1922] 



Vaughan: Geology of San Bernardino Mountains 



331 



into the hills in much the same way as the old valley floor of 

 the Santa Ana. A consideration of the atmospheric agencies explains 

 the greater removal of the old floor from Mill Creek Canon. The 

 precipitation here is fully as great as in Santa Ana Canon, since 

 the moisture comes from the south and is intercepted by the summit 

 ridge. In the spring the quick melting of the snow on the south or 

 Mill Creek side of this ridge affords a great carrying power for the 

 removal of detritus, while on the north or Santa Ana side the snow 

 melts more slowly, some remaining throughout the year, which does 

 not make for rapid erosion. Near the mouth of Potato Canon are low 

 ridges of fanglomerate which appear to have been deposited at the 

 same time as the fanglomerate in Mill Creek Canon, as they are of 

 similar composition and lie at about the same elevation. 



The crest of the ridge between Mill Creek and Potato canons is 

 certainly a portion of an old surface much reduced and covered with 

 soil. It is so well preserved that one gets the impression of being in 

 a broad rolling country rather than on a high ridge. It has crooked 

 gullies with low divides and also small inclosed basins, a good example 

 of which is found just above the head of the west fork of Birch Creek. 

 All are abnormal in their present position ; they must have been de- 

 veloped much nearer base level. Away from the crest the ridge is 

 deeply incised by V-shaped gullies and canons. Toward the west the 

 ridge is much lower and the crest line becomes more serrate, as would 

 be expected where the ridge is reduced below the old surface. The 

 correlation of this topography with that to the north is uncertain. 

 The relief is greater than that on the summit ridge and the surface is 

 certainly distinct from that developed when Mill Creek and Santa Ana 

 canons were floored with fanglomerate ; it must occupy some interme- 

 diate position. This fact, together with the actual resemblance of the 

 two surfaces, suggests that it belongs to the subcycle represented by 

 the streams along the ridge west of Sugarloaf. 



It has already been noted that the mountains have a general slope 

 to the east. The western portion was probably raised the highest at 

 each uplift, and hence must have developed the greatest relief as the 

 streams approached base level. Therefore we find the second cycle 

 well developed around Bear Valley, while to the east it is not so dis- 

 tinct, and still farther east, in the vicinity of The Pipes, large areas 

 of the first cycle are preserved. It is therefore clear that a certain 

 correlation of scattered remnants of any surface is exceedingly diffi- 

 cult and in some cases the evidence is hardly more than suggestive. 



