396 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
face blocks that remained in the cavity being accountable for as fragments from the 
broken edges subsequently giving way and being carried only a short distance as the 
upper end of the flow came to rest. In this way, probably, the walls were trimmed, 
for the cut in general was left remarkably clean. 
Another flow of similar character took place 3 miles north-northwest of the town 
of Half Moon Bay, on the creek next west of Frenchman Creek. It is shown in plates 
1328 and 133A. On the morning of the earthquake an acre of the gently sloping allu- 
vial floor of a broad, short valley tributary to the main creek on the east caved and 
flowed out, leaving an excavation 10 feet deep, where before it had been almost level 
and where there had been no stream channel. In this case, the water already gathered 
in this basin-like valley, which here had had no means of prompt escape, was an important 
aid in the formation of the flow, aside from the sudden accession of water that probably 
caused the earthquake. The presence of a large amount of water and the forcible 
movement during the earthquake shock resulted in the loosening and undermining of 
the ground and its transportation as a fluent mass. The angle of slope was about 5°. 
The flow carried out thousands of tons of earth in this manner and spread it over 
about 2 acres of meadow land, to an average depth of 1.15 to 3 feet. 
Plate 1328 gives a view of this earth-flow, showing the pit from which it was derived. 
Covering much of the surface of the flow and the floor of the hole are to be seen blocks 
of sod which have been carried right side up as if the material had moved en masse. 
The amount of water in evidence shows clearly how the earth was softened and enabled 
to move. The picture was taken two weeks after the earthquake. At that time water 
was still seeping up from underground, and out of the lower portions of the broken 
walls, while the ground near the surface of the valley was quite dry. The water had 
formed two definite rivulets thru the débris, at an elevation above the surrounding 
meadow, and was running in continuous streams, fast cutting a channel for itself and 
removing the soft material. Considerable water was dammed back in the hole by a 
4-foot ridge of débris piled across the mouth of the hole, as in the case of the previously 
described earth-flow. This mound of earth, along the line where the stream left the 
caved-in area and flowed over the preexisting slope, was probably piled up at the last 
by the remnants of the flow gliding down and heaping themselves up as a barrier at 
the mouth of the hole. 
“ The cavity, about an acre in extent, has 10-foot walls which gradually decrease in 
height lower down the valley, the bottom of the hole being more nearly level than the 
valley-floor. Plate 1334 shows part of this flow in detail. 
Some of the great blocks of sod around the edges have not been removed, altho the 
material from underneath has gone. Concentric cracks not visible in the pictures 
extend around the edge of the hole and for 50 feet above its upper end, showing that 
the area affected is broader than appears at first sight, and that the work is not yet all 
accomplished. ‘The material of the valley-bottom is a coarse, arkose earth, derived 
from decomposing granite, and containing many rock fragments. 
A flood of earth covers about 2 acres of the meadow. Water was present in this 
earth-flow in greater amount than in any other that was examined. The nature of the 
material may be judged of by the abrupt face of the stream where it stopt. The edge 
makes a steep angle with the meadow and rises to an average height of 2 feet above 
it. Yet the fact that this mass of earth was able to move more than 300 feet after it 
left the lower end of the hole, and spread into an even and thin layer over a wide extent 
of nearly level meadow, shows that it was fairly soft. It was moved on a basal layer 
of semi-fluid mud and sand, with the aid of the weight of the overlying and partly dis- 
integrated earth. 
