The principal points of water input to a glacier are the bergschrund in cirques, and crevasse fields lower on the glacier. Crevasse fields commonly occur over convexities at the heads of overdeepenings in glacier beds. The amplitude of subglacial water-pressure fluctuations is large just down-glacier from these points of water input. Erosion by quarrying is likely in such areas. Erosion is thus inferred to be localized on the headwalls of cirques and overdeepenings. In the case of overdeepenings, this leads to a positive feedback process in which a perturbation in the bed causes crevassing at the surface, resulting in erosional forces that accentuate the perturbation.
When subglacial water flows up an adverse bed slope leading out of a cirque or overdeepening, much of the viscous energy dissipated is used to warm the water to keep it at the pressure melting temperature as the ice thins and the pressure decreases. In such situations, subglacial conduits are maintained by high water pressures rather than by melting of conduit walls. In the limit, water pressures apparently become so high that water is forced out along the ice-bed interface and the conduits collapse. The products of erosion are then no longer flushed out, and a protective till layer accumulates. By limiting erosion on such adverse bed slopes, this till layer controls the geometry of these overdeepened basins.