Side-scan imagery and 3.5 kHz profiles in the Norway Basin reveal approximately 3-4X10 (super 4) km (super 2) of sea floor covered by regularly spaced (500-2000 m), low-relief hummocks previously attributed to bottom currents. Extrapolating from nearby borehole data, I attribute these mound fields to gravitational (Rayleigh-Taylor) instabilities resulting from superposition of 50-100 m of thick Pliocene-Pleistocene, glacigenic (glacier-derived) sandy clays (density 1800 kg/m (super 3) ) on a several-hundred-metre thickness of Miocene biosiliceous oozes having a density of 1300 kg/m (super 3) . This hypothesis predicts or explains similar hummock fields in many oceanic regions where such density inversions exist.