Near-bottom investigations with a deep-towed instrument package show that steep-sided, flat-floored furrows (1 to 100 m wide by 0.5 to 20 m deep) eroded into Holocene and Pleistocene hemipelagic mud are responsible for the characteristic hyperbolic echo traces on surface-ship echograms recorded over the Bahama Outer Ridge (water depth, 4 to 5 km). This echogram character, previously interpreted as being caused by side reflections from depositional wave forms, has been typically recorded in abyssal regions of contour-current activity. It is suggested that these remarkably straight longitudinal furrows are formed by secondary helical circulations in the bottom-boundary mixed layer.

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