A remnant of an old submarine erosion surface lies at a maximum depth of 1100 fathoms on the southwest slope of Eniwetok Atoll. Underwater photographs taken at a number of locations with a U. S. Navy Electronics Laboratory Camera have disclosed at least 50 square miles of ripple-marked calcareous sand and reef debris of Recent age at water depths ranging from 750 to 1100 fathoms; short cores penetrated fine-grained underlying Pliocene sediment at a number of stations. In at least two locations near the central area of the survey, the ripple marks are indurated by a cement which has given the bottom a dark appearance. Lighter-colored sediment shows through in irregular patches. Presumably formed in recently accumulated materials, these indurated ripple marks contrast sharply with the freshly rippled and sorted bottom sediments found over a greater part of the erosion surface. Manganese-coated bedrock or boulders project through coarse, sandy sediments near the outer break in slope. Although the photographs show an almost complete sediment cover, the short gravity cores indicate that the accumulated sediments are relatively thin. Water action has prevented bottom materials from building up any appreciable thickness since the Tertiary. A noticeable lack of benthic life in the photographs suggests not only strong water movements but a bottom environment of low organic content. These water movements on topographical highs appear to have originated below normal tidal wave action but apparently were related to mass eddy movements around submerged underwater features. Short-period oscillatory waves growing out of these mass water movements resulted in symmetrically shaped ripple marks at great depths.

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