Mountains of the South 229 



sular Plateau slopes in a general way toward the north to about 

 900 feet in the vicinity of Riverside. 



The San Gabriel and San Bernardino ranges together form 

 an east-west transverse range dividing Southern California into 

 north and south. These are the highest mountain ranges in 

 southern California. The San Gabriel Range extends from the 

 Santa Clara River on the west 50 miles to Cajon Pass on the 

 east. The San Bernardino Range extends east and south 80 

 miles to the Cottonwood Range, and separates the Colorado 

 Desert on the south from the Mojave Desert on the north. The 

 San Gabriel Range is the higher of the two. It has been sug- 

 gested by an eminent geologist that the two ranges may have 

 been originally continuous, the break at the Cajon Pass, through 

 which passes the San Andreas fault, marking a torsion stress in 

 the crust of the earth by which the mountain range was broken 

 in two. The character of the rocks suggests that earth stresses 

 caused a movement of the rocks by which the two ranges were 

 rent apart a distance of 15 to 25 miles."" 



The San Gabriel Range 



The San Gabriel Range is an uplifted block 50 miles in 

 length east and west, and 25 miles across. It is bounded by 

 faults or fault zones on three sides, the Sierra Madre on the 

 south, the San Andreas on the northeast, and the Soledad on the 

 northwest. The block covers an area of 1,200 square miles. 

 Uplifted thousands of feet, it stands out boldly above the sur- 

 rounding country. It is what is technically called a horst a 

 block of the earth's crust cut off at its sides by faults and up- 

 lifted as a huge wedge or block. The great block is clearly a 

 horst, though its mass is broken into many minor blocks by 

 faults which crisscross the range. The surface is exceedingly 

 rugged, being cut by streams into canyons and V-shaped gorges 

 to depths of a few hundred to several thousand feet. Alti- 

 tudes in the western part of the block rise to 4,000 to 7,000 



:; - William J. Miller, Univ. of Calif. (See Appendix.) 



