GREAT PLAINS. The Great Plains province is that part of the continental slope which extends from the foot of the Rocky Mountains eastward to the valley of the Mis sissippi, where it merges into the prairies on the north and the low plains adjoining the Gulf Coast and the Mississippi embayment on the south. The Great Plains present wide areas of tabular surfaces traversed by broad, shallow valleys of large rivers, most of which rise in the Rocky Mountains. They are also cut more or less deeply by the narrower valleys of tributary streams. Smooth surfaces and eastward-slop ing plains are the characteristic features of the province, but in some parts of it there are buttes, extensive escarpments, and areas of badlands (q.v.). Sand hills surmount the plains in some localities, notably in north western Nebraska, where they occupy an area of several thousand square miles. The region is made up of soft rocks, sands, clays and loans, in general spread in thin but extensive beds that slope gently eastward with the slope of the plains and lie on relatively smooth sur faces of the older rocks. The source of these deposits was mainly to the west and the ma terials were deposited layer by layer either by streams on their flood plains or in shallow lakes and, during earlier times, in the sea. The strata show but few local flexures, as the region as a whole has been alternately uplifted and depressed but not subjected to folding. During earlier epochs the surface was even smoother than at present. Owing to the great breadth of the plains and their relatively gentle declivity, general erosion has progressed slowly notwithstanding the softness of the formations; and as at times of freshet many of the rivers bring out of the mountains a larger load of sediment than they can carry to the Mississippi, they are now locally building up their valleys rather than deepening them.
The Great Plains province as a whole de scends to the east about 10 feet to the mile from altitudes approaching 6,000 feet above the sea at the foot of the Rocky Mountains to about 1,000 feet near Mississippi River. The altitudes and the rates of slope differ con siderably in different districts, particularly to the north, along the middle course of Mis souri River, where the general level has been greatly reduced. The Great Plains rise to an altitude of 6,200 feet at the foot of the Rocky Mountains west of Denver, and maintain this elevation far to the north, along the foot of the Laramie Mountains. High
altitudes are also attained in Pine Ridge, a plateau which extends from near the north end of the Laramie Mountains eastward through Wyoming, across the northwest cor ner of Nebraska, and for many miles into southern South Dakota. Pine Ridge marks the northern margin of the higher part of the Great Plains and its north side presents cliffs and steep slopes descending 1,000 feet into the drainage basin of Cheyenne River, one of the largest branches of the Missouri. From this basin northward the Great Plains comprise a succession of other basins with relatively low intervening divides, which do not attain as high an altitude as in the region to the south. This northern extension of the Great Plains is drained by the middle branches of Missouri River, of which the larger members are Yellow stone, Powder, Little Missouri, Grand, Cannon ball, Moreau, Cheyenne, Bad and White rivers. On the southern slope of Pine Ridge is Niobrara River, which rises in the midst of the high plains some distance east of the north end of the Laramie Mountains. Farther south are Platte River, with two large branches heading far back in the Rocky Mountains, and Arkansas River, both of which cross the plains to the southeast and afford an outlet for the drainage from a large area of mountains and plains. Between Arkansas and Platte rivers is Republican River, rising near longitude 105°, these having an extensive system of local drain age in eastern Kansas and Nebraska. South of the Arkansas are Cimarron River and numerous smaller streams that head in the western por tion of the plains.
The sandstones underlying the Great Plains contain water available for artesian wells and large flows have been obtained at some locali ties. The other natural resources. are lime stones and clay for cement, oil and gas, fullers earth and volcanic ash. To the east where rainfall is sufficient, agriculture is the most important industry, while the great semi-arid portion to the west is mostly devoted to cattle raising. Consult Darton, N. H., (Geology and Underground Resources of the Central Great Plains (U. S. Geol. Survey, Prof. Paper 32, 1905); Johnson, W. D., 'The High Plains and Their Utilization) (U. S. Geol. Survey, 21st Report, Pt. IV, and 22d Report, Pt. IV).
N. H. DAirrox, Geologist, United States Geological Survey.