THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 107 
Along this trail the direction of the cracks varied considerably. One an inch wide in 
places, elsewhere branching into several smaller ones, was traced for about 150 yards, 
chiefly along the crest of the ridge. Its direction varied from due east to southeast, and 
the upthrust on the west was sometimes as much as 3 feet. Going northwest down the 
crest of the ridge, numerous cracks crost in directions varying from southeast-northwest 
to northeast-southwest, several showing an upthrust varying from a few inches to a foot 
on the southwest side. At the foot of the trail, a large crack running down the center 
of the valley followed the road for about 100 yards, then cut across the fields. In places 
the crack was 2 feet wide, but in other places a ridge 3 feet high had been raised beside 
the road, and there were many parallel cracks within 50 feet of either side. There were 
upthrusts and downthrows, some as much as 1.5 feet, but the total change of level seemed 
to be nil. 
Alpine road. — A fault branches from the main San Andreas fault in the Portola Val- 
ley and crosses the Alpine road just where the Portola road leaves the latter. At this 
fork several cracks were formed at the time of the earthquake. A water pipe 2 inches 
in diameter was buckled and lifted out of the ground here, and farther along the Portola 
road this same pipe was pulled apart. Following southward along the Alpine road, the 
next evidence of disturbance by the earthquake was where the main fault-trace crosses 
the road 0.75 mile south of where the Portola road forks. Here the road was so 
badly broken and cracked that it was not possible to ride or drive across the fracture 
until the place was repaired. (Plate 63a.) The fracture followed along the south side 
of the road for a distance of 300 feet, tearing up the bank with cracks, some of which 
were a foot or more across. Where the road bends toward the south, the fracture crost 
to the north side of the road, making cavities several feet deep. These cracks continued 
toward the northwest thru the underbrush, pulling apart a barbed wire fence and leaving 
many well-marked furrows thru the adjoining fields. About 30 feet north of the road, 
a white oak, somewhat weakened by decay and fire, was jerked off by the violence of the 
shock. To the southeast the fault-line is traceable by a well-marked furrow thrown up 
in the fields. Where the fracture crosses the Alpine road, there appears to have been an 
uplift of about 2 feet on the northeast side of the fault. This appearance may be due to 
the settling of a part of the ridge of incoherent materials to the south, or it may be due 
to the lateral thrust along a sloping surface. 
Black Mountain. — The great mass of Black Mountain lies between the San Andreas 
fault and a branch fault (Black Mountain fault) which, starting in the Portola Valley, 
crosses the Page Mill road on the north side of the mountain about a mile south of Clarita 
vineyard. This area between the faults was badly shattered by the earthquake, tho it 
is not clear whether the abundant cracks found over the surface are to be attributed to 
the boldness of the topography or to the crushing of the wedge-shaped end of the fault 
block. Several days after the earthquake, 345 cracks, large and small, were counted 
along the county road (Page Mill) in a distance of less than 3 miles between these faults. 
These cracks ran in every direction, and some of them were clearly attributable to local 
topography, while others cut thru the mountains in apparent disregard of the topography. 
The main fault-trace crosses the Page Mill road in a topographic saddle near three 
frame houses. The displacement occurred along two parallel and well-defined cracks 
some 30 feet apart. These cracks can be traced across the fields on both sides of the road. 
Toward the northwest they converge until they are only a few feet apart. Where they 
crost the road, the fracture was not a single clean-cut break, but made up of a series of 
small short cracks from 3 to 5 inches across, parallel with each other and “splintering” 
across the general direction of the fracture. The fences on both sides of the road were 
displaced about 3 feet, and there was an apparent drop of 18 inches on the southwest 
side of the fault. The horizontal displacement showed the northeast side to have moved 
