574 C. R. KEYES RELATIVE EFFICIENCIES OF EROSIONAL PROCESSES 



track were washed out in less than an hour's time, and a canyon 75 feet 

 deep, 50 feet wide, and several miles in length was excavated in the 

 smooth surface of the sloping plain. By the time a permanent bridge 

 was built to span the deep trench the winds had filled the entire excava- 

 tion, so that where a yawning chasm had been was as smooth as the rest 

 of the plain, and the wing dams alsc had melted down into the general 

 smoothness of the desert surface. For several years, until it was finally 

 replaced by an earthen grade, travelers were wont to express great won- 

 derment at the possible utility of a fine iron bridge resting on the smooth 

 sands of the desert plain. 



In another instance an arroyo was partially obstructed by a wing dam 

 composed of large boulders and a ditch cut out laterally from the low 

 bank for the purpose of diverting over the adjoining plain some of the 

 flood waters for irrigation. The first time the dry creek filled with water 

 an impassable chasm 50 feet deep was cut for a distance of several miles 

 across the plain. 



As a corrasive power the floodsheet of the desert is so counteracted b}^ 

 the general leveling effects of the winds that its influence is as inappre- 

 ciable as are the eolian effects in humid lands. The heavy sediments car- 

 ried by the floodsheet often fill the hollows formed by wind scour in the 

 surface of the plains. The only noteworthy function that the floodsheet 

 performs is that of transporting the finer rock waste in large quantities. 

 Even this is not so important as it at first glance might appear. The 

 floodsheet merely carries down the plains slope for a distance of a few 

 miles, perhaps, the wind-formed soils which, as soon as dry again, are 

 blown back up the slope or out of the area. 



The great work of the sheetflood of the desert corresponds in a humid 

 clime merely to the movement of the idle, wind-blown sands of the sea- 

 shore. 



There are some phases of floodsheet effects that are of exceptional in- 

 terest. A most noteworthy result of constructive work and repeated 

 local occurrence of the sporadic "cloud-burst" on the plains is the forma- 

 tion of the playa. Under favorable conditions considerable sections of 

 stratified deposits may thus form. It may be that some of the extensive 

 Tertiary "lake" deposits of such districts as Death Valley, in eastern 

 California, belong to this class. Another instructive result is the forma- 

 tion of ephemeral lakes in the deserts. At intervals of a century or two 

 sheetflood or arroyo-running conditions sometimes prevail to an almost 

 unheard of extent. In the Carmen bolson, in the State of Chihuahua, 

 Mexico, unusual and repeated floodsheet waters produced a lake of large 

 size. Still farther to the south, at Luguna, in the Sanz bolson, the flood- 



