Tunnel Valley

A tunnel valley is a U-shaped valley that was originally cut beneath glacial ice near the edge of a continental ice sheet, and the largest examples reach roughly one hundred kilometers long, four kilometers wide and four hundred meters deep. It forms through erosion by meltwater flowing beneath the ice, carving channels that served as drainage pathways carrying large volumes of water away from the glacier. Geologists still debate the exact mechanism, with proposed explanations ranging from gradual erosion by steady subglacial streams, to sudden erosion during catastrophic meltwater floods, to valleys that lengthened progressively as erosion worked back toward the ice sheet during deglaciation. Former tunnel valleys survive today as dry valleys, lakes, depressions on the seabed, or valleys filled with sediment.