A significant exception to the rule of mature valleys occurs, however, in the case of the Missouri, the largest river, which is broken by several falls on hard sandstones about 50 miles (80 km) east of the mountains.
This peculiar feature is explained as the result of displacement of the river from a better graded preglacial valley by the Pleistocene ice sheet.
It extends northward from the Mexican boundary far into Canada.
Although the altitude of the plains increases gradually from 600 or 1,200 ft (370 m) on the east to 4,000–5,000 or 6,000 feet (1,800 m) near the mountains, the local relief is generally small.
The peneplain is no longer in the cycle of erosion that witnessed its production.
It appears to have suffered a regional uplift or increase in elevation, for the upper Missouri River and its branches no longer flow on the surface of the plain, but in well graded, maturely opened valleys, several hundred feet below the general level.
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.
All these reliefs are more plentiful towards the mountains in central Montana.
It also has currency as a region of human geography, referring to the Plains Indians or the Plains States.
If a boundary must be drawn where nature presents only a gradual transition, this rainfall line may be taken to divide the drier plains from the moister prairies.
The plains may be described in northern, intermediate, central and southern sections, in relation to certain peculiar features.
In terms of human geography, the term prairie is more commonly used in Canada, and the region is known as the Prairie Provinces or simply "the Prairies." The North American Environmental Atlas, produced by the Commission for Environmental Cooperation, a NAFTA agency composed of the geographical agencies of the Mexican, American, and Canadian governments, uses the "Great Plains" as an ecoregion synonymous with predominant prairies and grasslands rather than as physiographic region defined by topography.
The Great Plains ecoregion includes five sub-regions: Temperate Prairies, West-Central Semi-Arid Prairies, South-Central Semi-Arid Prairies, Texas Louisiana Coastal Plains, and Tamaulipus-Texas Semi-Arid Plain, which overlap or expand upon other Great Plains designations.