Hershey.] 



Quaternary of Southern California. 



3 



THE EARLY QUATERNARY OROGRAPHIC DISTURBANCE. 



Iii the hydrographic basin of the Santa Clara River of the 

 South, in Los Angeles and Ventura counties, there is an oval 

 Upper Pliocene area about twenty miles long and ten miles in 

 greatest width. After about 8,000 feet in thickness of gravel, 

 sand, and clay (in part marine and in part alluvial, but all 

 indisputably laid down in approximately a horizontally-bedded 

 position aud near sea-level) had accumulated, the basin was 

 uplifted, compressed and the strata plicated. Around the border 

 the gravel was tilted at angles prevailingly 20° to 80°, but near 

 the center there was formed a distinct anticline, in the southwest 

 limb of which the strata, even the very latest member, stand at 

 angles varying from 30° to 60°, but prevailingly 45°. The 

 orographic disturbance which thus tilted these late Pliocene beds 

 was not an unimportant or local one.* 



Another and smaller isolated basin of Upper Pliocene strata, 

 lying mainly in the extreme northwestern corner of Los Angeles 

 County, near Gorman's Station, has been elevated bodily to an 

 average altitude of 3,000 or 4,000 feet, tilted toward the center 

 at angles of 10° to 30°, and profoundly eroded. On the eastern 

 flank of Fraser Mountain, immediately northwest of this basin, 

 there are several high mesas (altitude about 6,000 feet). They 

 are remnants of old detrital slopes of light brown sub-angular 

 gneissic debris, built up over tilted and eroded pink Upper 

 Pliocene sandstone, and subsequently by uplift of Fraser Moun- 

 tain, tilted more steeply away from the center of the mountain, 

 raised high on its flank and deeply eroded. The detrital 

 capping is late Quaternary. 



Fraser Mountain and the neighboring Sierra de la Liebre 

 have beautiful dome shapes, and, as seen from a distance, 

 smooth contours contrasting strongly with the rugged and no 

 higher Alamo, San Emedio, Sierra Madre, part of the Tehachapi 

 Range and the Coast Ranges in general in that region. These 

 mountains and the similarly smooth ridge just north of Gorman's 



* These beds are unconformably above a series of sandstones and conglomerates 

 which are recognized as San Pablo in age, and on structural and lithologic grounds 

 they are correlated with the Paso Robles formation, the Salinas Valley equivalent 

 of the Merced series. 



