There are a series of basins off southern California, all presumably due to faulting of the broad continental borderland. Of these, San Diego Trough is the only one where sedimentation has gone on sufficiently to fill the basin depressions so that there is a continuous slope southward to where the trough connects with the deeper San Clemente Basin. This deposition has apparently come to a very great extent from turbidity currents moving down submarine canyons developing channeled fans at the canyon bases. All along the canyons and channels as well as across the fans and in San Diego Trough there are sequences of turbidite sands alternating with muddy sediments that represent the normal deep water sedimentation. Comparison of the coarse fraction from the sands and from the interbedded muds shows that the former are predominantly terrigenous in character whereas a large proportion of the latter are biogenous. Mica is quite low in the coarser sand layers but is abundant in the coarse fraction of the muds. Diatoms and radiolarians are largely confined to the mud layers where they may constitute a large percentage of the sand fraction.
Study of the heavy and light minerals shows that the sediments of San Diego Trough are derived largely from the nearby land masses rather than having a considerable admixture from the north as would be the case if sediments were being brought down by turbidity currents coming into the trough from the north. The alterites are appa-rently lower in San Diego Trough than in the near shore source area which suggests that a supply of sediments may be picked up from the canyon or channel walls along the way to the trough. Also the finding of the only coarse sands in the middle reaches of the channels suggests local sources as does the general make up of these coarse sands. Despite these locally derived coarse sands the prevalence of very fine sand and the “mud” layers between sand layers suggest that the turbidity currents supplying sediments to San Diego Trough have low velocities or at any rate little erosive capacity.