The alluvial bench grows upward by accretion of layers which are thickest at the mountain end, so that the steepness of the bench increases with time.
=Erosion in and about the desert.=—The violent cloudburst that is characteristic of the arid lands is a most potent agent in modeling the surface of the ground wherever the rock materials are not too firmly coherent. Under the dash of the rain a peculiar type of “bad land” topography is developed (plate 5 B and Fig. 226). Such a rain-cut surface is a veritable maze of alternating gully and ridge, a country worthless for agricultural purposes and offering the greatest difficulty in the way of penetrating it. When composed of stiff clay with scattered pebbles and bowlders, the effect of the “rain erosion” is to fashion steep clay pillars each capped by a pebble and described as “demoiselles” (Fig. 226).
FIG. 226.—A group of “demoiselles” in the “bad lands” (after a photograph by Fairbanks).]
Behind the mountain front the valleys out of which the torrents are discharged are usually short with steep side walls and a relatively flat bottom, ending headward in an amphitheater with precipitous walls (Fig. 227). In the western United States such valleys are referred to as “box cañons”, but in Mohammedan countries the name “wed” applies to the river valley within the mountains and to the distributaries as well.
=Characteristic features of the arid lands.=—It is characteristic of erosion and deposition within humid regions that all outlines become softened into flowing curves, due to the protective mat of vegetation. In arid lands those massive rocks which are without structural planes of separation, partly as a consequence of exfoliation, develop broad domes which are projected upon the horizon as great semicircles, broken in half it may be by displacement. The same massive rocks where intersected by vertical joint planes yield, on the contrary, sharp granite needles like those of Harney Peak (plate 8 A). Similarly, schistose or bedded rocks, when tilted at a high angle, may yield forms which are almost identical. Examples of such needles are found in the Garden of the Gods in Colorado.
At lower levels, where the flying sand becomes effective as an eroding agent, flat bedded rocks become etched into shelves and cornices, and if intersected by joints, the shelves and cornices are transformed into groups of castellated towers and pinnacles of a high degree of ornamentation. These fantastic erosion remnants are usually referred to as “chimneys” and may be seen in numbers in the bad lands of Dakota, as they may in Colorado either in Monument Park or in the new Monolithic National Park (plate 8 B).