130 University of California Publications in Geology [Vol. 7 



structural depression and that the south front of the El Paso 

 Range owes its main characteristics to deformation rather than to 

 differential erosion. The elevation of the El Paso Range might 

 conceivably have been produced either by warping or by faulting 

 or by a combination of these two. We have already concluded 

 that there has been tilting of the more gently sloping north flank, 

 where there is only a minor amount of faulting, none of which 

 exhibits itself in the topography. It is next necessary to deter- 

 mine whether the deformation along the range's south flank has 

 been accomplished by tilting or by faulting. 



In two localities along the south flank, separated from each 

 other by some eight or ten miles, the Rosamond is found dipping 

 to the northwestward, while between these two isolated exposures 

 and the Rosamond of the north flank lies the more elevated 

 portion of the range composed entirely of the older rocks. This 

 fact in itself would suggest the presence of a fault along the 

 south base. If we follow the two basalt flows in the Rosamond 

 series from Red Rock Canon southwestward we find that their 

 strike swings more and more to the south until we suddenly come 

 upon the straight escarpment of the range's southern front, 

 where both flows, together with the underlying sediments, 

 abruptly come to an end. We also find in the east wall of Red 

 Rock Canon just above the canon's mouth, on the south flank, 

 that the northward dipping Rosamond, traced along the bedding- 

 planes, suddenly comes to an end against the metamorphics with 

 a fault between the two. We therefore conclude that the south 

 flank of the El Paso Range is a fault scarp and that the range 

 is a tilted orographic block bounded on the south side by a 

 "block" fault. 



The deformation which formed the existing El Paso Range 

 was limited to a narrow area between the outcrop of the inter- 

 bedded basalt flows of the Rosamond and the south base of the 

 range. Along the course of Red Rock Canon this strip is no 

 more than three miles in width, extending only from Ricardo 

 post-office to the mouth of the canon. Westward from Red Rock 

 Canon the zone gradually becomes more and more narrow and 

 the range summits lower and lower until the range finally comes 

 to an end about two miles west of the canon. Viewed along the 



