Except in areas of gentle inclination, the continental slopes are cut by numerous submarine canyons. There is ample evidence that these canyons are loci of active erosion and represent the chutes down which sediments are transported to build the great fans that have

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formed along the base of the slopes. The numerous photographs of ripple-marked sand to depths of 3,500 m, and the observation from deep-diving vehicles of scoured hard rock walls at depths to 1,350 m provide evidence of this activity. The relative importance of turbidity currents and currents of unknown causes is still uncertain. Mass movements undoubtedly are an important contributing cause.

Most submarine canyons are located off relatively large land valleys. In some places, the canyons apparently originated as submerged river valleys, but submarine erosion appears to have played a major role in developing their present configuration. Evidence of more than one cycle of excavation of canyons exists in various places. Apparently canyons have become filled and then reopened by some type of rejuvenation that is not well understood.

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