28 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
crusht and broken along it. Back of Fort Ross, the surface rocks traversed by the Rift 
belong to the Walalla formation, and from this point for a number of miles to the north- 
west no other formation appears. 
Sw. 



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Fia. 1.— Geological section transverse to the Rift where it is followed by the Walalla (Gualala) River. 
At the point where the road from Stewarts to Geyserville crosses the Gualala River, fault- 
ing and erosion have exposed the underlying Franciscan formation. This appears upon 
the northeast side, showing that the opposite side, that toward the ocean, has dropt. 
The Franciscan occupies but a narrow strip and is replaced for some distance up the ridge 
upon the northeast, by Walalla sandstones. These relations are shown in the cross-section 
sketch shown in fig. 1. Near the mouth of the Walalla River the formation upon the coast 
side of the Rift still appears to be the Walalla sandstones; the rocks upon the opposite 
side are buried under the alluvium of the valley. After leaving the valley of the Garcia 
River, the Rift lies wholly within the Franciscan formation until it disappears in the ocean. 
The Monterey shales with sandstones at their base form nearly the whole of the coastal 
terraced plain in the vicinity of Point Arena> They rest unconformably upon the Francis- 
can rocks and dip at a steep angle to the southwest. The Monterey formation nowhere 
appears to come in contact with the fault. 
BODEGA HEAD TO BOLINAS BAY. 
General Note. — From the point 2 miles south of Fort Ross where the Rift in its south- 
easterly course leaves the shore, it passes beneath the Pacific for a distance of 12-.or 13 
miles. Its observed course to the northwest of Fort Ross, if projected southeasterly 
with a slight curvature, would strike the shore again at Bodega Head; and here it is 
found on the low ground of the isthmus that connects the head with the mainland. 
The Rift here coincides in position with a fault described by Osmont,’ whereby the Fran- 
ciscan rocks to the east are dropt down against the pre-Franciscan dioritic rocks of the 
headland. Immediately to the east of the fault-trace is a marsh. Across the mouth 
of the bay formed by the headland is a sandspit and the fault-trace should cross the spit 
near its abutment upon the shore line, but the drifting sands preclude its finding an 
expression here in geomorphic forms. 
To the south of Bodega Head the Rift follows Tomales Bay (plate 64) to its head near 
Point Reyes Station. This is a remarkably linear inlet of the ocean lying between Point 
Reyes Peninsula and the mainland, having a length of about 15 miles and not exceeding 
a mile in width. It has generally been regarded as a feature determined by a fault,’ 
the same as that noted by Osmont at Bodega Head, whereby the Franciscan rocks of 
the mainland were brought against the pre-Franciscan granitic and dioritic rocks of the 
peninsula. The bay is quite shallow, but both of the slopes above the shore line are 
rather precipitous, and the ridge crests on either side attain elevations of over 1,000 feet. 
On the mainland side of the bay there are some rather vaguely defined terraces, both in 
the form of wave-cut benches and delta embankments. On the same side of the bay 
there are marine deposits of late Pleistocene age, containing abundant molluscan remains 
which have been elevated to about 25 feet above sea-level, and which are the equivalent 
of similar deposits at a similar elevation on the east side of San Pablo Bay. 

-! Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3. 
? Cf. Anderson, Geology of Point Reyes Peninsula, Bull. Dept. Geol., Univ. Cal., vol. 2, No. 5. 
