LINEAMENTS OF THE DESERT 29 



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 

 eircumdenudation 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 wind- 

 drifted sands all local depressions are quickly filled. The tendency of 

 the surface mantle is thus merely to make the plains smoother than 

 thev 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 impor- 

 tant 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 



