A narrow, vertical or nearly vertical, cylindrical shaft with circular cross-section. It is formed in crevasses by the meltwater from glacier’s surface. If it gets wider with depth, it is called AVEN. At the bottom, it is either connected to a pit or a chamber, or transforms into a system of cascades/a horizontal corridor
https://polarpedia.eu/en/glacier-mill/
Meltwater pouring into a moulin on Greenland. J. Box.
http://www.esa.int/spaceinimages/Images/2011/01/Meltwater_pouring_into_a_moulin_on_Greenland