The Los Angeles Basin comprises an area of about 1,200 square miles. It is bounded on the north by mountains uplifted along east-west-trending faults. Most of the regional faulting trends northwest-southeast, resulting in roughly parallel block faults having varied histories of deformation and sedimentation. Paralleling or lying en echelon to these faults are topographic prominences varying from mountains along the easterly border to hills and knolls within the plain area. The knolls within this plain reflect anticlinal structures which are now developed as oil fields. Productive areas also occur along the borders of the Basin. Most of the oil deposits are within beds of lower Pliocene and upper Miocene age. Local accumulations also are present in beds of upper Pliocene a e, within middle Miocene, and in weathered or fractured schist of the basement complex.

The rocks within the stratigraphic section range in age from Jurassic (?) to Recent. The basement complex consists of various metamorphic and granitic rocks. Remnants of Upper Cretaceous sediments rest on these rocks in the Santa Ana Mountains area and vicinity, and in the Santa Monica Mountains. Eocene sediments also are limited to these two areas. Redbeds ranging in age from middle Miocene to probable Oligocene, inclusive, are rather widespread and were deposited as interfingers with marine sediments. Igneous activity was pronounced in middle Miocene time. Upper Miocene sediments are the most extensive of the marine deposits. Lower Pliocene sediments were deposited in deeper water. Upper Pliocene and lower Pleistocene sediments are rather extensive and were deposited in shallower wa er and under conditions of marginal oscillations which have resulted in only limited phases of these deposits being present at the edge of the Basin. Active orogenic movement took place at middle Pleistocene time and is continuing to the present. Physiographic features of the Basin are the result of this post-middle Pleistocene activity.