THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 93 
dam, the trace of the fault is slightly concave to the straight line connecting these two 
points on the fault, the concavity being to the southwest. Between San Andreas dam 
and the end of Upper Crystal Springs Lake, the trace of the fault is again slightly concave 
to the straight line between these points, but is on the opposite side of the fault, the 
concavity here facing the northeast. 
Characteristics of the fault-trace. — For this stretch of from 14 to 15 miles, Mr. Robert 
Anderson, who examined this territory under direction of Prof. J. C. Branner, describes 
the trace of the fault as marked by a belt of upturned earth resembling a gigantic 
mole-track. ‘The rupture may be traced along every foot of the way when not below the 
waters of the lakes. It varies in width from 2 or 3 feet to 10 feet, but at times branches 
out into several furrows that include a space of 100 feet or more in width. Such branches 
sometimes join again after a short interval. Sometimes it forms a crack 2 or 3 feet wide 
and several feet deep, and in other places shows a vertical wall of soil on one side or the 
other, several feet high. The typical occurrence in turf-covered fields is a long, straight, 
raised line of blocks of sod broken loose and partly overturned. It is thus shown in 
plate 614A, B. 
Associated with the fault fractures are many lateral cracks, extending away from the 
fault in a northward, or north slightly eastward, direction; that is, at an oblique angle to 
the northeast side. These cracks were especially abundant along the northeast side of 
the northern half of Crystal Springs Lake, and between there and San Andreas Lake. 
In places they run off every foot or few feet for a distance of 100 yards or more, and again 
they do not form for some distance. They vary in size from minute crevices in the 
earth to fractures a foot or more in width. Here and there they form lines of broken sod 
very like the main furrow in size, while they have a length of from a few feet to several 
hundred feet. At the great dam at the head of San Mateo Canyon, these cracks emerged 
from the lake and ran northward up on the hills for several hundred yards, breaking the 
fences where they crost. Plate 16a shows large lateral cracks of this description, already 
partly filled up, crossing a road that runs parallel to the fault at the upper end of Crystal 
_ Springs Lake. The main line of fracture is about 50 feet beyond the fence, and the 
cracks extend into the foreground at an angle of from 35° to 40° with the main fault- 
trace. The fence is pulled apart 40 inches in the two places which are shown in the photo- 
graph, and a total of 10 feet in ten different breaks in this locality, within a distance 
of 200 yards. Such lateral cracks as these were not noted on the southwest side of the 
fault. 
The lateral cracks described above make an angle of 45° to the general line of the fault 
fracture. They appear to have been produced very much like the fracture lines in com- 
pression tests of building stones. There was evidently great pressure holding together 
the two faces along the fracture. A dam made of earth and rock divides Crystal Springs 
Lake into two parts. This dam crosses the fault-trace at right angles, and was offset 
but not badly cracked or injured by the movement. The fences that line the road were 
warped and their boards buckled thruout the distance across the dam. The earthquake 
rendered them too long for the distance from the hills on one side of the valley to those on 
the other. The inference is that a strong compression took place. The slicken-siding 
shown in plate 62a furnishes further evidence of compression. In the same way the 
heaving up of the sod into a long, raised mound, for most of the extent of the furrow, 
suggests lateral pressure. The formation of cracks a few inches to 2 or 3 feet wide in 
places along the furrow seems to contradict the theory of compression; but these are 
regarded as due to the irregular, crooked fracturing of the surface and the faulting of 
irregularities into juxtaposition with one another near the surface. The open cracks 
