A large group of discrete peaks occurs on the northeastern surface of Orphan Knoll at water depths between 1800 and 2800 m. Long-range side-scan sonographs are used in conjunction with seismic reflection profiles to establish their flattened conical form. They commonly rise to 300 m above the sea floor and occupy basal areas up to 2 km in diameter at that level. Inclusion of the buried lower parts of these mounds may double estimates of both the height and diameter. The sonographs indicate that the mounds have a random distribution within an elongate northwesterly trending belt. Previous suggestions of their possible origin, such as remnants of dykes or ridges of resistant sedimentary strata, are rejected and an alternative explanation of a zone of partially buried Devonian reef knolls is proposed.

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