76 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
following the line of separation between the side slope and bottom slope. In some 
instances there was a crack on each side of the sag, but more frequently on one side only. 
Where the sag contained a pond the crack was usually present. With little or no excep- 
tion these cracks exhibit downthrow on the side toward the sag. (See plate 52nB.) At 
least two explanations of these cracks are possible. As the bottom of the sag usually 
shows no outcrop of rock and appears to consist wholly of soil washt down from the 
sides, it is possible that the earthquake caused a settling of the alluvium toward the 
middle of the sag and that the marginal crack is due to this settling. On the other hand, 
it is possible that a bedrock wedge underlying the sag was permitted to settle during 
the earthquake and that such settling caused the marginal crack. In the first case the 
cracks would belong to the superficial class; in the second, to the bedrock class. While 
the data at hand are not decisive, | am of opinion (as already stated) that the cracks 
resulted from some sort of readjustment of the small earth blocks whose upper surfaces 
determine the Rift topography. 
Springs. — The general testimony of residents is that the flow of springs was modified 
all thru the peninsula west of the Rift. As it was practically impossible to get quantita- 
tive data, I made few records of specific instances, but every farm owner or farm tenant 
of that region with whom I talked told me of some spring whose flow had been increased, 
diminished, or stopt at the time of the earthquake, the change being either temporary 
or permanent. Several lakes of the group near the coast (known as Seven Lakes) expe- 
rienced changes, the greater number having their levels lowered. A pond known as Mud 
Lake, on the divide at the head of Pine Gulch Creek and about a mile from the fault- 
trace, suddenly and permanently lost its water at the time of the earthquake. At the 
same time a small spring on the east side of the ridge and about 0.75 mile in a direct line 
from the pond, was suddenly enlarged, a torrent of water gushing from it for several 
hours and then gradually diminishing. It is suggested with much plausibility by resi- 
dents that these two phenomena were connected, the earthquake opening a subterranean 
course thru which the water of the pond was conveyed to the hillside spring. I heard 
of no changes in springs east of the fault-trace, altho a number of inquiries were made. 
Interpretation of bedrock cracks and springs. — The changes in springs are of course the 
results of changes in the conditions of underground circulation, and in a general way 
may be ascribed to the influence of newly-formed cracks. The spring phenomena and 
the visible cracks may be grouped together as indications of bedrock fracturing, and 
their distribution indicates the regions in which the rocky foundation of the land was 
more or less shattered. That region includes the Rift and extends from it to the ocean. 
The phenomena diminish somewhat with distance from the Rift, but the fracturing 
appears to have been important and general thru a belt 4 or 5 miles broad. 
Landslides. — The earthquake started a number of landslides. A few of these were 
on the line of the fault, especially where its trace intersected a cliff facing Bolinas Lagoon. 
Others were from cliffs of earth or weak rock bordering the ocean, one of the bays, or a 
creek. None were seen of unusual type or of great importance, except from the obstruc- 
tions to roads which they occasioned. South of Willow Camp a road overlooking the 
sea had been cut in the face of previous landslides, and the renewed movement put it 
out of commission. In the same manner roadways were obstructed at the entrance 
to Bolinas Lagoon, at two points near the head of the lagoon on the west side, and on 
the coast of Tomales Bay at Inverness. 
There were many landslides of the dry type on hillsides, masses of earth and rock 
breaking away on steep slopes and tumbling to the bottom. The largest seen were on 
the high ridge west of Tomales Bay, in the vicinity of Sunshine Ranch. Closely related 
to these were small falls of earth and rock from the low cliffs created in the construction 
of side-hill roads. (Plate 53s.) They occurred at a few places within the Rift and 
