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Victoria University Antarctic Research Expedition Science and Logistics Reports 2001-02: VUWAE 46

Beacon Valley

Beacon Valley

Beacon Valley together with its 12 side valleys (McKelvey and Webb 1959) have the most extensive and best defined polygonal ground in the Dry Valley area. Elevations of the main valley floor are between 1300 and 1500 m while elevations of the side valleys are about 200 m higher. Winds during the field visit were generally down valley and less than 10 knots, however, diurnal up-valley winds were also encountered. Our camp location was on a small patch of snow on the southwestern flank of University Valley (1650m; S77°51.368′ E160°41.987′) and was selected for the snow patch and central location to the valley. However, due to the rugged terrain and subsequent slow walking, it probably would have been more convenient and as climatically comfortable to camp adjacent to the main valley bottom on the southeastern flank.

The polygonal ground of Beacon Valley was studied in the 1960′s (Berg and Black 1966) and soils of the area have been described by (Bockheim 1982; Bockheim and Ugolini 1972; Linkletter et al. 1973; Potter and Wilson 1983; Ugolini et al. 1973). More recently, weather stations with ground temperature probes have been installed in the lower and central parts of the main Beacon Valley along with strain gauges across polygon troughs (B. Hallett and R. Sletten, pers. comm). In addition rock glaciers, which are uncommon in the Dry Valleys, emerge out of Friedmann and Mullins valleys onto the floor of Beacon Valley.

The origin of the debris material on the floors of Beacon Valley and its side valleys remains unclear. Although it appears that at one time a tongue of the Taylor Glacier must have occupied the valley floor, there is page 4 no obvious moraine to support this supposition. In addition, recent drilling and ground penetrating radar (GPR) indicate that debris-laden ice lies below 2 – 3 m of ice cemented debris on the floor of Beacon Valley. The thickness of this ice is unknown because surface salts obscure GPR results (R. Sletten, pers. comm.) but it may be over 150 m thick (A Hubbard, pers. comm.). (Marchant et al. 1996; Sugden et al. 1995) dated volcanic ashes associated with this ice and have suggested that it is more than 8 Ma old.

Polygons on the floor of Beacon Valley have a 10-20 m diameter and 2 – 3 m height differential between trough and polygon centres. Although the diameter of the polygons is not uncommon, the large height differential is and may result from a long development period or the glacial ice core of the valley. The height differential of the polygons in the side valleys is less and does not appear to exceed 1 m. Adjacent to and along the southeast flank of the main valley floor, is a lateral strip that is either absent of polygons or has polygons with the least amount of relief in the area. Depth to ice cemented ground appears to be 40 – 60 cm throughout the area. In one of the rock glaciers, clear ice was found below the ice cemented ground at about 30 cm.

The activity of any single polygon or part of it may be reflected by the distribution of the material in the troughs. Parts of troughs are flat having been filled with sand while other parts are steep and rocky with angular cobbles and boulders. This angular material may be sorted or unsorted. On the active part of a polygon, clasts may roll off the steep sides and into the trough. Sorting of clasts in the trough may occur by what the center crack is able to accommodate. On the inactive part of a polygon, wind blown sand may accumulate in the trough. This observation suggests that polygon activity may be dynamic so parts of it are active while at the same time other parts are inactive.

A major problem apparent from the field work is to understand what controls the age of the surface and the relationship to polygon development in Beacon Valley. Alternatively, it may be the ice content below the surface that controls polygon development. Soil development and age may be more of a function of the material, aspect and moisture regime, rather than the depositional age of the material in which the soil is forming. The absence of recognizable glacial deposits in Beacon Valley may reflect the activity of the polygons which has destroyed the structure of the moraines making them unrecognizable.