A deep shaft, nearly vertical and of roughly circular cross section, formed when surface meltwater enlarges a crack in the ice by transferring kinetic and thermal energy to its walls. Moulins connect to the englacial drainage network, facilitating transfer of surface meltwater to the bed. The meltwater resulting from enlargement of the moulin is an instance of internal ablation. Moulins may play a significant role in supplying lubricant to the bed. The word is French for mill, referring to the swirling motion of the water as it descends the shaft.