18 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
Santa Rosa and Russian River Valleys up to about Cloverdale. Here, however, evidence 
of faulting is lacking, altho it is known in places to be a line of flexure. Along the base 
of this line of scarp, between Oakland and San Jose, occurred the fault which caused the 
earthquake of 1868. It may be referred to as the Haywards fault, from the fact that it 
passes thru that town. 
An interesting and important fault traverses the peninsula of San Francisco, a little 
south of the city. The course of this fault can not be precisely determined, as its trace 
at the surface is obscured by Pleistocene and recent deposits. Its approximate position 
is at the southwest base of San Bruno Mountain, with a strike of about N. 48° W. By 
this fault the Merced strata, which are well exposed on the sea-cliffs south of Lake Merced 
to the thickness of over a mile, are dropt down against the Franciscan rocks, the throw 
being estimated at not less than 7,000 feet. To the northeast of the main fault, and 
close to the face of the mountain, is an auxiliary fault, and between these two faults 
there is a block of the Franciscan which has dropt only to a limited extent, and which 
is of the same character 4s the kernbuts of the Kern River.!. The bold and precipitous 
southwest face of San Bruno Mountain is thus a fault-scarp with two facets, one for the 
main fault and the other for the auxiliary, both being well exprest in the geomorphic 
profile of the mountain. ‘This fault-scarp appears to be the southern prolongation of the 
scarp which forms the coastal steep slope to the north of the Golden Gate, and seems 
to converge upon the San Andreas fault, off the Golden Gate, making a very acute angle 
with it. It affords an excellent illustration of the general fact above alluded to, that 
the northwesterly members of the fault system controlling the configuration of the 
coast are prolongations of fault-lmes within the Coast Ranges. Knowledge of the 
extent of this fault, altho its throw is so notable, is limited to the peninsula of San 
Francisco. 
Outside of Fort Point, at the Golden Gate, and a little south of the point, is a very 
well exposed fault which appears to strike southeast across the city of San Francisco. 
The fault is nearly vertical and has a throw of at least some hundreds of feet, whereby 
the serpentine on the north has been dropt against a formation of radiolarian cherts. 
The most interesting fault traversing the Peninsula of San Francisco is the San Andreas 
fault, on which movement was renewed on April 18, 1906, causing the earthquake. The 
extent and_course of this fault are described in detail elsewhere. To the southwest of 
the San Andreas fault, on the Peninsula of San Francisco, and in the Santa Cruz Moun- 
tains, are several other faults of notable extent. Of these may be mentioned the Fifield, 
Pilarcitos, Castle Ridge, Butano, Boulder Creek, and San Gregorio faults, all of which 
are important features of the structure of the region. 
On the southwest side of Montara Mountain is a very precipitous seaward slope, at 
the base of which strata of Miocene age are tilted at rather abrupt angles against the 
granite. The strata of arkose sandstone at the base still rest against the original floor 
of deposition, but it is difficult to see how such an acute uplift could take place in a 
granite massif without deformation of the granite. Such deformation might take the 
form of plastic flow if it were sufficiently deep-seated, or it might find its expression in 
a zone of faults; and as there is no evidence of plastic deformation, it is concluded that 
the uplift of Montara Mountain was effected.by faulting within the granite, the same 
deformation appearing as flexure in the stratified rocks which flank the mountain on 
this side. 
Northeast of the San Andreas fault are the: Belmont and Black Mountain faults, the 
latter a branch from the San Andreas fault. In the gap between the Santa Cruz and 
Gavilan Ranges is a fault followed by the canyon of Pajaro River near Chittenden, which 
drops the Tertiary formations on the north against the pre-Franciscan granitic rocks of 


1 Bull. Dept. Geol., Univ. Cal., vol. 3, No. 15. 
