1914] Dickerson: Eocene of the Santa Ana Mountains 



259 



The Santa Ana River, the main stream of this area, and Santiago 

 Creek, one of its tributaries, drain nearly all the southwestern part 

 of the range. There are no large trunk streams on the abrupt 

 eastern fault scarp of the range. 



The party was unable to work 

 out in detail the exact relations 

 of the range to an important 

 physiographic feature of this 

 region, the Perris Peneplain. In 

 the vicinity of Sierra Peak, at the 

 northern end of the range, there 

 are, at the crest, certain bits of 

 old topography which may be 

 remnants of this old surface. A 

 view looking southeast from 

 Santiago Peak shows rather ex- 

 tensive plateaus such as Mesa de 

 Burro and Mesa de Colorado. 

 These plateaus appear to be 

 directly connected with the Per- 

 ris Peneplain around the south- 

 east end of the Elisnore Moun- 

 tains, the extension of the Santa 

 Ana Range on the south. This 

 peneplain is best developed near 

 Perris, about ten miles south of 

 Riverside. The country around 

 Riverside and the San Bernar- 

 dino Valley appeal's to be an 

 extension of this old erosion 

 surface. 



The uplift of the Santa Ana 

 Mountains appears to have been 

 subsequent to the development 

 of Perris Peneplain. The Santa 

 Ana River and Murietta Creek 

 are the only streams which have 

 succeeded in maintaining their 

 courses across the general uplift 



