The Editor,
Journal of Glaciology
Sir,
The argument advanced by Messrs. Blake and Hollin on pages 792–93 of the October issue of the Journal of Glaciology depends on the premises that the patterned ground is all of one age and has taken a long time to develop. Both are assumptions of very doubtful validity. Apparently it has been assumed that the polygons showing in the photograph are ice–wedge polygons of the kind found commonly in Arctic regions and elsewhere, but Péwé has shown that they are quite distinct and has called them sand-wedge polygons.Reference Péwé 1
We have concluded from observations at many localities around the western and south–western sides of the Ross Sea that, under the present climatic conditions, sand–wedge polygons are forming and changing continuously and rapidly. They show a large range in radius, depth of trough, shape of the profile of the central core area, relative activity or stability, degree of infilling by wind-blown material, and other characteristics which can be related to site, to size, depth and lithology of superficial fragmental material, to a probable cycle of development through youth, maturity and old age, and to other factors some of which are not understood. Preliminary detailed studies of sand–wedge polygons by a number of colleagues are being prepared for publication, and their results appear to support our opinion that these polygons can form in less than 50 years, and possibly in only a few seasons. As noted by Péwé they even form in stagnant icé.
The dry valley shown in our photograph on page 652 of the March 1960 issue has been named Beacon Valley by McKelvey and Webb.Reference McKelvey and Webb 2 The photograph, which shows a small part of its lower end, is deceptive in indicating a uniform development of the polygons, for it was noted in the field and has been checked by inspection of a U.S. Navy vertical aerial photograph (0156, run Ant. 4) that there are considerable and important changes in the nature of the patterned ground up-valley from the terminal face of the glacier lobe. Between the ice–face and the nearest moraine ridge the polygons are well-defined and fairly regular in shape, and have a relatively constant radius, but even in this sector of the valley floor they have an increasing vagueness with increasing distance from the ice face. Between the moraine ridge that is nearest to the ice front and the one farthest from it the polygons are less well-defined, less regular and of variable radius. Beyond the farthest (fourth) distinct moraine ridge, there is an increasing vagueness and irregularity of shape with many very large polygons. We take these changes to represent mainly stages in a cycle from youth towards maturity, and to indicate that the youngest polygons are those closest to the ice front.
It is clearly desirable to have a greater understanding of sand-wedge polygons but in the meantime we favour the tentative working hypothesis of our original letter in preference to the stimulating alternative advanced less tentatively by Messrs. Blake and Hollin. Naturally, for the phenomenon is common in temperate regions,Reference Kolb 3 we consider that each terminal moraine could represent a minor advance superposed on a general period of retreat.
Our preferred hypothesis is supported by the occurrence in the middle and upper reaches of the Beacon Valley of moraines which were not mentioned in our original letter, partly because that locality was being described by colleagues,Reference McKelvey and Webb 2 and partly because we had viewed them only from a high point at a distance of several miles. We have obtained recently, however, a copy of a U.S. Navy aerial photograph (0189, run Ant. 16) of the head of the Beacon Valley which shows that the trunks of two small neighbouring valley glaciers are each bordered by four distinct loops of stranded lateral and terminal moraines (Fig. 1, p. 946). The fourth or latest loop is compound, and wraps around a tongue of moraine-covered stagnant ice continuous with active ice farther upstream. These closely-matching sets of moraines have a striking resemblance to nineteenth-century and present-day moraines of some temperate glaciers, such as those of the Hooker Glacier in New Zealand,Reference Kolb 3 and they provide a much more convincing suggestion of several phases of recent retreat than do those of the unusual glacier lobe reported in our original letter.
Fig. 1. Vertical aerial photograph of two small glaciers at the head of Beacon Valley, Victoria Land, Antarctica, showing stranded lateral and terminal moraines, and the form and distribution of sand-wedge polygons. Well-defined polygons cover the outer wall of the most recent arc of the fourth (compound) terminal moraine. U.S. Navy Aerial Photograph Antarctic run 16, No. 189; 15,700 ft. [4,785 m.]
