On the east it is strongly undercut by the retrogressive erosion of the headwaters of the Red, Brazos and Colorado rivers of Texas, and presents a ragged escarpment, 500 to 800 ft.
The central section of the Great Plains, between latitudes 42 and 36, occupying eastern Colorado and western Kansas, is, briefly stated, for the most part a dissected fluviatile plain; that is, this section was once smoothly covered with a gently sloping plain of gravel and sand that had been spread far forward on a broad denuded area as a piedmont deposit by the rivers which issued from the mountains; and since then it has been more or less dissected by the erosion of valleys.
Taking the average elevation of the central axial line of snowy peaks as 19,000 ft., the average height of the passes is not more than 10,000 owing to this process of cutting down by erosion and gradual encroachment into the northern basin.
The process by which the ancient tablelands have been trenched into valleys and confluent ridges is most instructively displayed among the higher mountains, where erosion proceeds at an accelerated pace.
In the south-west the results of this erosion are seen in an accentuated form in the region between the White river and the South Fork of the Cheyenne river, known as the Bad Lands or terres mauvaises.
Now the granite, continuing for long distances, forms the prevailing rock; then, again, it forms the foundation for thick strata of schist and sandstone, itself only appearing in valleys of erosion and river boulders, in rocky projections on the coasts or in the ridges of the mountains....
Uplift permitted renewed erosion to wear away the soft belts, leaving mountain ridges of hard rock separated by parallel valleys.
The coastal plain, however, is the result, not of a single recent uplift, but of movements dating back to Tertiary time and continued with many oscillations to the present; nor is its surface smooth and unbroken, for erosion began upon the inner part of the plain long before the outer border was revealed.
The crystalline highlands thereabouts, at altitudes of 8000 to 10,000 ft., are of so moderate a relief as to suggest that the mass had stood much lower in a former cycle of erosion and had then been worn down to rounded hills; and that since uplift to the present altitude the revived streams of the current cycle of erosion have not entrenched themselves deep enough to develop strong relief.
The horizontal strata of the plateau present equal ease or difficulty of erosion in any direction; the streams and the submature valleys of the plateau therefore ramify in every direction, thus presenting a pattern that has been called insequent, because it follows no apparent control.