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Cuspate Surfaces of Melting Ice and Firn

1948; Taylor & Francis; Volume: 38; Issue: 2 Linguagem: Inglês

10.2307/210861

1931-0846

John Leighly,

Geology and Paleoclimatology Research

IN HIS recent article on the Wolf Creek glaciers of the St. Elias Range, Robert P. Sharp describes and illustrates a type of surface of melting glacier ice that he calls scalloped.' His term was evidently suggested by the profile of a section normal to the surface. A more suitable name for a three-dimensional surface such as he describes might be or negatively mammillate. According to Sharp, surfaces are associated with fluted ones, and always face downward. Scallops appear on a horizontal or nearly horizontal surface and pass into flutings as the inclination of the surface from the horizontal increases. Sharp is unable to suggest a satisfactory explanation ... for the form and pattern of flutes and scallops. I propose here a hypothesis of the origin of these surface forms of melting ice. As a supplement to the illustrations in Sharp's article, Figure I is included, reproduced from a photograph of part of the roof of a stream tunnel under a firn bank. I took the photograph on August 7, 1940, in the canyon of Lowell Creek, which flows into Resurrection Bay at Seward, Alaska. In winter, snow accumulates to great depths in parts of this canyon, as a result of the piling up of avalanches that have slid down its steep tributary valleys. As late in the summer as the date when the photograph was taken, three such accumulations had not yet melted. All the exposed snow had been changed into firn, and the stream flowed through tunnels under the remaining banks. Figure I was taken from the upper end of one of these tunnels, looking downstream. The avalanche snow is rather dirty, so that many of the ridges in the melting surface of the firn are lined with bits of rock and plant material. Most of this foreign matter has been carried to the ridges and cusps by the water that flows as a film along the melting surface toward the cusps and there drips off. In the upper right of the picture, melting has exposed an ice crust containing debris; it has an appearance different from that of the true cuspate or scalloped surface.