GEOLOGY OF THE COAST SYSTEM OF MOUNTAINS. 17 
the ranges. The coincidence of many of the larger valleys with a synclinal axis is very 
marked. 
Faults. — In the Coast Ranges there are numerous faults, but our knowledge of them 
is limited, owing to the small amount of geological mapping which has been done in the 
region. With the extension of cartographic work, many more than are now known will 
doubtless come to light. Of those at present known, the great majority have a general 
northwest-southeast strike, but there are several minor faults which trend transverse 
to the general strike. The faults of the Coast Ranges, as well as those of other parts of 
California, are indicated, as to position and extent, on Map No. 1. A summary reference 
to them is all that will be here attempted. 
The most northerly fault of the Coast Ranges is one which Mr. O. H. Hershey calls 
Redwood Mountain fault. It is an overthrust, according to Mr. Hershey, heading to 
the northeast and having a throw of probably over 5,000 feet. It trends southeast 
along the southwest flank of South Fork Mountain for scores of miles, and doubtless 
determines the very straight trend of this great ridge. Parallel to it, on the southwest 
side of Redwood Creek, near Acorn, there is another fault having a throw of at least 
1,000 feet, according to Mr. Hershey. Its extent is unknown. ‘The precipitous south- 
west front of Mount St. Helena has been shown by Osmont'* to be a degraded fault- 
scarp; and the downthrow on the southwest side of the fault is estimated by him to be 
not less than 2,500 feet. The western edge of the Sacramento Valley, from Benicia to 
Cordelia, is probably determined by a fault with an easterly downthrow. 
In the Mount Diablo region, there is a pronounced overthrust fold which causes Mio- 
cene strata to rest upon Pliocene strata with a dip of 30° to 45° to the northeast. Louder- 
back’s work on the structure of Mount Diablo has shown that this over-tipt fold passes 
into a thrust fault whereby a considerable proportion of the mountain has been shoved 
to the southwest. The west side of San Ramon and Livermore Valleys is bounded for 
the most part by a steep mountain wall at the base of which, near Pleasanton, the Ter- 
tiary rocks are faulted down against the Franciscan. This fault extends southward 
thru Calaveras Valley and past Mount Hamilton. Its general course is about N. 35° W. 
It has an extent of at least 60 miles and may be very much longer. In the Berkeley 
Hills to the east of this there are many minor faults, both overthrust and normal, which 
will not be described in detail. In the Mount Hamilton Range, between the crest and 
the Santa Clara Valley, there are several faults, notably the Mission Creek, Mission 
Peak, Mount Hamilton, and Master’s Hill faults, which have a more or less regular 
northwest-southeast trend; and there are several shorter faults transverse to these, and 
of variable strike. 
The valley of the Bay of San Francisco and its prolongation southward in the Santa 
Clara Valley is bounded on the northeast side by a range of hills which presents a very 
even, straight, and on the whole, but little dissected, front to the southwest. This even 
front extends from near Point Pinole, on San Pablo Bay, to the vicinity of Hollister, a 
distance of about 100 miles, forming a very striking geomorphic feature of the Coast 
Ranges. At Berkeley and Oakland, and southeast of the latter, there is evidence that 
this even front represents a somewhat degraded fault-scarp, or series of scarps, and this 
interpretation may with very probable truth be placed upon it for its entire extent. 
Near Berkeley the slope of this degraded scarp is traversed by supplementary step faults, 
which are not improbably characteristic of it in other places; so that in regarding the 
feature as a fault-scarp it is not intended to apply that term too narrowly, but to include 
rather the idea of a zone of acute deformation traversed by step faults. This line has a 
course of about N. 35° W. North of San Pablo Bay, on the geographic prolongation of 
the line, a similar feature, tho by no means so straight, is found on the east side of the 

1 Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3, p. 78. 2 Results not yet published. 
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