134 University of California Publications in Geology [Vol.7 



GEOLOGIC HISTORY POSTERIOR TO THE TIME OP DEPOSITION OP THE 

 ROSAMOND SERIES AS INFERRED FROM THE STRUCTURE 

 AND PHYSIOGRAPHY 



The Two Epochs of Post-Miocene Uplift with the Intervening 

 First Cycle of Post-Miocene Erosion. — The Rosamond series both 

 on the north and south flanks of the El Paso mountains was 

 tilted to the northward at a moderate angle after its deposition. 

 It was then subjected to long erosion which bevelled off the tilted 

 beds to an essentially level surface. The peneplain formed by 

 this cycle of erosion is correlated with the one developed on the 

 folded Rosamond beds in the vicinity of Barstow, in San Ber- 

 nardino County. At the end of the first cycle of post-Miocene 

 erosion or more likely after renewed deformation had caused a 

 new cycle to begin a thin veneer of alluvial debris was spread 

 over the bevelled surface of the Rosamond strata northeast, 

 north, and northwest of Ricardo post-office. This alluvium was 

 derived from the erosion of the recently uplifted Sierra Nevada 

 Range. Because of the coarseness of the arkosic material of the 

 alluvial mantle and the almost total absence from it of the 

 products of mature chemical decomposition, and because an 

 erosion surface similar to that produced by the first cycle of 

 post-Miocene erosion in the El Paso Range was also developed in 

 the Sierra Nevada it is thought most probable that the deposition 

 of the alluvium mantle on the evenly eroded surface of the Rosa- 

 mond was brought about by a mountain-making uplift in the 

 southern Sierra Nevada. An uplift, to which the present El Paso 

 Range owes its existence, followed the development of the pene- 

 plain and caused the development of a great fault along the 

 southern base of the range at the same time that an uptilting 

 took place on the northern flank. But probably before this 

 uplift occurred the basalt flow of Black Mountain was erupted, 

 covering the substantially flat and even erosion surface developed 

 on the uptilted Rosamond series. 



Origin of Bed Rock Canon and Last Chance Gulch. — These 

 drainage channels cross the main southern ridg^e of the El Paso 

 Range in deep, narrow, and precipitous canons although in 

 their upper reaches they have developed broader, shallower, and 



