Cuspate Surfaces of Melting Ice and Firn

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. American Geographical Society is collaborating with JSTOR to digitize, preserve and extend access to Geographical Review. 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 "cuspate" or "negatively mammillate." According to Sharp, "scalloped" 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, I940, 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. MELTING ON OVERHANGING SURFACESA part of the heat that melts downward-facing surfaces of ice and firn is probably derived from the absorption of reflected solar radiation and of I R. P. Sharp: The Wolf Creek Glaciers, St. Elias Range, Yukon Territory, Geogr. Rev., Vol. 37, I947, pp. 26-52; reference on pp. 46 and 47 (Figs. 27 and 28).

On the Origin of Ablation Hollows (Polygons) on Snow

“…In the area examined , the polygons are a common phenomenon on walls and inverted surfaces, such as on roofs of snow tunnels, under natural and artificial bridges, and on vertical and overhanging walls of snow drifts. This probl em has been discussed by Leighly (1948), Richardson (1954) and Lliboutry ( 1964-65 ). Lliboutry advanced an hypothesis that ablation by infra-red and dispersed rays, as well as turbu lence in air masses contained in the glacier and snow tunnels, provide conditions for developing hollows similar to those on bare (uncovered ) snow surfaces.…”

“…M elt wa ter penetrating into the , but the related forms on the ablation surface would then appear as protuberances instead of patterns of polygonal ridges. The initial stage of the formation of polygonal hollows due to ablation can be referred to the action of meteorological factors, especially to turbulence currents of warm air, as demonstrated by Leighly (1948) and Richardson (1954), Ashwell and Hannell (1966) noticed the connection between polygons and winds but, since they regarded the dispersion of dirt as the cause instead of an effect of the development of the hollows, they were thus prevented from properly accounting for the origin of these form s. * The turbulence theory seems to be closest to the truth though we are not quite familiar with the details. The following evidence may be adduced to support this theory .…”

“…However, there have been more recent attempts to resolve the question of the origin of these forms. Leighly's ( 1948) work described some hitherto unknown ablation forms on downward -facing surfaces of firn and ice ("overhanging surfaces" ) . He put forward the idea of "cellular convection" of the air in contact with the surface of the snow and explained thereby the process of the formation of polygonal hollows.…”

Theory of structure formation in snowfields motivated by penitentes, suncups, and dirt cones

“…Leighly [11] argued that heat from air (delivered by wind) leads to the formation of ablation polygons (cf. Figure 3).…”

“…Some observers who advocate uniform-heating driven formation of ablation hollows insist that dirt on the snow surface is not required [11,13,16,17]. Indeed, some photographs show ablation hollows in clean snow inside a tunnel or on other inverted surfaces, suggesting that neither dirt nor solar illumination are necessary.…”