The Great Plains slope gently eastward from the foothills of the Rocky Mts. at an elevation of 6,000 ft (1,829 m) to merge into the interior lowlands at an elevation of roughly 1,500 ft (457 m). The 1,500 ft (457 m) contour line, the 100th meridian of longitude, and the 20-in. (51-cm) isohyet of precipitation are arbitrarily used to mark the region's transitional eastern border. 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 Great Plains region has generally level or rolling terrain; its subdivisions include Edwards Plateau, the Llano Estacado, the High Plains, the Sand Hills, the Badlands, and the Northern Plains.
The Black Hills and several outliers of the Rocky Mts. interrupt the region's undulating profile. 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. (51 cm) of precipitation annually. 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. The dominant type of vegetation consists of shortgrass prairies; trees grow in moister areas and along water courses.