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



361 



THE FIRST CYCLE OP POST-MIOCENE EROSION 



The deformed Rosamond has everywhere been extensively 

 eroded. In the areas where the old erosion surface has been 

 preserved by later lava flows and alluvial cappings it is seen 

 to be one of virtual peneplanation (pis. 35b and 37b). In the 

 locality north of Barstow, in the Calico Hills, and at Black 

 Mountain, the tilted, folded, and faulted beds of the Rosamond 

 have been beveled to an essentially even surface. A very notable 

 and peculiar feature is the almost total absence of the effects 

 of mature weathering in the superficial layer of the old erosion 

 surface. 



That portion of the western Mohave Desert situated south 

 of a line connecting the towns of Mojave and Barstow and west 

 of the Mojave River is a gently rolling or nearly flat country. 

 All of the hills in this western part of the desert, with the ex- 

 ception of the range extending west from Rogers dry lake to 

 northwest of Rosamond, are low-lying residuals. East and north 

 of Mojave River the ranges are more numerous, more rugged, 

 and higher. In this portion of the desert there is a greater 

 amount of later lava flows, while in the southwestern part there 

 has not yet been found any evidence of this later vulcanism, 

 and it is probable that none — or but very little at the most — will 

 be found. The present differences in altitude and relief of the 

 two portions, therefore, is probably accounted for by the lava 

 flows in the northern area and by their subsequent deformation. 

 The country to the north and east has perhaps been more re- 

 cently uplifted and has not yet reached the advanced stage of 

 erosion possessed by the country to the west, where the former 

 cycle of erosion still reigns in the main. 



Hershey has expressed the belief that the detrital filling of 

 the western portion of the Mojave Desert is a relatively thin 

 veneer over the old bedrock surface, and the data collected re- 

 cently support this belief. His remarks are quoted in full : 



The only extensive portion of Southern California, so far as seen by 

 the writer, apparently remaining nearly in its late Pliocene condition is 

 the Mohave Desert. Professor N. S. Shaler* says of it : " The most 



* Broad Valleys of the Cordilleras, Bull. Geol. Soc. Am., vol. 12, p. 290. 



