GEOLOGY OF THE COAST SYSTEM OF MOUNTAINS. 23 
In the valley of the Santa Ynez, there is a marked departure from the northwest- 
southeast trend which characterizes the geomorphic features of the Coast Ranges in 
general, and a more striking instance than any yet cited of the obliquity of those features 
to the general trend of the Coast Range belt. The valley lies nearly east and west and 
its general slope is southward to the base of the precipitous northern face of the Santa 
Ynez Range. This face is, as has been indicated, a fault-scarp; and the course of the 
valley is thus seen to be in intimate relation to this dominant structural feature. To 
the west the valley opens widely to the sea, while to the east it loses its individuality in 
the headwater canyons of eastern Santa Barbara County and western Ventura County. 
Between the Santa Ynez Valley and the upper Cuyama is the rugged and deeply dis- 
sected country culminating in the San Rafael Mountains on the northern side of the tract. 
This mountainous belt has a trend intermediate between the pronounced east-west trend 
of the Santa Ynez Range and the northwest-southeast trend of the Coast Range ridges 
and valleys to the north. For a portion of its length the belt is bounded on the south 
by the Santa Clara Valley, with a general east and west course; but across the head- 
waters of Santa Clara River the mountainous tract persists and finds its prolongation, 
with the same general trend, in the San Gabriel Range, and beyond Cajon Pass in the 
San Bernardino Range, both bold and lofty sierra. It may even be considered as ex- 
tending, under the name of the Chocolate Mountains, to the Colorado River above 
Yuma. From Tejon Pass southeast to Cajon Pass, the northern side of this mountain 
tract presents a very abrupt front with a very straight course. At the base of the abrupt 
slope lies the San Andreas Rift. To the north of this, and between it and the southeast 
scarp of the southern Sierra Nevada, lies the Mojave Desert. To the south of the south- 
east end of the San Bernardino Range and west of the Chocolate Mountains lies the 
Colorado Desert. As has been already indicated, the south side of the San Gabriel 
Range is determined by a profound fault. Lying thus between two faults, the range is 
a magnificent example of a horst which has been thrust up between its bounding faults. 
It is the convergence of these two bounding faults which segregates the San Gabriel 
Range from the San Bernardino Range in the vicinity of Cajon Pass. The latter range 
is similarly bounded on the south by the same fault as that which determines the south 
front of the San Gabriel Range, but here it is coincident with the Rift. Between Los 
Angeles and Ventura lie the short ranges known as the Santa Monica and the Santa 
Susannah Mountains, inclosing San Fernando Valley. The Santa Monica Range is 
probably on the same line of orogenic uplift which finds its expression farther west in the 
Santa Cruz and Santa Rosa Islands. 
South of the San Gabriel Range lies the fruitful valley of southern California, extend- 
ing with an east-west course from the sea to San Bernardino. South of this valley, and 
between the Colorado Desert and a somewhat elevated coastal plain bordering the 
Pacific, is a mountainous tract, the ridges of which swing around into a more northwest- 
southeast trend, and so conform again with the prevailing trend of the ridges and val- 
leys of the Coast Ranges north of the San Rafael Mountains. The valleys in this region 
are, however, less regular in their orientation than those of the northerly Coast Ranges, 
and the geomorphic features are less mature, if we except certain very old features which 
have survived from an earlier cycle of geomorphic evolution. The consequent char- 
acter of the streams on the seaward slope is much more pronounced than in any part 
of the northern Coast Ranges, and on the whole the geomorphy of the region must 
be regarded as less advanced than to the northward, and more closely allied in its 
morphogeny with the Sierra Nevada than with that of the Coast Ranges of northern 
California. 
The notable ranges of this region are the Santa Ana Mountains and the San Jacinto 
Mountains. The former present the features of a seaward sloping, tilted, orographic — 
