138 University of California Publications in Geology [Vol. 7 



these drainage channels one finds oneself on a summit upland 

 which is noticeably broad in places and can be traced to the 

 north, east, and west in neighboring summits. The presence 

 of considerable remains of the summit upland between the valleys 

 indicates that this higher portion of the range has only reached 

 the stage of early maturity, since the whole surface is by no 

 means entirely reduced to slopes. We see, therefore, that since 

 the time of formation of the old erosion surface the surface of 

 the summit upland of the Sierra has been uplifted and a new 

 cycle of erosion begun which has reached the stage of late 

 maturity or early old age in the outer slopes of the range and 

 a stage of early maturity near the heads of the drainage courses 

 in the summits. 



It was during this last cycle that the debris apron was spread 

 out beyond the foot of the range by deposition of the materials 

 removed by erosion from the mountains' southern slopes. The 

 erosion folowing the last uplift has dissected the old surface of 

 the Sierra summits and the alluvium derived from this erosion 

 has covered the surface of the peneplain developed on the tilted 

 Rosamond series. So we can conclude that the post-Miocene 

 peneplain of the Mohave Desert region has its contemporaneous 

 counterpart in the old erosion surface of the summit uplands of 

 the Sierra, tentatively correlated with the Chagoopa Plateau 

 surface of the Upper Kern Basin. And since it has become neces- 

 sary, in deciphering the later Cenozoic history of the Great Basin, 

 to refer to these erosion cycles as datum planes in much the same 

 way as rock formations are used, the writer proposes the name 

 "Ricardo erosion surface" for the product of this post-Miocene 

 cycle of erosion, which antedated the great deformation of the 

 southern Sierra Nevada that has given to the range the altitude 

 and major orographic features which we see in it to-day. 



The Ricardo erosion surface where it is developed on the less 

 resistant Rosamond series, and, for that matter, on some of the 

 granitic bedrock surfaces, of the Mohave Desert is an unques- 

 tionable peneplain. North of Walker Pass in the southern Sierra, 

 disregarding the effects of tilting, of possible faulting, and of 

 later erosion, it has a maximum differential relief of a thousand 

 feet or more and is an erosion surface in approximately the 



