66 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
TOMALES BAY TO BOLINAS LAGOON. 
By G. K. GILBERT. 
The Fault-trace. — The trace traverses the zone of the Rift. Its general course is N. 
35° W. and it nowhere departs more than a few hundred feet from the straight line 
connecting its extreme points. For considerable distances it is a single line of rupture; 
elsewhere it is divided into parts which separate and reunite; and in yet other portions 
it is composed of unconnected parts arranged en échelon. ‘There are no vertical 
sections exhibiting hade, but the relation of the trace to sloping surfaces indicates that 
the fault-plane is approximately vertical. 
For considerably more than half its length the surface expression is a ridge from 
3 to 10 feet wide and ranging from a few inches to about 1.5 feet high. (See plates 37B 
and 40a.) The ground constituting the ridge is in fragments, loosely aggregated, so 
that there are considerable voids. Where pasture lands are crost the turf is torn into 
blocks, and these, in conjunction with the cracks which separate them, make up a pat- 
tern. This pattern is always irregular and sometimes gives no evidence of system, but 
usually its lines have a dominant direction, traversing the ridge obliquely, the northern 
ends of the cracks pointing toward the eastern boundary of the ridge, and the southern 
ends toward the western boundary. The cracks have resulted from stresses connected 
with the horizontal faulting, in which the southwest block moved northwest with 
reference to the northeast block. (See plate 39.) 
In other places, and usually for short distances, the surface expression is a shallow 
trench (plates 408 and 468), with ragged vertical sides from 2 to 5 or 6 feet apart, and 
occupied by loosely aggregated fragments of the ground, the pattern of the fragments 
and interstices being similar to that observed in the case of the ridges. This phase sug- 
gests that just below the surface the fault may be somewhat open, so that there has been 
an opportunity for fragments to drop into it. 
In a third phase the ground is not notably elevated nor deprest but is traversed by a 
system of cracks obscurely parallel one to another and making an angle of about 45° 
with the general direction of the trace. Their orientation is such that they run nearly 
north and south. The cracks do not meet, but leave the intervening strips of ground in 
full connection with the undisturbed ground outside the trace. This phase occurs chiefly 
in wet alluvium. 
There are a few spots where for short distances the surface expression is a simple 
straight fracture along which horizontal motion took place. 
In the detailed descriptions which follow, the first three phases described above will be 
spoken of as the ridge phase, the trench phase, and the echelon phase. 
The most southerly observation of the fault-trace was on the spit separating Bolinas 
Lagoon from the ocean. Near the west end of the spit its surface is covered by small dunes, 
and among these the trace was seen in its echelon phase. After a lapse of nine months 
the drifting of the sand had obliterated most of the cracks, but a few were still visible. 
Inside the spit lie a number of islands, the largest of which, Pepper Island, has a nucleus 
of sand (the vestige of an ancient spit), but superficially consists mainly of a fine tidal 
deposit. In the earlier field excursions the fault-trace was here overlookt, the echelon 
cracks by which it is represented being mistaken for secondary cracks, but at the present 
time (spring of 1907) it is easily traced, even from a distance, because the vegetation on the 
two sides of it has acquired different colors. Unfortunately the camera does not dis- 
criminate these colors. (Plate 414.) The echelon phase here dominates, but the ground 
east of the trace is deprest about a foot, and this depression has so changed the relation 
of certain plants to the tides that they now find the conditions of life unfavorable and are 
dying out. This matter will be considered more fully in another connection. 
