_:b1217043 . _:b1217043 . _:b1217043 . _:b1217043 "A deep but narrow valley with a 'U' shaped cross-section and frequently a 'U' shaped plan which is usually found filled with glacial till. It is formed when the edge of an ice mass advances up a slope. Its length may be appropriately measured in centimetres or kilometres, depending on the circumstances of formation."^^ . _:b1217043 "Wikipedia:Tunnel_valley"^^ .