Mass movements in submarine canyon heads

The repetition of sounding profiles along precise ranges at the heads of the submarine canyons in the La Jolla are a has given a sequence of depth changes during the past three years. Changes up to a maximum of 21 ft have been observed. Fill and deepening have alternated, but without any seasonal relationship such as occurs on the local beaches as a response to storm waves. The shallow valley heads are shown to shift in position, and at least one new valley with a maximum depth of 16 ft was formed during the period on an undissected slope. These changes have taken place on the sand‐covered slopes inside the rock gorges and appear to be explained by sedimentation and landsliding. The latter may be a combination of creep and sudden shifting of sediment. Sufficient material is being moved by these slides to fill the outer gorges in a matter of a few centuries, but the material is certainly being carried out of the gorges, probably going to form the sand layers in the deep outer trough at the mouth of the canyon. The canyon heads are thought to trap a large proportion of the sand that is carried along the shore by currents. This prevents the formation of sand beaches directly down current from the canyons.