While the final effect of the subaerial denuding agencies is to sweep away all relief, and to cut the land surface down to low-lying base-levels or peneplains, yet in the process great irregularities are produced by the more rapid removal of some parts than of others. The topographical forms generated by this differential erosion vary much according to circumstances. We have already considered some of these differences with regard to the agencies which have produced them. Now we have to examine the differences with a view of learning how topographical forms are determined by the character and arrangement of the rocks which are undergoing degradation.

Forms In Horizontal Strata

When a peneplain or plain of marine denudation is lifted high above sea-level, without folding or steep tilting of the strata, streams are soon established upon the new land, and proceed to cut deep trenches across the plateau, which are gradually widened out under the influence of weathering, and the arrangement of hard and soft rocks finds expression in the resulting forms. If the surface layers resist weathering, the plateau will be gradually dissected into flat-topped mesas and table-mountains, which in the progress of denudation are converted into pyramidal shapes; while if the whole mass of rocks be easily destructible, they weather down into dome-shaped and rounded hills, which are smallest at the top, the part longest exposed to weathering. The wild and grotesque scenery of the Western bad lands, with their chaos of peaks, ridges, mesas, and buttes, is merely the result of the differential weathering of horizontal strata, some beds and parts of beds yielding more readily than others.

The bad lands are carved out of soft and scarcely indurated rocks, but firm rocks in climates of similar aridity give rise to many vertical-sided mesas (see Frontispiece), as is so notably the case in the sandstones of New Mexico. In pluvial climates the slopes are gentler.

If a series of more resistant beds underlies a mass of softer strata, a change in the topographical forms will occur when the underlying harder rocks are partially exposed. In the soft rocks the valley sides have gentle slopes, but when the harder mass is penetrated, the slopes become steep, or even vertical. When hard and soft strata alternate in a valley wall, the harder beds form cliffs. This is accomplished by cutting away the softer beds and thus undermining the harder ones, until the latter can no longer support their own weight, and masses fall from the face of the cliff, thus maintaining the verticality. The talus blocks form a slope, connecting the successive cliffs by gentler inclines. The Uinta Mountains in northern Utah are formed by a great anticlinal arch, so broad and gently curved that in a given section the strata appear almost horizontal. Out of these immensely thick and nearly level masses the subaerial denuding agencies have carved an infinite and most picturesque variety of peaks, pinnacles, columns, and amphitheatres, while the streams have cut profound and gloomy canons.

Vast talus slopes remain to indicate the amount of destruction.