MINOR GEOLOGICAL EFFECTS OF THE EARTHQUAKE. | 393 
fault at Mussel Rock. The steep slope of the scarp is underlain by hard sandstone of the 
Franciscan series, with but a thin vencer of soil, or none at all. At the base of the scarp 
is the gentle slope of Merced Valley, underlain here by Pleistocene and recent sands. 
The sands, partly eolian, lap up on the lower flanks of the scarp, and mantle the trace 
of the auxiliary fault which follows its base. The sands thus vary in thickness from 
a feather edge to an unknown thickness, which it is believed may be as much as a few 
hundred feet at no great distance from the base of the scarp. ‘Traversing the gentle 
slope of the valley-floor are several shallow arroyos, which head in incipient ravines in 
the face of the scarp. At the moment of the earthquake there was a sudden outgush 
of sand and water at a point at the upper end of the cemetery, close to the base of the 
scarp and quite near, if not immediately upon, the line of the buried fault-trace. This 
stream of sand and water, admixed with the loam of the slope, flowed rapidly down the 
course of a shallow arroyo on a grade of about 1:25 with a depth of from 13 feet in its 
upper part to about 3 feet in its lower. The front of the stream stopt abruptly at a point 
just beyond the roadway about half a mile from the origin. The flow was so rapid that 
it carried away many small trees; a wind-mill was wrecked and the heavy concrete 
blocks which served for its foundation were swept down, with other débris. One of 
the pumping stations of the cemetery was demolished by it, and 2 horses were carried 
off their feet, and were extricated afterwards with difficulty. (See plates 1304, B and 
131.) 
According to Mr. M. Jensen, the superintendent of the cemetery, the entire flow had 
been accomplished within 3 minutes from the time of the shock, and he was at its source 
within 20 minutes after it occurred. The height of the flow within a few hundred feet 
of its source was attested by the mud upon the trunks of some eucalyptus trees near its 
margin. This mud extended up to 13 feet above the bottom of the arroyo. ‘This, 
however, doubtless indicates the height of the front of the stream as it past this point. 
As the flow advanced, its surface near its source rapidly dropt; and by the time the 
front had reached the roadway the stream was probably no deeper at its source than 
at its terminus. Indeed, it seems to have been somewhat less, as there was a marked 
tendency for the sand to pile up at the front by reason of the negative acceleration at 
the front due to loss of water. After the moving mass had come to rest and partially 
dried out, it was found that it had left a streak of muddy sand on the bottom of the 
arroyo averaging 100 feet wide and about 3 feet thick. Taking the length of the flow 
as 900 yards, this gives the total volume of the compacted wet sand as 89,100 cubic 
yards. The cavity in the slope caused by the evacuation of this sand and loam was not 
measured, but was estimated to have a width of 150 yards, a length of 300 yards in the 
direction of the flow, and an average depth of 2 yards. On this estimate, its volume 
would be about 90,000 cubic yards, which agrees quite closely with the estimated volume 
of the material ejected. 
The sand, after it had ceased flowing and had been drained and com pacted, undoubtedly 
held in the voids between the grains not less than 25 per cent of its volume of water. 
An additional 15 per cent would probably give it the necessary fluidity for flow down 
a slope of 1:25. But as the flow was swift, there was an excess of water, so that probably 
25 per cent would have to be added to give it the properties manifested in the actual 
flow. The sand, however, in its original position before the time of the earthquake, 
probably did not contain more than 20 per cent of water, since the upper or soil layer 
_ had been somewhat dried out by the air. To the original sand of the slope, therefore, 
there must have been added 30 per cent of its volume of water to cause it to behave as 
it did. This amounts to 27,000 cubic yards. This water came from ground immediately 
below the source of the flow; and it came in a moment, at the time of the earthquake. 
It is only another way of stating the facts to say that it was squeezed out. There was 
