Fast-flowing ice streams and outlet glaciers currently account for as much as 90% of the discharge from the Antarctic and Greenland Ice Sheets. Although the deformation of subglacial material has been proposed as the mechanism for this rapid motion, such sediment is usually hidden under several kilometers of ice. Marine-geophysical records have allowed reconstruction of the three-dimensional thickness of the sedimentary bed beneath a large Antarctic paleo–ice stream for the first time. Fast flow is indicated by streamlined seafloor lineations that form the surface of a layer of low shear strength, unsorted sediment, averaging 4.6 m thick. Rapid motion of the paleo–ice stream was a result of subglacial deformation within this layer.