In recent years the concept of large-scale strike-slip faulting, involving movements ranging up to hundreds of miles, has been employed in an attempt to solve some of California's major structural and stratigraphic problems. It may well be that ultimately an irrefutable case will have been made for this sort of fault movement. At present, however, the history of these faults has not been certainly resolved. It appears, also, that the method of multiple working hypotheses has not always been given its due, in analyzing the geology adjacent to these faults.

The San Gabriel fault is one of the "major strike-slip faults" of California. In earlier publications, horizontal movement of 20-30 miles has been suggested for this fault. Such movement was based on the contrasting petrologic features of upper Miocene coarse clastic sedimentary rocks, which are locally found in proximity to each other on opposite sides of the San Gabriel fault. An alternative explanation for the origin of these facies, involving minor or no strike-slip movement, can be deduced from the geological record. Also, the suggested interfingering of upper Miocene sediments across one segment of the San Gabriel fault, the adjacency of upper Miocene marine basins, the apparent matching across the fault of pre-upper Miocene paleogeologic elements, and other features point to a ominantly dip-slip history.

Numerous unconformities in the blocks on either side of the San Gabriel fault attest to the unstable nature of the terrane, and to a Tertiary history which involved several major vertical oscillations. It is proposed that this demonstrable series of oscillations is reflected in the history of the fault, which may also have been subjected to reversals of vertical--i.e., dip-slip--movement in the course of its history.