A large, elongate, overdeepened depression cut into bedrock or glacigenic sediment, up to 4 km wide, over 100 m deep, and 30–100 km long. In long profile they contain enclosed hollows or isolated and elongated basins, sometimes displaying convex-up or undulating longitudinal channel profiles and anabranching channel networks (Beaney (2002) Qt. Int. 90). Ó’Cofaigh (1996, PPG 20) attributes their formation to the creep of deformable subglacial sediment into a subglacial conduit, followed by the removal of this material by meltwater flow.