SCIENTIFIC BACKGROUND 167 



physiographic lineaments in Southern Cahfornia. Sev- 

 eral hot springs also occur along its course, and it has 

 provided several minor shocks in historic times. 



The course of this fault zone, which in places is 

 composed of parallels, is marked by infacing escarp- 

 ments and a great depressed valley trough or graben. 

 Among the valleys which owe their existence to it 

 may be mentioned, beginning at the northwest, the 

 Chino Creek Basin, the Elsinore Basin, which lat- 

 ter includes the continuous valleys of the Temescal, 

 Elsinore Lake and Temecula drainages, the San Luis 

 and Banner drainage valleys and the great Vallecito 

 and Jacuba valley embayments, which extend as north- 

 westward indentation from the desert into the east 

 edge and summit of the plateau. All of these features 

 are adjusted to the lines of the rift zone. 



This rift is associated with the border escarpments 

 of many adjacent highlands which are uplifted blocks 

 along its course. Some of these face to the northeast 

 and others in opposite or southwest directions, sug- 

 gesting a condition which is technically called rota- 

 tional. Among the east facing features are the east 

 borders of the Puente Hills, the great, titled block of 

 the Santa Ana Range, and the gigantic escarpment 

 which leads down from the summit region of the Pen- 

 insula Highlands to the desert in the cross section along 

 the Mexican border. The several conspicuous west- 

 facing escarpments along the line include those of the 

 Agua Tibia, Volcan, Santa Ysabel and Vallecito Moun- 

 tain blocks and also the belt of country between the 

 San Jacinto and Elsinore Rifts at the north end of the 

 Peninsula Ranges, which has been termed the "Perris 

 Peneplain." No detailed studies have been made of 



