340 University of California Publications in Geology [Vol. 13 



East of Blackhawk Canon many alluvial fans creep high on the 

 hills and in two eases they have been faulted. One of these is about 

 a mile west of the mouth of Rattlesnake Canon and the other is about 

 three miles east of Rock Corral. 



West of Cushenberry Spring the fans are well developed but do 

 not creep high on the hills in the same manner as those farther east. 

 Another important difference is that more large boulders have moved 

 far down the slope. In this connection it is well to note that west of 

 a north-south line passing through Negro Butte there are numerous 

 yucca trees, while to the east there are none ; the vegetation there con- 

 sisting of greasewood, sagebrush, catclaw, and mesquite. Even these 

 are sparse, and five miles east of Old Woman Springs there is a great 

 area of sand dune country. The significance of these facts seems to 

 be that the rainfall is materially greater to the west. This, of course, 

 would explain the difference between the alluvial fans. 



Playas are the beds of intermittent, undrained lakes and are typical 

 of a region so arid that no outside drainage is established. They are, 

 of course, the places where the finest silt and dissolved salts in the 

 run-off from the surrounding country are deposited. By far the 

 larger part of the material is fine silt, but thin calcareous and gyp- 

 siferous seams are not uncommon. When these playas dry up, the sur- 

 face is hard and smooth and light grey to white in color. At a dis- 

 tance they look like sheets of water. 



A more or less continuous patch of green vegetation extends from 

 Cushenbury Springs to Rabbit Springs. Numerous springs flow 

 forth along a fault and the moisture has served to retain the finer dust, 

 so that areas of fine loamy soil are found which are in marked contrast 

 with the coarse fan material on all sides. To a less extent the same 

 tiling is found at Old Woman Springs. 



Alluvial Fans in San Gorgonio Pass 



Along the south front of the mountains the streams are building 

 up alluvial fans. One at the mouth of Mission Creek is well developed 

 and shows the features characteristic of these accumulations. Near 

 the mouth of the canon the material which constitutes the fan is hardly 

 less than a mass of boulders. Some of these are well rounded, but most 

 are subangular although worn fairly smooth. Toward the southeast 

 the material is finer and finally, just north of Palm Springs station, 

 there is a great deal of fine alluvium ; but even here there are some 



