1922] Vaughan: Geology of San Bernardino Mountains 339 



of the hills a considerable strip along the upper portion of the sub- 

 alluvial bench has been laid bare by the removal of fanglomerate. 

 Along its lower or northern edge this uncovered rock platform is 

 rather smooth, but higher up to the south there are small gullies devel- 

 oped with portions of the flat surface between. Still higher the gullies 

 become deeper and wider and the surfaces along the tops of the ridges 

 become broken. It seems probable that this stripping has been brought 

 about by an increased gradient due to a sinking of the area to the 

 north. Had it been due merely to a local uplift of the hills, part of 

 the bench on the south side would also be denuded. The hills to the 

 east of Negro Butte exhibit the same general character. 



The recency of the uplift of Fry Mountain has already been men- 

 tioned. The alluvial fans are in accordance with this, for they are 

 poorly developed as compared with those around the hills to the south- 

 west. The long hill three miles east of Fry Mountain has a well 

 developed, alluvial embankment on the southwest side and the upper 

 part of the suballuvial bench has been stripped and eroded in exactly 

 the same manner as that west of Negro Butte. The northeast side of 

 the hill is straight and steep, which leads to the conclusion that it is 

 a fault scarp. This idea is further strengthened by the fact that it is 

 in line with the northeast side of a similar hill four miles to the south- 

 east. An interesting feature in connection with this hill is the way 

 in which the wind has affected the alluvial embankment. On the 

 east side it is perfectly normal fanglomerate, but on the west there is 

 a great deal of finer wind-blown sand creeping high on the slopes. 

 At the hill two miles to the southeast this material nearly reaches 

 the top. 



The two hills northwest of Means Wells are similar to Fry Moun- 

 tain in that they are steep and have rather poorly developed fans. 

 These observations naturally lead to the conclusion that they too are 

 of recent uplift. Contrasted with these are the hills farther north, 

 where again we have well developed fans and many isolated salients. 



Extensive alluvial fans are being developed along the north front 

 of the main mountain mass. These present certain peculiarities which 

 show that the uplift has been by stages. On the north side of Black- 

 hawk Mountain an old fan creeps high on the slope and extends more 

 than three miles from the base of the mountain. It is composed almost 

 wholly of fragments of limestone so recemented as to be nearly as firm 

 as the original. This fan is now deeply dissected, probably due to 

 increase in gradient by uplift. 



