Folding and shearing are the two types of deformation which may result from compressive stress in the earth's crust. The structures found in a given region depend on the type of deformation, which in turn depends on the original condition of the rocks at the time of deformation. Folding requires special conditions for its development, and only rarely is it the controlling type.

In the California Coast Ranges deformation has been governed by shearing, particularly along the great faults such as the San Andreas. This type of deformation began after the intrusion of the Sierra Nevada and Lower California batholiths, and was determined and controlled by them. The history of the California Coast Ranges is the history of a large number of fault blocks, each of which has acted differently under the stresses to which it was subjected, but is nevertheless related mechanically to the forces which govern the regional movements.

Deformation by shearing may produce faulting on a large scale, the fault blocks may again be sheared by secondary stresses, or the conditions for folding may exist within them, giving folds of the type found in the Coast Ranges. In addition to these structures, shearing from deep-seated compression may produce vertical uplift, resulting in tension at the surface and normal faulting. Most of the structural features of the earth's crust find their origin in one of these types of deformation.

California offers examples of all of them, and there seems to be some relation between oil accumulation and the type of structure. On the western side of the San Joaquin Valley there has been folding due to compression. Possibly this compression and the heat involved in the deformation has influenced the origin of the great oil fields of this region. On the east side of the valley, where the structures are due to tension following bulging by deep-seated compression, no oil has been found.

A more penetrating analysis of the mechanical development of oil-bearing structures is invited in the hope that it may lead to a better understanding of the problems of oil distillation and accumulation.