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The Great Plains region extends from northwestern Canada southward through the entire conterminous United States to the Rio Grande River on the United States-Mexico border.  (Some consider the Arctic Slope of northern Alaska to be a northward extension of Great Plains topography.)  The portion of the Great Plains in the conterminous United States has a minimum width of 200 miles near the southern end and has a maximum width of 550 miles near the northern end, with an average width of about 375 miles; it covers about 450,000 square miles–very slightly less than the Atlantic-Gulf Coastal Plain province.  Nearly all of its western edge borders the Rocky Mountains, but a portion of it touches the Basin and Range province at the southern extent.  This western border is very distinct and often abrupt and dramatic.  In contrast, much of the eastern border, especially along the Central Lowlands portion, has few topographic distinctions.  Because of this general lack of prominent border features, the junction of the Great Plains and Central Lowlands has not been consistently defined.  Sometimes the junction is generally delimited by a line coinciding with 1,500 feet in altitude (at least in Kansas and Nebraska), and sometimes with the 2,000-foot contour.  It is also said to coincide loosely with 97-98° west longitude, which is close to the 1,500-foot contour, or 100° west longitude, lying close to the 2,000-foot contour.  Regardless of which value is used as the general location, the easternmost extent of the border is 97° west longitude at the Nebraska-Kansas border and at the southern end in central Texas, and the westernmost edge is 103° west longitude at the United States-Canada border. 
Despite the general lack of major topographic breaks along much of the eastern border of the Great Plains, there are distinctive physical features at and near the northern and southern ends of the border.  In North Dakota and South Dakota the eastern border is demarcated by the 300- to 600-foot-high Missouri Escarpment, which is a steeply-sloping area generally 5-20 miles wide.  The border in Texas just south of the panhandle is at the east-facing Caprock Escarpment, which is composed of a very erosion-resistant layer of carbonate.  The southern portion of the border, where the Great Plains province contacts the Atlantic-Gulf Coastal Plain, is marked by the prominent Balcones Escarpment, which in places is 1,000 feet high.  The middle portion of the border, in eastern Kansas, is sometimes placed at the Flint Hills cuesta, a prominent east-facing escarpment locally referred to as the Break of the Plains, but others place the border a little farther west along the Smoky Hills (Smoky Hill Buttes) near the center of the state and near the eastern end of the Red Hills in the southern part of the state). 
 
Distinctions between the Great Plains and Central Lowlands
Although it is possible to cross from the Great Plains to the Central Lowlands in many places without encountering prominent topographic features, there are several differences between the two provinces.  Much of the Great Plains region is characterized by shorter grasses than is the Central Lowlands.  Grama grass, which typically grows to heights of 6-18 inches, is dominant in the Great Plains, while bluestem grass, which commonly reaches 6 feet in height, is the primary grass in the Central Lowlands.  Fewer trees occur in the Great Plains than regions to the east; forests within the Great Plains, commonly pine, are generally confined to river valleys and the highest elevations.  The shorter grasses and dearth of forested areas are primarily a result of the drier conditions that occur in the Great Plains compared with wetter conditions to the east.  The 20-inch rainfall line coincides closely with the eastern edge of the Great Plains; the semiarid Great Plains west of this line generally receive 12-20 inches of precipitation annually, compared with more than 20 inches received by almost all regions east of the Great Plains.  Although there is almost no true desert in the Great Plains, the region was often referred to as the Great American Desert in the early 19th Century.  Rainfall varies greatly from year to year in the Great Plains, with an average annual variability of as much as 25 percent, compared with more consistent rainfall east of the 20-inch rainfall line.  Also, the regions differ in soil characteristics.  The Central Lowlands province consists primarily of the alfisol soil order in the east and mollisol in the west.  The Great Plains region consists of several types of soils: mollisol and entisol orders in the eastern portion, and mollisol, entisol, alfisol and aridisol in the western part of the province.
 
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