398 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
by the Nunez flow. One of these 2 earth-flows, that at the right of the picture, started 
near the top of the ridge in a depression in the slope, formed a hole 75 feet long and 
40 feet wide, and coursed down a narrow runnel having a gradient of 25° to the bottom 
of the hill, a distance of 600 feet. Enough earth issued to fill up the rather deep ditch 
in the gully clear to the bottom of the hill and to bury the grain field on both sides to 
a depth of 1 to 2 feet. In this case, as in the preceding one, there were formed lateral 
ridges higher than the center, so as to leave a groove between. Down this channel 
there flowed softer material, which lined the sides of the lateral ridges with a smooth 
coat of mud and left conspicuous flowage marks. The flow thus raised a ditch for itself 
above the level of the slope. The earth-flow probably assumed this form by leaving 
behind, at the sides, the material least capable of flowing, and by concentrating its 
most liquid parts along the deep central line. 
The other earth-flow was near by, on the convex face of the knoll in the center of 
the picture. <A similar cavity was produced, from which the contents were spread out 
broadly. It is a good example of the starting of a gully, as there was no depression 
before. One branch of this earth-flow came straight down the hill and slightly toward 
the canyon on the left; the other branch came down toward the gully in which the 
first-mentioned of these 2 earth-flows occurred. Thus drainage lines were started 
which ultimately may separate the central hill from the ridge on the right, of which 
it is now a continuation. The left arm of the flow on the hill may develop a channel, 
as explained below, which will cause the drainage from this hill, which is now toward 
the foreground, to pass into the canyon on the left. 
Similar landslides, tho usually of smaller size, occurred thruout the region neighbor- 
ing the fault visited by the writer, and even in districts at a considerable distance from 
the fault. Frequently they were not definitely referable to the earth-flow type, but 
resembled more closely earth-slumps formed without the aid of a suddenly increased 
water supply. It was often difficult, especially in cases where the movement was 
slight, or the slide was in the embryonic stage, to determine whether the earthquake 
at those points had caused a flow of water or not. In the instances so far described, it 
was pretty certain that it had; but in many others the phenomena were explainable 
as being the result of moisture that was already collected before the earthquake. Many 
slips were formed on hillsides and along the embankments of mountain roads, and along 
the cracks formed by the shock in moist and loosened soil. Often these slips were 
arranged one above another, the perpendicular faces due to slipping having the appear- 
ance of step faults. In such cases the weight of the moved mass and the amount of water 
was not sufficient to cause the material to flow. There were examples of such slips 
along the coast hills north of San Pedro Point, near the road halfway between San 
Bruno and San Andreas Lake, near the road from Belmont to Crystal Springs Lake, 
0.5 mile southeast of the San Mateo Alms House, and in many other places on the San 
Francisco Peninsula. In some places bare ridges had their lines of symmetry broken 
into little knolls and irregularities by these slips, a common occurrence in the hills of soft 
sand formations in the northern part of the San Francisco Peninsula. All the slips 
just referred to illustrated the gradation between earth-slumps and earth-flows. !Doubt- 
less in many of them a small amount of water did gather as a result of the earthquake. 
Relation of earth-flows to rainfall (Robert Anderson). — The rainfall previous to the 
earthquake, tho possibly of little importance in connection with the more extreme types 
of earth-flows, in which practically all the work was done by a head of water brought 
from underground by the shock, bears a close relation to the less extreme types, and 
to the geologically very important doubtful types intermediate between the earth- 
flows and earth-slumps. In a dry year the number and size of all of these would prob- 
ably have been much less. Had covering of slopes been unsaturated, areas might not 
