Vol. 6] Baker: Cenozoic History of the Mohave Desert. 365 



relief is directly due to the circumstance that the southwardly 

 flowing streams have been much aided in their work by recent 

 faulting' along- the south flank of the range, which has uplifted 

 that flank relative to the region to the south, but this faulting 

 belongs to the recent uplift which began the present cycle of 

 erosion. Recent stream-cutting has exposed twenty feet, with 

 lower limit unknown, of well decomposed black loam, free from 

 pebbles, boulders, and other arkosic material, in upper Holcomb 

 Valley, near the head of Van Dusen Canon. It is said that the 

 bedrock was never reached by the extensive placer mining in 

 the broad marshy meadow of this valley and that consequently 

 it must lie at a considerable depth. Some of these wide flats 

 at the heads of water courses, such as Holcomb Valley, Cactus 

 and Broom flats, and the unnamed region about Mound Springs, 

 constitute the divides for streams flowing in opposite directions. 



Although the hills surrounding Holcomb and Bear valleys 

 have well-rounded, fairly gentle slopes and rounded exfoliated 

 knobs of granitic rocks, there is approximately a relief of two 

 thousand feet between their summits and the valley flats. The 

 high terrace near the head of Santa Ana River contains a large 

 amount of coarse boulders and gravels which are certainly not 

 the products of anything like an extreme old-age stage of erosion. 

 Even accounting for a considerable portion of the present relief 

 by tilting, warping, and possible faulting, it is yet evident that 

 the surface of the older sub-cycle had several thousand feet relief 

 between the Big Meadows of the Santa Ana and the surrounding 

 mountain summits. The stage of erosion reached in the San 

 Bernardino Range at the end of the sub-cycle was that of late 

 maturity or early old age. A picture of this range at the end 

 of this sub-cj'cle would probably duplicate in large measure the 

 present aspect of the Wichita Mountains of Oklahoma, rising, 

 as the latter mountains do, abruptly from a nearly flat plain and 

 penetrated by rather broad, open valleys. 



There is a disposition to correlate the surface developed 

 during the first cycle of post-Miocene erosion with the surface 

 of the summit ridge of the San Bernardino Mountains. The 

 sub-cycle does not seem to have advanced to the stage one would 

 expect if it is the same as the more advanced stage in the Mohave 



