108 
OIL DISTRICTS OF SOUTHERN CALIFORNIA- 
STRUCTURE. 
GENERAL STRUCTURAL RELATIONS OF THE FIELDS. 
The structure of the Puente Hills is that of an anticline, contracted 
in the western part, expanded in the eastern. The main axis of the 
flexure is not everywhere easy of recognition, owing to the prominence 
of some of the nearly parallel secondary folds that exist throughout 
the length and breadth of the hills, but the line laid down on the map 
(in the eastern half of the hills—the northern branch) approximately 
indicates the axis of the anticline. The general trend of the Puente 
fold is N. 65° W., varying but a few degrees in either direction. 
Besides the main and parallel or slightly divergent secondary folds, 
there are several flexures, with a trend approximately northeast and 
southwest. These are particularly developed on the northern slope 
of the main anticline in the eastern half of the hills. The most impor¬ 
tant is the broad, gentle arch occupying the region of the forks of Brea 
Canyon, and to this is due, in considerable degree, the areal expansion 
of the Puente Hills in their eastern part. The eastern rim of the hills 
presents several complications of strike and dip, doubtless more or 
less connected in structure with the Santa Ana Range to the south. 
(See PI. XI.). 
The south side of the Puente Hills is devoid of folds other than those 
secondary to and parallel with the main anticline. On this side of 
the hills, however, the force which produced the anticline has most 
severely manifested itself, the folds being sharp and numerous and 
the strata badly crushed. Indeed, the data at hand suggest the pos¬ 
sibility of a fault of considerable magnitude, extending from the 
extreme west end of the hills nearly or quite to Santa Ana River. The 
amount of throw is indeterminable, but local displacements of several 
hundred feet are indicated by the relation of the beds. The con¬ 
tinuity of the fault, considered as a single fracture, is not established. 
On the contrary, it is probable that within the zone of maximum dis¬ 
turbance there are a number of fractures, more or less connected, it 
may be, yet throughout a part of their course apparently distinct—in 
fact, a zone of faults instead of a single uninterrupted break. The 
chief evidences of faulting are the continuity of a zone of marked 
crumpling, the overturned and crushed condition of the beds at many 
points, variation in the succession of beds adjacent to the line of sus¬ 
pected fracture, and a divergence between trend of break and strike 
of strata, particularly well defined in the Whittier field. On the other 
hand, the line of possible fractures is at no point far distant from the 
contact between the Fernando and the Puente formations, and it is 
well known that throughout the Coast Range there is a conspicuous 
unconformity at this horizon. Without doubt the unconformity is 
