1922] 



Vaughan: Geology of San Bernardino Mountains 



341 



large boulders, brought down doubtless during cloudbursts. The 

 stream is constantly changing from one radiating channel to another 

 across the fan. 



Out from Millard Canon the same thing is to be seen. At the 

 mouth of the canon the fan is a mass of subangular boulders, while 

 at Cabezon the alluvium has been cultivated and planted to orchards. 

 Again at Banning the upper part of the fan contains many boulders 

 and south of the town the soil is rather heavy. 



It is interesting to note that the fans spreading out from the 

 canons on the north side of the pass cross it and the streams actually 

 wash against the bedrock on the south ; in fact, the only alluvial fans 

 on the south side are at the mouths of Snow Creek and Blaisdell Canon, 

 and these are protected by prominent salients. A study of the relative 

 amounts of water coming down the canons on each side of the pass 

 seems to contribute to an explanation. Planimeter measurements 

 show that the catchment area for Whitewater River is 60 square miles, 

 while that for Snow Creek is only 32 and for Blaisdell Canon it is 

 only 7 square miles. For Potrero and Millard canons it is 42 square 

 miles and for the opposing streams from the south only 19 square 

 miles. The total area draining into the San Gorgonio Pass from the 

 north is 198 square miles and that from the south is only 74 square 

 miles. It is therefore clear that with the same precipitation the 

 streams from the north would be the stronger and build out the larger 

 fans. There are reasons, however, for believing that the precipitation 

 on the north is the greater. In this region the moisture-bearing winds 

 are from the southwest and on striking the high mountains they drop 

 a large part of their burden ; hence the south and west slopes will 

 receive the more. This being the case, the precipitation is probably 

 much greater on the north side of the pass than on the south. 



While the foregoing considerations explain the greater development 

 of the fans on the north side, they do not fully account for the fact 

 that fans on the south side are almost entirely absent. A factor which 

 may be of importance in this connection is the effect of the fault on 

 the north side of the pass. This fault is certainly very recent, for its 

 topographic expression is still clear despite the fact that the rocks on 

 the north, or upthrown side, are but loosely consolidated and therefore 

 not resistant to erosion. The lowering of the south side would nat- 

 urally cause the fans there to be buried by those from the north. 



In addition to the extent of the fans there are other effects of the 

 greater flow of water from the north. The slopes of the fans at the 



