374 



University of California Publications. 



[ Geolouy 



The remainder is carried away by the wind to the adjacent slopes 

 from which a portion again returns to the playas during the 

 succeeding rainy seasons. It is a moot question whether in 

 many cases the playas are increasing their thickness of sediment 

 or even holding their own, for sedimentation upon them is very 

 slow and the wind is almost perpetual in its action and is always 

 provided with tools of sand. The wind-blown sand is mainly 

 deposited in dunes and hillocks on the stoss sides (here the 

 western and northwestern slopes) of debris aprons, mountain 

 slopes, and larger valleys. Occasionally under the impetus of 

 a strong wind the sand blows into rippled drifts like dry snow 

 in a prairie blizzard. 



DISSECTION AND CANON-CUTTING IN ALLUVIAL 



SLOPES 



For the trenching of some of the alluvial slopes of the South- 

 west an explanation is apparently recpiired to account for the 

 dissection of a graded surface by some process which involves 

 neither the recent upwarp of the surface nor the change in 

 differential altitude of the base-level. The hypothesis of recent 

 climatic change has commonly been advocated to explain this 

 dissection. It has been assumed that abundant evidence for this 

 view is found in the drying up of the large Quaternary lakes of 

 the Great Basin and in the diminution of glaciation. In the 

 western Mohave Desert no evidences of Pleistocene lacustral 

 conditions have yet been found. Neither are there any remains 

 of Pleistocene organisms nor of maturely weathered soils. And 

 so in the absence of supporting criteria it is desirable to test the 

 value of the dissection of alluvial slopes as an indicator of recent 

 climatic change. 



The familiar process of the formation of alluvial terraces 

 by the erosion of a formerly aggraded flood plain is a process 

 which works in arid regions as well as in regions of greater 

 rainfall. That such terraces are developed as well under condi- 

 tions of local base-level as under direct control of sea-level is 

 patent. The Great Basin, in common with other arid regions, 

 is essentially a region of local base-levels, in which the same 

 great principles of erosion and deposition hold as in other more 



