A moulin or glacier mill is a narrow, tubular chute, hole or crevasse through which water enters a glacier from the surface. They can be up to 10 meters wide and are typically found at a flat area of a glacier in a region of transverse crevasses. Moulins can go all the way to the bottom of the glacier and can be hundreds of meters deep, or may reach the depth of common crevasse formation (about 10-40m) where the stream flows englacially.
Moulins are a part of a glacier's internal "plumbing" system, to carry meltwater out to wherever it may go. Water from moulins often exits the glacier at base level, but occasionally the lower end of a moulin may be exposed in the face of a glacier or at the edge of a stagnant block of ice.
Water from moulins may help lubricate the base of the glacier, affecting glacial motion. Given an appropriate relationship between an ice sheet and the terrain, the head of water in a moulin can provide the power and medium with which a tunnel valley may be formed.