Page:Popular Science Monthly Volume 74.djvu/33

Rh the beveled edges of the strata. The plateau-plain thus represents a former position of the general plains-level. It is the best example of circumdenudation through vigorous wind-scour.

The soil mantle of the interment plains is everywhere relatively thin. These plains instead of being areas of great accumulations of recent rock-waste, as might very naturally be expected, appear to be, as a rule, only thinly veneered. Often extensive areas are swept clean by the winds so that the rock-floor is exposed.

Nearly all of the finer surface detritus is transported from a greater or less distance. It is rare for the surface materials of the desert plains to give any suggestion of the rock-composition immediately beneath.

The gravely character of the intermont plains of the desert, to which travelers commonly allude, is largely only apparent. Most of the gravel-surfaced areas when upturned by the plow give excellent loamy fields. It is not generally recognized that the great abundance of pebbles on the surface of many plains is due to the fact that the strong and persistent winds blow away the finer materials, leaving a pebble mosaic behind.

While the rock-floor of the plains is itself a plain there are many inequalities in the surface. Between sheet-flood erosion and winddrifted sands all local depressions are quickly filled. The tendency of the surface mantle is thus merely to make the plains smoother than they otherwise might be.

One of the most remarkable features which at once attracts the attention of the traveler is the general absence of distinct waterways in the valleys or intermont basins. Notwithstanding the fact that the gradients are high, no drainage systems are developed. Channel-ways that are corraded by unusual freshets, which sporadically occur, are quickly obliterated by the drifting sands and soils.

The degradation of the desert regions is not to be regarded as all accomplished by wind-scour. At times water plays a minor but important part locally. In the loftier mountain ranges normal torrential water-action takes place, much the same as it does in humid regions. The streams which are occasionally formed by heavy rainfalls soon sink into the ground on reaching the plain and become lost rivers. At other times, the stream-ways are without water for the greater part of their courses; arroyos, or dry creeks, the Spanish term them. McGee's vivid description of the advance of a flood-sheet over a piedmont slope is in reality the running of an arroyo as it enters a plain.

Water sometimes assumes another strange phase. The excessive local rainfalls which occur at rare intervals, "cloud-bursts" they are called, often form in the intermont valleys extensive shallow lakes. Most of these bodies of water are of short duration. When they are