THE SAN ANDREAS RIFT AS A GEOMORPHIC FEATURE. 29 
To the south of Tomales Bay the Rift lies in a remarkable defile with abnormal and 
ill-adjusted longitudinal drainage, which extends thru to Bolinas Bay, a distance of 
about 14 miles. On the east side of the defile is the steep coastal slope of the mainland, 
rising to a ridge crest from 1,000 to 1,700 feet in height. The transverse gullies in this 
slope are shallow, and detract but little from the general effect of a fairly regular but 
uneven steep slope. On the west is an even steeper but more incised and rugged slope, 
which forms the eastern edge of the peninsular land mass. This slope culminates in 
crests having an altitude of about 1,500 feet. The most striking geomorphic feature 
of the bottom of the defile is the presence of low ridges with intervening ravines or gul- 
lies elongated parallel to the general axis of the depression. More or less hummocky 
surfaces, with hillocks and hollows having no regular orientation, also occur. In the 
hollows ponds are fairly common features. The chief drainage is to Tomales Bay by 
Olema Creek, which heads within 2.5 miles of Bolinas Lagoon; and the divide between 
this stream and the parallel one which flows to the southeast has an altitude of about 
400 feet above sea-level. The southeast end of the depression is submerged beneath 
sea-level, and is cut off from Bolinas Bay by a sandspit. The very shoal water inside 
of the sandspit is known as Bolinas Lagoon. (See plate 6n.) 
The rocks on the east side of the defile belong wholly to the Franciscan series. On 
the west side, at the north end, we have chiefly the granitic and dioritic rocks of the 
peninsula with limited masses of crystalline limestone into which these rocks are intrusive. 
Farther south the granitic rocks are overlain by the shales of the Monterey series, and 
these rocks form the west side of the defile for several miles. The shales have inconstant 
and often very high dips. Still farther south the sandstones of the Merced series lie 
unconformably upon the Monterey shales, and near the town of Bolinas dip uniformly 
at moderately low angles toward the axis of the defile. It is thus apparent that the axis 
of the defile crosses more or less obliquely or transversely the contact between the Mon- 
terey and the granitic rocks, and also the contact between the Merced and the Monterey. 
It is also a remarkable fact that altho on the east side of the defile the Franciscan rocks 
constitute the mountain mass to a thickness of several thousand feet, this entire series, 
together with the Knoxville, Chico, Martinez, and Tejon, is almost entirely absent be- 
tween the Monterey and the granitic rocks on the peninsula in the immediate vicinity. 
This indicates clearly that in pre-Monterey time the peninsular mass had been uplifted 
on a fault along the present coastal scarp, so that the granite was brought against the 
Franciscan and denuded of its unconformable mantle of sedimentary strata before it 
was submerged to receive the deposits of Monterey time. It is also clear that inasmuch 
as there is a great volume of Monterey shales on the peninsular or seaward side of this 
fault line, and no trace of the same formation on the mainland to the east of the fault line, 
one of two things must have happened. Either the submergence which permitted the 
deposition of the Monterey shales was confined to the peninsula and was effected by a 
downthrow of that block on the same fault as that upon which it had earlier been upthrust, 
so that there was no sea over the territory east of the fault; or, if the regions on both 
sides of the fault were submerged together, then in post-Monterey time the east side of 
the fault was lifted into the zone of erosion and denuded of its covering of Monterey 
shales so thoroly that no trace of them now remains. There is no escape from one or 
the other of these conclusions, and each of them involves a movement on the fault with 
relative downthrow on the southwest side, or the reverse of that which occurred in earlier, 
pre-Monterey time. From this interpretation it follows that the defile extending from 
Tomales Bay to Bolinas Bay lies along the trace of a fault which dates from pre-Miocene 
time, and that upon this fault there have been large movements in opposite directions 
so far as the vertical component of such movements is concerned. ‘The trace of this 
ancient fault is also the line of the modern Rift. 
