230 Adventures, in Scenery 



feet, and in the eastern from 6,000 to 10,500 feet. The orig- 

 inal faulting by which the great block was outlined dates back 

 probably to early Tertiary time, or possibly earlier. That the 

 region has long been subject to erosion is shown by the deeply 

 dissected character of the surface. The upheaval which 

 brought the range to essentially its present altitude occurred, 

 however, in later (Quaternary) time. The great horst block 

 is itself broken into many smaller blocks by faults. These 

 minor faulted blocks were uplifted to different heights, and 

 thus the surface of the range is broken into higher and lower 

 peaks and ridges. Located in the heart of Los Angeles County, 

 15 to 25 miles from the population center of the city of Los 

 Angeles, the region is an almost inaccessible wilderness, for the 

 most part untouched by the foot of man. The southern part 

 of the region is more accessible.* 



Rocks of the San Gabriel Range 



The rocks of the San Gabriel Range are exceedingly com- 

 plex. In a general way the range is a great block of pre- 

 Cretaceous metamorphic and igneous rocks thousands of feet 

 high, bounded on all sides by Tertiary and Quaternary depos- 

 its. All the rocks making up the San Gabriel Range proper 

 are crystalline. The oldest rocks are schists, crystalline lime- 

 stones, and quartzites, probably pre-Cambrian in age, originally 

 shales, limestones, and sandstones. The old sedimentary rocks 

 have been invaded by igneous intrusions of diorite, granite, 

 quartz syenite, granodiorite, and other molten intrusives. 

 These upwellings of molten rock have occurred at repeated in- 

 tervals, and the rocks now exposed at the surface are broken and 

 intermixed in great confusion, so that over large areas the rocks 

 make up a complex of igneous and metamorphic sedimentary 

 and metamorphosed igneous rocks of a very complicated na- 

 ture. Large areas of mixed rocks consist of schist cut and 

 injected by granite or other intrusive. Dikes cut sharply all 



* William J. Miller, op. cit. 



