Great Plains

The Great Plains (french: Grandes Plaines), sometimes simply "the Plains", is a broad expanse of flatland in North America. It is located west of the Mississippi River and east of the Rocky Mountains, much of it covered in prairie, steppe, and grassland. It is the southern and main part of the Interior Plains, which also include the tallgrass prairie between the Great Lakes and Appalachian Plateau, and the Taiga Plains and Boreal Plains ecozones in Northern Canada. The term Western Plains is used to describe the ecoregion of the Great Plains, or alternatively the western portion of the Great Plains.

The Great Plains lies across both Central United States and Western Canada, encompassing:
* The entirety of the U.S. states of Kansas, Nebraska, North Dakota and South Dakota;
* Parts of the U.S. states of Colorado, Iowa, Minnesota, Missouri, Montana, New Mexico, Oklahoma, Texas and Wyoming;
* The southern portions of the Canadian provinces of Alberta, Saskatchewan and Manitoba.

The term "Great Plains" usually refers specifically to the United States portion of the ecozone while the Canadian portion is known as the Canadian Prairies. In Canada it covers southeastern Alberta, southern Saskatchewan and a narrow band of southwestern Manitoba, these three provinces collectively known as the "Prairie Provinces". The entire region is known for supporting extensive cattle-ranching and dryland farming.

Grasslands are among the least protected biomes with vast areas having been converted for agricultural purposes and pastures.

Usage

The term "Great Plains" is used in the United States to describe a sub-section of the even more vast Interior Plains physiographic division, which covers much of the interior of North America. It also has currency as a region of human geography, referring to the Plains Indians or the Plains states.

In Canada the term is rarely used; Natural Resources Canada, the government department responsible for official mapping, treats the Interior Plains as one unit consisting of several related plateaus and plains. There is no region referred to as the "Great Plains" in the ''Atlas of Canada''. In terms of human geography, the term ''prairie'' is more commonly used in Canada, and the region is known as the Canadian Prairies, 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 Tamaulipas-Texas Semi-Arid Plain, which overlap or expand upon other Great Plains designations.

Extent

The region is about east to west and north to south. Much of the region was home to American bison herds until they were hunted to near extinction during the mid/late-19th century. It has an area of approximately . Current thinking regarding the geographic boundaries of the Great Plains is shown by thi
map
at the Center for Great Plains Studies, University of Nebraska–Lincoln. This definition, however, is primarily ecological, not physiographic. The Boreal Plains of Western Canada are physiographically the same, but differentiated by their tundra and forest (rather than grassland) appearance.

The term "Great Plains", for the region west of about the 96th 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'' brought the term Great Plains into more widespread usage. Before that the region was almost invariably called the High Plains, in contrast to the lower Prairie Plains of the Midwestern states. Today the term "High Plains" is used for a subregion of the Great Plains. The term still remains little-used in Canada compared to the more common, "prairie".

Geography

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:
* Missouri Coteau or Missouri Plateau (which also extends into Canada), 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 – southeastern Wyoming, southwestern South Dakota, western Nebraska (including the Sand Hills), eastern Colorado, western Kansas, western Oklahoma, eastern New Mexico, and northwestern Texas (including the Llano Estacado and Texas Panhandle);
* 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.

Further to this can be added Canadian physiographic sub-regions such as the Alberta Plain, Cypress Hills, Manitoba Escarpment (eastward), Manitoba Plain, Missouri Coteau (shared), Rocky Mountain Foothills (eastward), and Saskatchewan Plain.

The Great Plains consist of a broad stretch of country underlain by nearly horizontal strata extending westward from the 97th meridian west to the base of the Rocky Mountains, a distance of . It extends northward from the Mexican boundary far into Canada. Although the altitude of the plains increases gradually from 600 or on the east to 4,000–5,000 or near the mountains, the local relief is generally small. The semi-arid climate excludes tree growth and opens far-reaching views.

The plains are by no means a simple unit. They are of diverse structure and of various stages of erosional development. They are occasionally interrupted by buttes and escarpments. They are frequently broken by valleys. Yet on the whole, a broadly extended surface of moderate relief so often prevails that the name, Great Plains, for the region as a whole is well-deserved.

