THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE. 30 
of both Bolinas Lagoon and Tomales Bay such features grade into dislocation terraces of 
greater magnitude, which originated at earlier dates but may have been recently accentu- 
ated. There are also narrow terraces of displacement on the comparatively steep face 
of the ridge southwest of the Rift, and at two points there are minor crests and asso- 
ciated canyons parallel to the main crest and to the Rift. So little is known of the local 
details of geologic structure that a different explanation of these creases, terraces, and 
spurs is not altogether barred; but their physiographic relation to the Rift features is 
so intimate as to leave little question in my mind of their genetic similarity. Assuming 
that they are correctly explained as the product of minor faulting of only moderate 
antiquity, they serve to connect the great trough containing the bays with the narrow 
belt of peculiar and striking topography, and indicate these as parts of a single great 
phenomenon — a belt which has been the locus of complicated fissuring and dislocation 
during the later geologic epochs. 
MUSSEL ROCK TO PAJARO RIVER. 
From Bolinas to the vicinity of Mussel Rock, about 8 miles south of the Golden Gate, 
the course of the Rift is beneath the waters of the Pacific, across the bar in front of the 
entrance to the harbor. Near Mussel Rock it intersects the shore at a great landslide 
(plate 12a) in rocks of the Merced series. At Mussel Rock, the basal beds of the Merced 
series rest directly upon an old land surface of worn-down Mesozoic rocks, and the basal 
bed contains abundant cones of Pinus insignis resting upon cemented alluvium. The 
cone-bearing bed immediately underlies marine strata and numerous fossils occur near 
the base of the series at the top of the ridge. The Merced strata here have a dip of about 
15° to the northeast. ‘The contact between the Merced and the older rocks trends south- 
east across the peninsula; and for some miles the Rift is approximately coincident with 
the trace of the contact and, for some portions of this distance, exactly so. From the 
shore line the course of the Rift is the same as that of the steep cliffs which rise at the 
back of the Mussel Rock slide to an altitude of over 700 feet. From the top of these 
cliffs, at an elevation of about 500 feet above sea-level, the course of the Rift as far as 
San Andreas Lake is marked by a line of shallow longitudinal depressions, ponds, and 
low scarps. (See plate 128, 13, and 14.) There are eight ponds in this stretch of about 
4.5 miles. This portion of the modern Rift was recognized as such in 1893." 
At a point about 4 miles from the Mussel Rock slide, the longitudinal depression which 
marks the course of the Rift becomes much more pronounced and passes into a remark- 
ably straight and deeply trenched valley, the greater part of which has been converted 
by, large dams into the San Andreas and Crystal Springs Lakes, used as reservoirs by 
the Spring Valley Water Company as water supply for the city of San Francisco. This 
straight valley (see plate No. 15) has an extent of 15 miles with a steady course of 
S. 34° E. to a flat divide southwest of Redwood City, whereby one passes over into the 
end of a similar but less pronounced valley, in which are situated Woodside and Portola. 
The San Andreas and Crystal Springs Lakes valley is almost wholly in the Franciscan 
terrane and the axis of the valley is discordant with the structural lines and contact 
planes of its constituent formations and intrusive masses. At the upper end of San 
Andreas Lake, however, the southwest edge of the Merced terrane forms in part the 
boundary of the valley on the northeast side for a short distance. The valley as a geo- 
morphic feature (plate 16a) dates back fairly well into the Pleistocene. It is drained 


1“ The line of demarkation between the Pliocene and the Mesozoic rocks, which extends from Mussel 
Rock southeastward, is in part also the trace of a post-Pliocene fault. The great slide on the north side 
of Mussel Rock is near the land terminus of this fault-zone, where it intersects the shore line. Movement 
on this fault-zone is still in progress. A series of depressions or sinks, occupied by ponds, marks its 
course. Modern fault-scarps in the Pliocene terrane are features of the country traversed by it.” The 
Post-Pliocene Diastrophism of the Coast of Southern California, by Andrew C. Lawson, Bull. Dept. 
Geol., Univ. Cal., vol. 1, No. 4, pp. 150-151. 
