual ascent here and there to the crest of a ragged escarpment, the escarpment-remnant of a resistant stratum.
It has an area of approximately 500,000 sq mi (1,300,000 km The term "Great Plains", for the region west of about the 96th or 98th meridian and east of the Rocky Mountains, was not generally used before the early 20th century.
Nevin Fenneman's 1916 study Physiographic Subdivision of the United States Today the term "High Plains" is used for a subregion of the Great Plains.
There are also the occasional lava-capped mesas and dike formed ridges, surmounting the general level by 500 ft (150 m) or more and manifestly demonstrating the widespread erosion of the surrounding plains.