The western boundary of the plains is usually well-defined by the abrupt ascent of the mountains. The eastern boundary of the plains (in the United States) is more climatic than topographic. The line of of annual rainfall trends a little east of northward near the 97th meridian. 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. However, in Canada the eastern boundary of the plains is well defined by the presence of the Canadian Shield to the northeast.

The plains (within the United States) may be described in northern, intermediate, central and southern sections, in relation to certain peculiar features. In Canada, no such division is used: the climatic and vegetation regions are more impactful on human settlement than mere topography, and therefore the region is split into (from north to south), the taiga plains, boreal plains, aspen parkland, and prairie ecoregion regions.

Northern Great Plains

The northern section of the Great Plains, north of latitude 44°, includes eastern Montana, eastern Wyoming, most of North Dakota and South Dakota, southwestern Minnesota and portions of the Canadian provinces including southeastern Alberta, southern Saskatchewan and southwestern Manitoba. 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. There are also the occasional lava-capped mesas and dike formed ridges, surmounting the general level by or more and manifestly demonstrating the widespread erosion of the surrounding plains. All these reliefs are more plentiful towards the mountains in central Montana. 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. 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 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. Here, the ice sheet overspread the plains from the moderately elevated Canadian highlands far on the north-east, instead of from the much higher mountains nearby on the west. The present altitude of the plains near the mountain base is .

The northern plains are interrupted by several small mountain areas. The Black Hills, chiefly in western South Dakota, are the largest group. They rise like a large island from the sea, occupying an oval area of about north-south by east-west. At Black Elk Peak, they reach an altitude of and have an effective relief over the plains of 2000 or This mountain mass is of flat-arched, dome-like structure, now well dissected by radiating consequent streams. The weaker uppermost strata have been eroded down to the level of the plains where their upturned edges are evenly truncated. The next following harder strata have been sufficiently eroded to disclose the core of underlying igneous and metamorphic crystalline rocks in about half of the domed area.

Intermediate Great Plains

In the intermediate section of the plains, between latitudes 44° and 42°, including southern South Dakota and northern Nebraska, the erosion of certain large districts is peculiarly elaborate. Known as the Badlands, it is a minutely dissected form with a relief of a few hundred feet. This is due to several causes:
* the dry climate, which prevents the growth of a grassy turf
* the fine texture of the Tertiary strata in the badland districts
* every little rill, at times of rain, carves its own little valley.

Central Great Plains

The central section of the Great Plains, between latitudes 42° and 36°, occupying eastern Colorado and western Kansas, is mostly a dissected fluviatile plain. That is, this section was once smoothly covered with a gently sloping plain of gravel and sand that had been spread far forward on a broad denuded area as a piedmont deposit by the rivers which issued from the mountains. Since then, it has been more or less dissected by the erosion of valleys. The central section of the plains thus presents a marked contrast to the northern section.

While the northern section owes its smoothness to the removal of local gravels and sands from a formerly uneven surface by the action of degrading rivers and their inflowing tributaries, the southern section owes its smoothness to the deposition of imported gravels and sands upon a previously uneven surface by the action of aggrading rivers and their outgoing distributaries. The two sections are also alike in that residual eminences still here and there surmount the peneplain of the northern section, while the fluviatile plain of the central section completely buried the pre-existent relief. An exception to this statement must be made for the southwest, close to the mountains in southern Colorado, where some lava-capped mesas ( Mesa de Maya, Raton Mesa) stand several thousand feet above the general plain level, and thus testify to the widespread erosion of this region before it was aggraded.

Southern Great Plains

The southern section of the Great Plains, between latitudes 35.5° and 25.5°, lies in western Texas, eastern New Mexico, and western Oklahoma. Like the central section, it is for the most part a dissected fluviatile plain. However, the lower lands which surround it on all sides place it in such strong relief that it stands up as a table-land, known from the time of Mexican occupation as the Llano Estacado. It measures roughly east-west and north-south. It is of very irregular outline, narrowing to the south. Its altitude is at the highest western point, nearest the mountains whence its gravels were supplied. From there, it slopes southeastward at a decreasing rate, first about , then about 7 ft per mile (1.3 m/km), to its eastern and southern borders, where it is in altitude. Like the High Plains farther north, it is extraordinarily smooth.

It is very dry, except for occasional shallow and temporary water sheets after rains. Llano is separated from the plains on the north by the mature consequent valley of the Canadian River