During four dives in the Tongue of the Ocean in the deep submersible Alvin, we observed extensive evidence for erosion, including cliffs several tens of metres high of horizontally bedded pelagic chalk, crescentic scour marks around boulders in channel bottoms, and bands of freshly exposed rock at the bases of cliffs that were otherwise stained dark brown by manganese oxide. In the central part of the Tongue of the Ocean, the change from a depositional to an erosional regime must have occurred only recently, as the axial valley is cutting headward into bedded chalks of mid-Pleistocene age. Such changes may have occurred several times in the history of the Tongue of the Ocean, and the steep walls may be the result of an interplay between upbuilding of the platforms and erosion of the troughs.

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