THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. . 67 
In the U. 8. Geological Survey map of this region the islands are not represented. 
In fig. 28 they are represented as they appear at half-tide, or, more strictly, the parts 
shown are those covered by vegetation. This figure also shows the corresponding part of 
the delta of Pine Gulch Creek. After crossing Pepper Island and a smaller island imme- 
diately adjacent, the fault-trace disappears under the water of the lagoon and it was next 
seen on the mainland of the southwest shore near the head of the lagoon. In the interval 
it probably crosses the delta of Pine Gulch Creek between the lines of high and low tide, 
but this tract was not examined until after the floods of March, 1907, which overspread 
it with alluvium. A disconnected group of cracks opening in the alluvial plain of the 
creek about 400 yards to the west (plate 394) probably marked the position of a divergent 
branch of the fault. This line of disturbance crost the creek and road near the bridge 
in the northern settlement of Bolinas, trending approximately north and south and fading 
out in both directions. 
The trend of the fault-trace on Pepper Island is about N. 34° W., and if continued would 
bring the trace to the shore at the head of the lagoon, but its actual position on the 
mainland is farther west, indicating that there is either a swerving or an offset in the part 
not seen. Near the shore the fault occasioned a number of landslides which obstructed 
the road until removed; and beyond the confusion occasioned by the landslides the trace 
consists of a number of subparallel cracks occupying a belt several yards in width. 
There is also a nearly parallel branch of the trace in a fault-sag lying a little farther west, 
but this could be followed only a short distance, and has since been largely obliterated by. 
plowing. Mr. Nunes, who cultivated this sag, states that it once contained a pond or 
marsh, and this he had drained, but the water stood there again after the earthquake, 
showing that the earthquake had caused a depression of the bottom of the sag. 
The diffused cracks on the main line soon gather into a narrow belt and descend into a 
narrow sag, containing the barn and other farm buildings of the Steele place. After 
following the sag for a short distance, the trace gradually rises on its eastern wall, crosses 
obliquely an intervening ridge, and enters a parallel sag toward the east. In this sag, 
which also is narrow, the trace intersects one of the roads leading from Bolinas to Wood- 
ville and immediately begins to ascend the narrow ridge bounding the sag on the east. 
Crossing this ridge obliquely, it skirts for 0.25 mile the western border of the much broader 
sag in which the water of Pine Gulch Creek gathers before it enters the canyon from 
which it is named. This wall it descends obliquely, and, just before reaching the bottom 
of the sag, intersects and offsets a line of eucalyptus trees marking a property and town- 
ship boundary. The ridge phase dominates in this region (plate 378), and near the line of 
eucalyptus trees the trace itself has a small offset to the west. (See fig. 18.) 
Now for nearly half a mile the trace follows a valley-bottom, being divided on the way 
between two or three branches. The ridge phase obtains, but there are several places in 
flat alluvial ground where the ordinary group of cracks is replaced by a single crack with 
clean shear. On Mr. Strain’s place two fences were crost which afforded measurement of 
horizontal displacement. Beyond them the fault-trace becomes once more single, and, 
after passing a group of very small ridges and sags, begins to climb the eastern wall of a 
larger sag, which here contains Pine Gulch Creek. (See plate 418.) Along its line there 
soon develop a small sag and ridge constituting a sort of shelf or notch on the wall of the 
deeper sag (plate 424), and in this small sag are several ponds. (Plates 10a, 544, and 
55a.) The sag first rises for a distance and then gradually descends. The fault-trace 
exhibits here in alternation the ridge and trench phases, and at many points there is an 
apparent vertical displacement with throw of 1 or 2 feet toward the northeast. (Plates 
10B and 48n.) Near Bondietti’s house the individuality of the sag is lost, and the 
fault-trace swerves somewhat to the east. A parallel trace develops west of it, and the 
two come together near Beisler’s place. Northwest of Beisler’s is a relatively high fault 
