The area mapped is located in the portion of the Puente Hills just east of the town of Puente. The problem was to map the region shaded in brown on the index map. The area hatched in brown is the Puente Quadrangle, which is the map used.
Geographical names are vaguely defined in this part of Southern California. The United States Geological Survey has called the nearly isolated group of hills between Puente and the Pass and Covina Road, part of the Puente Hills, and the large group of hills between this road and the city of Pomona, the San Jose Hills. Other definitions will be omitted from this paper in order to avoid confusion, so the name Puente hills, as far as this region is concerned, applies only to the small cluster of hills mentioned above.
The area chosen was bounded by the San Gabriel Valley on the north, Puente Valley on the south, the east border of the Puente Quadrangle, and the town of Puente on the west. Field work was conducted in this three and one-half square miles plot for sixteen days, with the purpose being a structural field problem for a Bachelor's thesis at the California Institute of Technology. The base map, the Puente Quadrangle of the Unites States Geological Survey, edition of 1927, was excellent to work with, as the scale was 1:24,000, and the contour interval in the hilly country was twenty-five feet.
Strikes and dips, accurate in most cases to plus or minus three degrees, were obtained with a Brunton Compass. The estimate does not account for the slumping of surface outcrops, nor does it apply to the attitude of poorly bedded outcrops. A few dip and strike readings do not fit in the general picture; nor can they be explained in any way other than mistakes or minor flexures.
All of the contacts were traced as accurately as natural conditions allowed. In the parts of the region devoted to pasturage, the only hindrance to exact tracing of the contact was cactus, as outcrops were abundant and most of the rock was in place. Unfortunately, much of the area was under cultivation which removed all of the surface outcrops and destroyed any trace of the contact. The change between conglomerate and shale in these places could only be determined to within two hundred feet.