LOS ANGELES DISTRICT : STRUCTURE. 
157 
where its trend changes to N. 70° W. Thence it passes westward at 
least as far as a point 1 mile southeast of Colegrove, where it is proba¬ 
bly deflected still farther north of west and merges with the great 
anticline which tilts the Puente sandstone in a northeasterly direc¬ 
tion on the west side of Cahuenga Pass. (See PI. XX, sec. C-D.) 
Between Lake Shore and Bellevue avenues the fold either blends 
with the Elysian Park anticline or is replaced by some undiscovered 
fault or other form of disturbance. The latter theory is strengthened 
by the evidence obtainable both east and west of the Sisters’ Hospital 
where directly in line with an eastward extension of the fold there is a 
sharp line of disturbance (probably a complicated thrust fault) which 
extends to the east end of the field at the Catholic cemetery. (See 
PI. XX, sec. G-H.) Another line of fracture, consisting of several 
more or less disconnected minor reverse faults, lies south of the sub¬ 
ordinate flexure just described and toward the west diverges slightly 
from it. (See PI. XX, sec. E-F, and fig. 13, p. 171.) This «fault 
zone appears in general to mark the northern boundary of the pro¬ 
ductive belt. As a rule the beds lie nearly horizontal along the axis 
of the flexure, but dip more and more steeply toward the south as 
they approach the productive zone. The oil appears to have accumu¬ 
lated in the sands of the southern limb of the anticline just below the 
point where the steeply dipping beds bend toward the horizontal 
before passing over the axis of the fold. The structure in the Salt 
Lake field, in the southern part of the Rancho la Brea, appears to be 
that of a minor flexure developed on the flanks of the fold along the 
southern limb of which the other Los Angeles, fields are located. 
(See fig. 17, p. 180.) Apparently the axis of this minor flexure trends 
northeast and southwest. The brea along the south side of the 
Salt Lake field is formed by oil from underlying beds which probably 
has been forced upward by gas pressure along lines of fracture accom¬ 
panying this flexure. 
JOINT CRACKS. 
All the rocks in the Los Angeles district are intersected by numerous 
joint cracks, along which, in many instances, slight displacements 
have taken place. These tiny fissures have doubtless played a most 
important part in the accumulation of the oil, for it is probable that 
whatever was its original source the transverse joint cracks in the 
shale and sandstone are largely responsible for its transference to the 
upper porous beds of the formation. Without these cracks the numer¬ 
ous impervious beds in the series would have precluded the passing of 
any fluid, least of all low-gravity oil, upward through the strata. 
