Internal Geometry and Evolution of Moulins, Storglaciären, Sweden

Abstract The initial conditions needed for formation of moulins are crevasses and a supply of melt water. Water pouring into a crevasse may fill it until it overflows at the lowest point, which is normally near the margin. However, as the crevasse deepens, it intersects englacial channels through which the water can drain. These channels may be finger-tip tributaries in a dendritic system such as that described by Shreve (1972) and observed by Raymond and Harrison (1975). When the crevasse closes, heat in the melt water keeps the connection open and a moulin is formed. The englacial channel enlarges rapidly by melting, utilizing mechanical energy released by the descending water. Descents into moulins, and mapping of structures exposed at the surface after many years of melting, demonstrate that the drainage channels leading down from the bottoms of the moulins have inclinations of 0–45° from the vertical. These channels trend in the direction of the original crevasse but appear to be deeper than the expected depth of the crevasse. They have not, even at depths of 50–60 m, become normal to the equipotential planes described by Shreve.