Paleozoic rocks form an escarpment up to 35 m high at the border with the Frontenac Axis of the Canadian Shield in southeastern Ontario. The escarpment, which lies nearly transverse to the flow direction of Pleistocene ice and subglacial meltwater in the region, is cut by a series of deep channels locally incised into the bedrock at the escarpment. A subbottom acoustic survey of two of these channels and mapping of a third, partly flooded channel revealed the shape of the bedrock surface beneath the water and glacial and postglacial sediments of lakes now occupying the channels. Most cross sections show a smooth-walled, dominant depression eroded up to 100 m below the land surface nearby, and flanked by one or more secondary depressions. The pattern changes considerably in detail, making secondary depressions especially difficult to trace along the length of the lake. The pattern is similar to that of subglacial fluvial erosion marks more than three orders of magnitude smaller found in bedrock throughout the region. We ascribe the origin of these channels cut across the escarpment to periodic, large flows of water beneath the Laurentide Ice Sheet. The escarpment formed a barrier through which the flow was forced at regular intervals, eroding large-scale channels at these sites of confined and accelerated flow, while the ice was still largely in contact with the surrounding surface. At the highest flows the entire surface was flooded and eroded.