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OIL DISTRICTS OF SOUTHERN CALIFORNIA. 
however, exceptional. Oil occurs at intervals from a depth of less 
than 300 feet to nearly 2,000 feet. The shale lies at the very heart of 
the Puente anticline and is, therefore, except in one or two places, 
not only the oldest body of rock exposed northwest of Santa Ana 
River, but the lowest formation in which oil has been found in the 
general region of the Puente Hills. 
Flanking the shale on the north is the Puente sandstone, which is 
overlain by a thin body of the upper shale, and this in turn at the 
periphery of the hills by the Fernando conglomerate and associated 
beds. The dip from the crest of the ridge northward is to the north 
with marked regularity. South of the crest, opposite the developed 
territory, the shale extends to the valley level, the younger formations 
lying buried beneath the more recent wash. The dip on the south 
side of the ridge, especially in the eastern half of the held, is variable, 
indicating a rapid succession of synclines and anticlines, each lower 
as the edge of the prairie is approached. Opposite the western half 
of the held the folds are less pronounced and the prevailing dip on 
the lower slope of the hills is northward, changing to southward 
higher up. 
Opposite the Puente oil held, for a distance of 2 or 3 miles along 
the southern base of the hills, the Fernando formation is wanting in 
outcrop, this being the only break in its continuity from a point near 
the Santa Ana to San Gabriel River. The ridges which usually mark 
the outcrop of the Fernando are also wanting, having been carried 
away, doubtless, by erosion at the time when the Pacific washed the 
base of the hills. It may be due to this that the Puente sandstone 
lying buried with the younger formation beneath the terrace gravels 
of the valley is also lacking along this same stretch of country. On 
the other hand, the disappearance of the Fernando and Puente sand¬ 
stone may be by faulting, which is known to have disturbed the 
structural relations both to the east and to the west. North of the 
hills the strike of the Fernando and underlying formations changes 
from its normal direction of N. 65°-70° W. to nearly northeast. The 
Fernando passes from the flanks of the hills to the valley of San Jose 
Creek, while the Puente sandstone and the underlying shale enter into 
those subordinate folds of northeasterly trend which characterize the 
northern side of the hills from this point eastward. 
STRUCTURE. 
Without a detailed survey it is impossible to delineate the individual 
flexures of the Puente field. They are many in number, parallel with 
one another, of varying length and amplitude, and some are but 
slightly less important than the main anticline itself. There is no 
doubt, however, that the axis of the principal fold is here nearly coin- 
