98 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
lake for the first 0.875 of a mile of its length. It then enters the water (plate 61p) and 
follows the northeast side of the lake, a little distance from shore, to the San Andreas dam 
at the lower end of the lake. In this distance of nearly 2 miles, the fault-trace emerges 
from the water at a number of points where little capes project into the lake. The cross- 
ing of these capes by the fault-trace indicates that it follows a very straight course beneath 
the water of the lake. On the last of these promontories traversed by the fault, the main 
fault-trace has associated with it a number of auxiliary cracks. Between the main 
fault-trace and one of the diverging cracks, on the southwest side of the fault, is a brick 
and cement gate-well in connection with the tunnel which takes the waters from the lake 
toward Millbrae. This gate-well was circular in cross-section, the inside diameter being 
about 26 feet. The nearest point of the structure to the main fault-trace is within 5 feet. 

CONCRETE SHAF 7 
7 
of ve 
CONCRETE 
X aa TE FOREBAY 
a <2 
YCONCRETE SHAFT 
\NERICK FOREBAY 
Note: Dotted circle inside 
of Brick Forebay represents 
original pos/tion of inside 
circumference of Forebay 
400feet aofeet 

Fic. 35.— Main and auxilia-y faults, San Andreas Lake. A. General Plan. #8. Detail. After H. Schussler. 
The walls are about a foot thick, and are strongly buttressed. As a result of the shock 
this gate-well was shattered and deformed so that it became oval in cross-section, the east 
and west diameter becoming 30 feet and the north and south diameter about 21 or 22 
feet, as shown in the accompanying figure. A new concrete gate-well a few feet to the 
north, rectangular in cross-section and having three compartments, each 2.5 x 2.5 feet, 
was uninjured, altho on the line of the same branching crack. A concrete manhole 45 
feet northeast of the damaged gate-well, also on an auxiliary crack, was similarly unaf- 
fected. (See fig. 35.) 
