SHEETFLOOD EROSION 79 



regions it is one of the most potent of degradational agencies, second only 

 to erosion by the winds. . 



In humid lands recently elevated, where most of us have been accus- 

 tomed to picture the evolution of general land forms, the most striking 

 effect is the development of perfect stream systems with their accompany- 

 ing valleys. In the arid regions the main effect of running water ap- 

 pears to be the very reverse. Instead of the attainment of ever more 

 perfect drainage systems, the general tendency is toward complete obliter- 

 ation of all evidences of distinct waterways. In place of sharply incised 

 types of topography, there is the marked planation type of surface relief. 



This plain-forming erosion, if it may be so called, has been so gener- 

 ally overlooked and of recent years the relief effects in the arid regions 

 so generally explained on principles applicable only in a humid climate 

 that it appears pertinent at this time again to direct special attention to 

 some geologic factors peculiar to the dry countries. In the semiarid and 

 arid districts of western America and of Mexico the inhabitants have 

 long recognized this special erosive cause which tends to produce the 

 plain effects. They have also given this phase of erosion a distinctive 

 title. The provincial name is the floodsheet; and, as will be seen later, 

 a very appropriate term it is. 



The encounter with a floodsheet in a dry desert is something quite dif- 

 ferent from every other experience, and when once met with never to be 

 forgotten. The sporadic but severe thunderstorms produce diametrically 

 opposite effects in the mountains and on the plains. These "clo^^d- 

 bursts," as they are suggestively called by those dwelling in the arid 

 regions, give rise in the hills, and often for a short distance from the foot 

 of the ranges, to normal stream action of a very vigorous character. As 

 the ephemeral torrents debouch from the canyons at the base of the moun- 

 tains they quickly spread out into typical, though very local, floodsheets. 

 McGee's account is of one of the latter and is as follows : 



"During the 1894 expedition a moderate local rain occurred while the party 

 were at a Papago rancheria near Rancho de Bosqne. some 1.^) miles north of 

 the international boundary at Nogales ; the rainfall was perhai)s one-fifth of an 

 inch, sufficient to moisten the dry gi'ouud and saturate the clothing, despite the 

 concurrent evaporation, and was probably greater in the adjacent foothills of 

 the Santa Rita range. The road was sensibly level, having only the 20-foot-to- 

 the-mile grade of the Santa Cruz valley ; it ran across the much stronger slope 

 from the range toward the river, and an arroyo embouched from lowterraci- 

 form foothills not more than 200 or 300 yards up the slope. Thus the arroyo 

 opened not on a perceptible fan, but on a sensibly uniform plain of sand and 

 silt with occasional pebbles, sloping perhaps 150 feet to the mile. The shower 

 passed in a few minutes and the sun reappeared, rapidly drying the groimd to 

 whiteness. Within half an hour a roar was heard in tlie foothills, rapidly 



IX — Bull. Geol. Soc. Am., Vol. 10, 1007 



