| Although areal scour typifies continental ice sheets, these large masses of ice can also erode linear channels and troughs. Such troughs resemble the troughs seen in alpine settings in their overdeepened basins (like the one occupied by Seneca Lake, NY, at right) and "U-shaped" cross-profiles) but unlike mountain valleys, this landscape was entirely covered with ice. Erosion of troughs like those of the Finger Lakes (shown here) probably involved glacial scour of preexisting stream valleys, subglacial fluvial erosion, and positive feedback at the glacier bed (the thick ice in the valleys remained warm-based, thus eroded, while thin, cold-based ice outside the trough did not). |  |