THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 75 
Bedrock cracks occurred at many points within the Rift, usually appearing as branches 
from the faults. ‘They were seen also at a number of points west of the Rift, their dis- 
tribution reaching to the ocean in the vicinity of Point Reyes, ten miles from the fault- 
trace. At the more remote points they were quite small, often barely discernible, and 
no system of arrangement was discovered. They are peculiarly prominent along the sum- 
mit of the ridge constituting the southwestern rim of the main Bolinas-Tomales trough. 
This summit was visited on four lines of road, and at each locality conspicuous cracks 
were found. On the road from Inverness to Point Reyes Post Office, about a mile in a 
direct line from Tomales Bay, a crack was traced for more than 800 feet. Its general 
trend is east and west, but its course is not straight and it has a branch diverging at 45°. 
Along this crack there is a horizontal throw of from 2 to 6 inches, the south side having 
moved westward with reference to the north side. 
On the next road to the southward a group of cracks was seen at a point a mile from 
the shore of Tomales Bay. These cracks occur on a crest trending northwest and south- 
east, and their trend makes a small angle with that of the crest. The arrangement of 
the cracks suggests horizontal shear, but no definite observation was made on this point. 
They extend for several hundred feet at least, but were not traced out. 
On Mount Whittenberg there are two bedrock cracks. One of these crosses the north- 
eastern spur of the peak near its junction with the main crest. Its trend is approxi- 
mately northwest and southeast and at one point it margins a fault-sag. As it assumes 
in one place the ridge phase of the fault-crop, I infer that it has horizontal displacement. 
On the opposite side of the main crest is a crack which was traced for about 1,000 feet. 
Its general course is northwest-southeast, but it is not straight and exhibits a vertical 
throw of 1 or 2 feet to the southwest. At one point it touches a fault-sag. Between 
these two long cracks a group of short cracks occurred, with similar trend, on a knob 
constituting a portion of the main divide. 
About 6 miles farther south, at the head of Pine Gulch Creek, another road crosses the 
range, and in following this a group of cracks was seen. A short distance west of the 
divide, and about a mile in a direct line from the fault-trace, is a fault-sag trending 
northwest-southeast. On each side of it a crack was seen, the eastern crack being the 
wider and showing a small throw to the southwest. This crack was traced for about 
0.75 mile and found to curve thru an arc of nearly 90° from southeast to southwest. 
At its southwest end, or at least the southwestern limit of tracing, it is on a ridge, and it 
there expands into, or else is replaced by, a group of cracks diverging fan-wise. On each 
member of the group faulting took place, the downthrow being toward the northwest 
except in the case of two apparently short cracks with downthrow to the southeast. 
On four of these cracks the throw was greater than 1 foot, and at one place it was 
about 5 feet. Each crack was associated with a preéxistent bluff or scarp, indicating 
that earlier movements have occurred at the same place. The field in which the prin- 
cipal phenomena occur is cultivated with the exception of the steeper scarps, whose 
faces retain a bushy growth. (See plates 52a and 53a.) 
A tract lying between this locality and the coast, and extending several miles in each 
direction, exhibits a peculiar topography intermediate in type between that of the Rift 
and that commonly associated with landslides. Near the coast are a number of basins 
with ponds or lakes of much larger size than those along the Rift, and in association 
with these are seen a number of sags similar to the fault sags of the Rift. On several 
lines which were thought from the physiography to represent partings between dis- 
located blocks, earthquake cracks were seen, and on one of these near the coast there 
was a vertical displacement of 3 feet, the downthrow being to the southwest. 
All thru the Rift there is association of earthquake cracks with fault-sags; probably 
half of the sags were bordered by such cracks on one side or the other, the crack usually 
