Today, all that remains of the High Plains in eastern Colorado thrusts westward like a blunt wedge, the point of which rests near Limon (Fig. 2). Westward from Limon, a low divide extends to the foot of the Front Range of the Rocky Mountains, between Denver and Colorado Springs, separating the northeasterly oriented drainage of the Platte river system from the southeasterly oriented system of the Arkansas. At one time, the High Plains extended to the Rocky Mountains, but during the geologic past these two great river systems, which rise in the Rockies and flow eastward across the High Plains of Colorado, have cut broad, deep valleys, largely destroying the Plains which once encroached upon and buried the lower slopes of the mountains. This area of stripped alluvial plains is known as the Colorado Piedmont (Fenneman 1931). On the surface of the High Plains, streams flow eastward down the long, gentle slope toward the Plains border in central Kansas (Fig. 2).

The northwestern border of the High Plains is formed by a low escarpment carved by the northand northeastward-flowing waters of the South Platte River. The southwestern border is formed by another low escarpment cut by the southward- and southeastward-flowing tributaries of the Arkansas River.