132 SOUTHERN CALIFORNIA GEOLOGY 



Shaffer's Well. The Pliocene marine sediments which 

 are found below sea level to the south near West- 

 moreland and elsewhere, have been here extensively 

 uplifted, faulted and folded. 



While no conspicuously identifiable, single fault is 

 visible on the west side of the depression, several 

 north-south extending faults may be seen which are 

 evidently vestiges of a great step-down zone. These 

 lie in line with trends of both similar faults of the 

 Whitewater type on the east side of the San Bernar- 

 dino highlands and the great Sierra Nevada fault 

 far to the north, facts which suggest that there may 

 once have been a long line of such displacements along 

 this general longitude. 



The border between the west side of the desert de- 

 pression and the east side of the Peninsula highland 

 is made ragged and jagged by a number of long and 

 narrow valleys between southeasterly trending divides 

 of the highland, which extend towards and into the 

 main body of the desert. These features are adapted 

 to the extensions of major southeast rift lines which 

 cross the highlands diagonally from the Pacific side. 



The origin of these north-southerly extending rift 

 lines along the west borders of the Great Basin and 

 Colorado Desert Regions certainly antedates the late 

 Miocene in age, although the movements may have 

 begun in an earlier epoch and have been later revived. 

 The aggregate displacement along this indefinite, 

 west side, rift line, as seen in the east front of 

 the San Jacinto Range, must in places exceed 11,000 

 feet. At least three thousand feet of this move- 

 ment must have been accomplished in Pleistocene time, 

 as is testified by the Westmoreland Pliocene sediments 



