142 University of California Publications in Geology [Vol. 7 



or change of climate, a possible explanation which has been 

 more fully considered by the writer in an earlier paper. 11 The 

 depression between the Sierra Nevada and the El Paso Mountains 

 may have been a higher temporary base level, since drained by 

 valleys tributary to the lower base level of the Kane basin, in 

 which case the courses of Red Rock Canon and Last Chance 

 Gulch would not be antecedent but have developed their canons 

 across the El Paso Range by heaclward erosion. But a piedmont 

 alluvial fan spread out during or soon after the uplift of the 

 Sierra, when slopes were steep and debris abundant, would sub- 

 sequently be dissected when the supply of debris became less, 

 allowing running water to erode in places which were formerly 

 the sites of aggradation. 



SUMMARY 



1. The oldest known rocks of the El Paso Range are a series 

 of metamorphics cut by intrusives. 



2. Unconformably overlying the basement complex is the 

 Rosamond series of fossiliferous sedimentary rocks of an age 

 probably not older than the Upper Miocene. 



3. The Rosamond series was tilted at a moderate angle and its 

 strata bevelled by peneplanation. This peneplain is named the 

 Ricardo erosion surface and is tentatively correlated with the 

 Chagoopa plateau of the Upper Kern Basin. 



4. Following peneplanation came an eruption of olivine basalt 

 and the uplift of the present El Paso Range. The range has the 

 form of a faulted monocline with its south flank a fault scarp. 

 Two drainage courses crossing the entire range are believed to 

 have been antecedent to the uplift. 



5. The south front of the Sierra Nevada directly north of 

 the El Paso Range has not been affected by an uplift which 

 affected the rest of the southern Sierra to the east and west. 



6. The piedmont alluvial slope of that portion of the Sierra 

 Nevada situated north of the El Paso Range is at present under- 

 going dissection. 



11 Notes on the later Cenozoic history of the Mohave Desert region in 

 southeastern California, Univ. Calif. Publ. Bull. Dept. Geol., vol. 6, pp. 374- 

 377, 1911. 



