CHAPTER XVIII 

 THE VALLEY OF THE SOUTH 



The Great Valley of the South 

 a Structural Basin 



Lying south of the San Bernardino and San Gabriel ranges, 

 embracing the rolling plain between the San Jacinto and Santa 

 Ana mountains, is a vast structural basin called the Valley of 

 the South. It is cut off abruptly at the north by the fault walls 

 of the San Bernardino and San Gabriel ranges, and is sharply 

 delimited on the east by the San Jacinto fault and on the west 

 by the Elsinore fault, which forms the eastern wall of the Santa 

 Ana Mountains. Transversely across the northern side of the 

 great basin lies the sunken San Bernardino Valley, which in- 

 cludes the deeply depressed San Bernardino Basin, the Riverside 

 basin, and the Chino and Cucamonga plains. To the west the 

 alluvial San Gabriel Valley extends to the Los Angeles Basin 

 and the Santa Ana coastal plain, beyond the low Puente, Merced, 

 and Repetto hills. The San Jose Hills, regarded as the northern 

 continuation of the Santa Ana Mountains, project into the 

 valley northeast of Pomona, about 25 miles from the Pacific 

 coast. Southward from the San Bernardino Valley and River- 

 side an undulating plain extends to Lake Elsinore and the towns 

 of San Jacinto and Hemet, and includes the broad Perris plain. 

 This is an old worn-down plain, interrupted by ridges and 

 knobs of granite, "stumps" of granitic and schistose rocks, rem- 

 nants (monadnocks) of an "old" landscape. The whole region 

 from the northern end of the San Jacinto mountains westward 

 to the Puente and San Jose Hills was at one time not very far 

 back in geologic history an undulating worn-down plain (a 

 peneplain). The granitic knobs and ridges are the stumps of 

 mountains, remnants of the old landscape. The "floor" of the 



