Cape Denison landscape
Cape Denison is a rugged, 1.5 km wide tongue of ice, snow, rock and moraine projecting into Commonwealth Bay from the steeply rising ice cap of continental Antarctica. The ice cliffs at either end of the Cape (Land’s End and John O’Groats) and the sea hemming the northern shore form a natural sense of enclosure.
The most striking feature on approach to Cape Denison is the towering height, behind the cape, of the massive Antarctic ice cap. It extends as far as the eye can see up and down Commonwealth Bay and terminates in huge ice cliffs which periodically calve into the sea as icebergs.
The topography is defined by a series of four rocky ridges, running south-southeast to north-northwest, and three valleys filled with ice, snow, and glacial moraine. The largest, most westerly valley contains the four AAE huts. At the seaward end of this valley is Boat Harbour, a 400 m long indent in the coast.
The landscape is strewn with glacial deposits that have left large boulder fields, many of which are now covered with colourful lichen, the only flora on Cape Denison. There are six melt water lakes associated with glacial action.
Cape Denison provides suitable habitat for breeding colonies of Adélie penguins – the principal fauna on Cape Denison during summer – primarily at the eastern and western edges of Cape Denison and on Penguin Knob at the northern edge of Boat Harbour. The perimeters of the colonies are guarded by scavenging south polar skuas. Other nesting sea birds include snow petrels and Wilson’s storm petrels. Weddell seals (and the occasional elephant seal) rest around the margins of Boat Harbour.
Wind is the dominant feature that has shaped the Cape’s occupation, and continues to define the landscape. It makes it different from most other Antarctic landscapes, and sets it apart from the sites of other Heroic Era huts. Sun, cloud and seasonal changes in daylight and darkness are largely irrelevant compared to the cycle of katabatic winds that creates an annual average daily maximum wind speed of 71 km per hour (the windiest place on earth at sea level). Frequent blizzards and gusts exceed 100 km per hour: in 1913 the wind was recorded at 143 km per hour for twelve continuous hours.
Humans perceive this wind as a constant force (rather than the perception of eddies and gusts usually associated with winds). This force, always from the south, carries huge amounts of drift snow and ice before it, often creating blizzards–hence Mawson’s Home of the Blizzard – and whips the sea into a chop, topped with fierce spume only metres from the shore. Surface temperatures, not including the wind chill factor, generally range from –21° and lower in winter, to –3° in summer, with occasional days approaching zero.
The average wind at Commonwealth Bay is thus a gale … A remarkable feature of the wind is its extraordinary steadiness… After some practice the members of the expedition were able to abandon crawling, and walked on their feet in these 90-mile torrents of air, ‘leaning on the wind’.
– Cecil Madigan, Meteorological Records of the Cape Denison Station, AAE Scientific Reports Series B (1929)
The area contains many geological features that are important for the understanding of the Gondwana break-up 55 million years ago, as it was roughly opposite what is now the Gawler Craton in South Australia. Little has been studied on the geomorphology of the area since the AAE report of Stillwell (1918). The area can be divided into a ‘lower zone of relatively polished rock and a higher zone of relatively unpolished rock’. Glacial plucking is common and generates a roches moutonnee effect with gentler, smoother surfaces towards the ice source and a rougher, more plucked downslope area. There are abundant glacial erratics and striated surfaces.
The ‘upper’ moraine, close to the ice edge and containing a great diversity of rocks, many unknown in outcrop in the area, is a genuine moraine. Boulders are more angular and sorting less obvious than in the ‘lower’ moraine. The rocks, including little studied red sandstone and crystalline limestone, from which no fossils have been recovered, may provide an insight into the rocks that underlie the ice of this part of Antarctica.
The ‘lower’ moraine is generally below 12 m above sea level and is dominated by local rocks: this may be a result of ‘ice push’ from the sea rather than a genuine glacial moraine. The boulders are more rounded and sorted to some extent into bands where grain size is more constant. This material shows some signs of having been water worn and includes some lithified beach sand with foraminifera and other organic remains.
Valleys and lakes (with one exception) are generally oriented parallel to the foliation of the basement rocks. The basement of the Cape Denison area consists mainly of partly migmatised, massive felsic orthogneiss intruded at about 2350 million years ago into an older metamorphosed sequence, originally of mudstone, perhaps Archaean in age. This entire sequence was intruded, probably at about 2350 million years ago, by mafic dykes which were metamorphosed at about the same time.
The AAE was the principal expedition to describe and map the natural features of Cape Denison. Additional work was done by expeditions primarily focused on cultural heritage features, including ANARE in the 1970s, Project Blizzard in 1984–1986, Don and Margie McIntyre in 1995 and the AAP Mawson’s Huts Foundation expedition in 1998.
The carbonate sediments from which these rocks have been derived were characterised to a greater or lesser degree by the presence of detrital material, which in the process of metamorphism has reacted with the carbonate minerals. This group of rocks, however, with two exceptions, still possesses a content of free carbonate material. Its quantitative amount is dependent in some cases on the degree of metamorphism of the rocks concerned, and in others on the quantity of foreign material present in the original sediment, capable of chemical reaction with calcite or dolomite.
– C.E. Tilley, Geology, AAE Scientific Reports Series A (1923)