In places, however, it is clearly marked by an escarpment. Much of the Great Plains was once covered by a vast inland sea, and sediments deposited by the sea make up the nearly horizontal rock strata that underlie the area. Intrusive igneous rocks account for sections of higher elevation. The Black Hills and several outliers of the Rocky Mts. The Saskatchewan, Missouri, Platte, Republican, Arkansas, Cimarron, and Canadian rivers flow in wide beds, generally from west to east, and are important sources of water.
Rainfall decreases from east to west.
Except for its easternmost margin and the elevations, the Great Plains have a semiarid climate, averaging less than 20 in. There are wide seasonal temperature ranges and winds of high velocity. In the westernmost sections the chinook, a warm winter wind, brings relief from bitterly cold and snowy winters. Licensed from Columbia University Press. All rights reserved.