92 SOiUTHERN CALIFORNIA GEOLOGY 



path of the last mentioned stream between Redlands 

 and its mouth approximately follows this line. This 

 border also corresponds more or less with the south 

 boundary of the Ontario Valley Plain. 



There are many wonderful physiographic features 

 associated with this northern end of the Highlands, 

 which in places has been badly indented by erosion 

 so that it is marked by many isolated, outlying hills 

 of the Rubideaux, Riverside, Colton and Jurupa type. 



These features include the San Jacinto Mountains 

 and the Badlands Range, the San Jacinto Valley Gra- 

 ben, the Lakewood Horsts, the Grapevine and Reche 

 Canyon Mountain Blocks, the Gavilan Country, the 

 Elsinore Graben, the Santa Ana Range and the 

 Downey Plain. That portion of the area between the 

 San Jacinto Mountain and the Elsinore Graben Valley 

 has been termed the Perris Peneplain by Mendenhall, 

 and is supposed to be a remnant of the same, ancient, 

 land-surface as that which is now also represented in 

 the summit of the San Bernardino Plateau. 



The San Jacinto Range is one of the three master 

 Highlands of Southern California, and presents a more 

 or less precipitous slope on the Desert side of 11,000 

 feet above the bottom of the Salton Sea at its foot. 

 It is bordered on three sides by master faults, so that 

 it is included in a long triangle with its narrowest 

 angle to the south. The faults on its northern side 

 were probably made in pre-Pliocene time, while the 

 one on the east was probably initiated in the Miocene 

 and greatly rejuvenated in the Pleistocene, as attested 

 by the uplift of the marine Hathaway formation on 

 its west side of the fault, to a height of 3000 feet above 

 its former position at or below sea level. The San 



