Observations from Deepstar, a research submersible, indicate that submarine erosional processes are actively modifying the main channel of La Jolla fan-valley. Internal terraces, slump scars, scour depressions around isolated erratic boulders up to 3 feet across, and a lack of talus deposits at the base of the steep walls of the main channel indicate an over-all downslope movement of the sedimentary fill on slopes of less than 1° in depths to about 4,000 feet. Cobble beds, probably deposited as part of the La Jolla fan in depths greater than 2,000 feet, presently are being eroded. The redistribution of the eroded products is evident in box cores which contain rounded balls of semi-consolidated deep-sea clay in a coarse sand matrix. The patchy distribution of differen types of sediment along the axis of the fan-valley indicates that the processes currently active in moving coarse-grained sediment down the canyon were not a continuous event. The occurrence of a 2-4-inch-thick layer of fine-grained silt and clay overlying a medium- to coarse-grained sand indicates an abrupt change in the type of sedimentation active in the fan-valley approximately 1,100 years ago.

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