1922] Vaughan: Geology of San Bernardino Mountains 343 



the valley floors of the second cycle ahove Banning Heights, which 

 was probably at the base of the mountains at that time, is a measure 

 of the uplift. This is approximately 3000 feet, somewhat less than 

 the amount, of the first uplift. 



Since the most recent uplift is evidenced by rejuvenation, with no 

 evidence of the streams having even approached base level, it is impos- 

 sible to determine its actual amount. 



Glaciation has locally modified the topography of the summit ridge 

 extending from San Bernardino Mountain to San Gorgonio Mountain. 

 Well preserved cirques and moraines are found along the north side. 

 The record they afford shows that the period of ice action was of 

 considerable duration and had a history of some complexity, one 

 glacier having left five terminal moraines. The glacial till retains a 

 considerable amount of water which feeds a large number of springs. 

 These water several meadows and give rise to at least one important 

 stream. 



The physiographic history of the desert north of the San Bernar- 

 dino Mountains differs considerably from that of the mountains them- 

 selves. Numerous areas of basalt, each resting on a nearly flat surface 

 of granite or schist, because of their resemblance to similar areas in 

 the mountains east of The Pipes, indicate that the desert and the 

 present mountains were at one time a continuous surface of low relief. 

 There has been considerable faulting in the desert as well as in the 

 mountains, but the movements were much smaller and the topography 

 does not bear the record of a history divisible into parts corresponding 

 to the second, third, and fourth cycles in the mountains. The changes 

 taking place in the desert during the development of the second cycle 

 in the mountains are still going on. Because of the erosional processes 

 peculiar to arid conditions, desert topography consists of three ele- 

 ments: rock masses of characteristic forms, alluvial fans, and playas. 

 These elements are well developed in this area and display certain 

 variations due to the fact that faulting has taken place at the same 

 time. 



Extensive alluvial fans are being built along both the north and 

 south flanks of the main mountain mass. Those on the south flank 

 spread out across the San Gorgonio Pass. They are much greater than 

 the fans extending northward from the south side of the pass, the 

 difference probably being due to a greater amount of water contrib- 

 uting its load to them. 



