572 C. R. KEYES RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



clearly indicates. The relatively small volume of coarse detritus brought 

 down from the highlands and deposited in the fans at the base further 

 shows how remarkably unimportant is stream work. This is one of the 

 great surprises to one leaving the humid region and entering the desert 

 country. 



ROLE OF THE PLAINS FLOOD-SHEET 



In recently^^ discussing some of the effects of the desert floodsheet, I 

 described the phenomenon as it appears on the plains — those vast noticea- 

 ably inclined interment plains of the Mexican tableland. This is the 

 true fioodsheet, as it is understood and is called by the dwellers of the 

 arid region of southwestern United States. McGee's account^^ of the 

 sheetflood is really a picture of "cloud-burst" effects in a desert mountain 

 range, and the advancing flood front which this author so graphically 

 portrays is the temporary mountain torrent debouching from a canyon 

 and spreading out over a great fan. This phase of local flood waters is 

 by the desert dwellers distinguished as arroyo-running. The effects of 

 these sporadic but severe thunder-storms in the mountains and on the 

 plains are diametrically opposed. 



It does not seem advantageous to group both phenomena under a single 

 heading, so very different are the effects of the two from each other. The 

 floodsheet, as understood by the people of the region in which it takess 

 place, is, as already stated, a strictly plains phenomenon. The sheet- 

 flood described by McGee is, as noted, a phenomenon of the mountains. 

 Gradationally the first is constructive, the second destructive. The latter 

 in its workings corresponds to normal stream corrasion in the humid 

 land. The former in its constructive effects is merely a means of local 

 transportation of wind-deposited dust; its corrasive powers are slight at 

 best and usually merely accidental. 



The plains floodsheet has little general corrasive effect, for the reason 

 that the cloud-burst is of too infrequent occurrence to enable it to ex- 

 tensively erode the rock floor. The materials which it transports are 

 mainly the finer soils which the winds have already drifted about over 

 the plains. The flowing mud which marks its course is soon dried to the 

 same pulverulent condition that it was before, and it is again carried 

 away by the winds. The only noteworthy effects of the floodsheet is the 

 filling of the wind-blown hollows in the surface of the plains. Thus it 

 tends to make the plains smoother. This also accords with Passarge's 

 observations in the South African desert plains. 



22 Bulletin of the Geological Society of America, vol. 19, 1908, p. 78. 



23 Ibid., vol. 8, 1897, p. 87. 



