THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE. 39 
displacement along the upper line, which has raised a ridge of the old crystalline rocks 
across the valley. This dam must first have given rise to a lake, but as this filled up 
with the wash brought down from the mountains, a marshy meadow took its place. The 
oldest resident in the district says that the earthquake of 1868 formed a small lake in 
the lower portion of the cienega. The great body of gravels filling the old valley acts as 
an important reservoir of water. The city of Hollister has taken advantage of this fact 
to secure a water supply. By tunneling thru the rock barrier, the gravels are reached 
and the water led away in pipes. 
The Rift comes out upon the San Benito River 4 miles above Paicenes P.O. For 
several miles up the river from this point, the Rift line is masked by the recent flood 
plain. Above Mulberry P.O., and just before coming to the bridge across the river, a 
most striking and interesting feature appears. Upon the east side of the river, and 
separated from it by a ridge, is a narrow depression half a mile long and 75 feet deep, 
without any external drainage. The ridge between it and the river extends a mile 
northwest of the sink, and presents a steep face to the northeast. The road passes along 
the eastern base of the ridge and opposite the sink makes use of its even crest. The 
river makes a sharp bend at the bridge, and the Rift crosses to the west side. Faulting 
has here brought to the surface, upon the west side of the Rift, limestones associated 
with the crystalline schists and granitic rocks of the Gavilan Range. 
In order to follow the Rift beyond the mouth of Willow Creek, we leave the San Benito 
River road at the mouth of the creek and follow to its head a long narrow canyon which 
has evidently been eroded on the line of fracture. At the head of the canyon we come 
out upon a bit of open rolling country which, but for a low ridge, would drain into the 
San Benito. This ridge has evidently been raised along the Rift, diverting a stream 
which would naturally be tributary to the San Benito, so that now it forms the head of 
Bear Creek and flows down past the Chelone peaks into the Salinas River. Several 
undrained hollows (plate 188) mark the Rift as it follows the ridge between Bear Valley 
and San Benito River. The formation of both walls is probably of Tertiary age up to 
a point near San Benito P.O., where the Franciscan series constitutes the southwest side 
and the Tertiary the northeast. South of San Benito P.O., there is a considerable area 
where the surface has been much changed as a result of some one of the movements along 
the old Rift. A fertile valley, perhaps 0.5 mile long, appears to have been formed thru 
subsidence, while on the southwest is an abrupt ridge 200 feet high and fully a mile long. 
The ridge without doubt has been produced by faulting. Its abrupt northeastern face 
and long, gentle, southwesterly slope suggest in’a remarkable manner the great fault 
blocks of the west, such as the Sierra Nevada Range. The ridge gradually sinks in a 
southeasterly direction, blending with Dry Lake Valley. The latter is about 2 miles 
across and has no external drainage. The fault-scarp already mentioned extends as a 
low ridge part way across the valley and is utilized by the road. 
Looking southeast across the valley in the direction which the Rift pursues, a moun- 
tain is seen which seems to have been sharply cut off. Descending a narrow valley to 
the southeast of Dry Lake Valley, we reach the foot of a steep escarpment (plate 19a) 
where there are apparent two, and possibly three, lines of displacement. The middle 
one passes at the foot of the main cliff, which is between 400 and 500 feet high. It 
can not be said with certainty that the whole cliff is the result of faulting, altho it is 
certainly so in part. The formation in the cliff is sandstone of either Tertiary or Cre- 
taceous age. About 5 miles northwest of Bitterwater there is an interesting valley which 
has been so disturbed that it has no external drainage, while thru its center passes a 
ridge formed along the Rift. The ridge forms a fine roadbed. Descending toward 
Bitterwater Valley and P.O., another ridge appears which is as even and regular as a 
railroad grade. - Bitterwater Valley is occupied during the wet season by a marshy lake. 
