30 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
The dip of the Merced beds at Bolinas toward the Franciscan rocks of the mainland 
is quite analogous to the dip of the same beds toward the Franciscan of San Bruno 
Mountain on the San Francisco Peninsula,’ and has the same significance, viz., that the 
Merced beds have been relatively downthrown on the west against the older rocks. 
The fault in the Tomales-Bolinas defile has usually been regarded as identical with and 
a continuation of the San Bruno fault of San Francisco Peninsula, and there seems to 
be no good reason for changing this judgment, altho, as will appear shortly, the modern 
Rift to the south of the Golden Gate does not coincide with the trace of the San Bruno 
fault, but leaves it at a small angle and pursues a course nearly parallel, but to the 
southwest of it. It is noteworthy, also, that while on the Point Reyes Peninsula, par- 
ticularly in the vicinity of Bolinas, there is a magnificent wave-cut terrace at an altitude 
of about 300 feet, with a width of 1 to 1.5 miles between the base of its sea-cliff and the 
brink of the present sea-cliff, no such feature is to be found on the landward side of 
the fault-line on the coastal scarp between Bolinas Lagoon and the Golden Gate. 
Characteristics of the Rift (G. K. Gilbert, pp. 30-35).— In a broad sense the structural 
trough in which lie the two bays is a feature of the great Rift. In a narrower sense 
the Rift follows the lowest line of the trough, controlling the topography of a belt 
averaging 0.75 mile in width. The physiographic habit of the trough is that of a 
depression occasioned by faulting. It is remarkably straight. One wall, the south- 
western, is comparatively steep; the other is comparatively gentle. The gentler slope 
is an inclined plateau with incised drainage. Viewing the trough from any command- 
ing eminence, the physiographer readily frames a working hypothesis of faulting and 
tilting. He sees in the southwestern wall a fault-scarp of moderate freshness, and in 
the northwestern wall a slope originally less steep, in which erosion has been stimulated 
by uplift and tilting. The general facts of the geology of the district, as worked out 
by Anderson,’ agree with this theory. The axial line of the valley is recognized by 
him as the locus of a fault, or fault-zone, and the rocks of the southwest wall are every- 
where older than those which adjoin them at the base of the opposite slope. The 
gentler slope is well shown by plate 7a. Plates 8B and 418 also show something of 
the gentler slope, and plate 7B of the bolder. 
In a general way the two slopes are drained by streams which descend to the axis of 
the valley, and are there gathered in two longitudinal trunk streams which flow severally 
to Tomales Bay and Bolinas Lagoon; but in a central belt following the lowest part of 
the trough the details of drainage are comparatively complex, and their complexity 
is associated with peculiarities of the relief which serve to distinguish the central belt 
from the bordering slopes. In the bordering slopes the subordinate ridges conform in 
normal manner to the drainage, having evidently been developed by the erosion of the 
canyons which separate them. In the axial belt the ridges are evidently independent of 
the drainage, often running athwart the courses which would normally be followed by 
the drainage. In part the ridges divert or control the drainage; in part the drainage 
traverses and interrupts the ridges. 
The influence of the ridges on the drainage is illustrated by the accompanying dia- 
grams. Fig. 2 shows the actual drainage system; fig. 3 the system which would be 
developed if there were no special conditions along the axial zone. The small ridges of 
the axial zone trend parallel to the axis, and their interference gives parallel courses 
to various streams which would otherwise unite. The influence of the drainage on the 
ridges is illustrated by fig. 4, which shows a small ridge resting on the side slope of a 
larger ridge. The drainage of the larger ridge breaks thru the smaller, making gaps. 
Plate 7B shows the slope of a greater ridge at the right; and at the left two bushy hills 


* Cf. A Sketch of the Geology of the San Francisco Peninsula. U.S. G.S., 15th annual report. 
* Geology of Point Reyes Peninsula, by F. M. Anderson. Bull. Dept. Geol., Univ. Cal., vol. 2, No. 5. 
