SIR, Ice mounds on frozen lakes in McMurdo Sound, Antarctica

In the course of a two-month stay in McMurdo Sound, thanks to assistance from United States expeditions, I observed on the surface of frozen lakes mounds of ice elliptical in shape and crevassed. They appear to be a common feature in this region, and it would be interesting to know how they originate.

One was seen at McMurdo Sound about 0.9 km. to the north-east of the U.S.A. Station at a height of 127 m. in the centre of a small crater lake. Further to the north-north-east of the Station, 5 or 6 mounds were found in a frozen lake at the terminus of the big glacier to the north-east, at an altitude of 140 m. At Cape Royds, 2–3 km. to the south-east of the hut, on two small elliptical lakes there was a mound on each. In Victoria Valley, at the end of the lower Victoria Glacier, there was one mound. On Lake Vida there was at least one big mound. Finally, on the McMurdo Ice Shelf below Brown Island, amid a series of moraines, a number of small frozen lakes were seen from the air, some of which contained radiating crevasses reminiscent of the design of the preceding mounds, but closer observation was not possible owing to the danger of landing.

In the little crater lake 0.9 km. to the north of the Station there was a mound about 12–15 m. in diameter, with its top about 0.5 m. above the level of the lake. The crevasses in it (Fig. 1) were 8–10 m. long, about 5 m. apart, and 10–15 cm. wide at the top. At the top of the mound the ice was in columnar crystals which, viewed from above, formed a cellular network, each crystal being about 2–3 cm. in diameter. The centre of the crystals was transparent. the margins more opaque or white (Fig. 2). Their upper surface in contact with the air was melting. On the lower margin of the top one could see nearly vertical crystals projecting upwards some 5–8 cm., about 2–5 cm. wide, with 3–8 cm. space between. In places on these small pillars there was a crust of snow probably belonging to a former surface before melting. In contrast to the ice crystals of these mounds, the surrounding snow was typical of that in the McMurdo region–loose, powdery and line grained.

Fig. 1. Crevasses in the ice mound

Fig. 2. Same place, upper melting surface, showing the lattice formed by the nearly vertical crystals

About 500 m. further north-north-east, on the frozen lake where the glacier from the north-cast peters out, one could see 5 or 6 mounds, some oval (Fig. 3), some more elongated. The ice was blue and clear in small vertical columns joined together. The surface showed the same cellular network. There were crevasses which cut across the mounds. At Cape Royds (Fig. 4) there was a much fractured mound. 6–8 m. in diameter, with radiating crevasses. In the lake at the front of the Victoria Glacier the mound was 1–2 m. high, and 10 m. in diameter. The radial crevasses were 20 cm. wide and 60 cm. deep. On the wall the ice was clear blue and furrowed by vertical lines which were very conspicuous. At the surface the design was the same as before with a wide cellular network on a transparent base.

Fig. 3. Ice mound on the frozen lake

Fig. 4. Strongly crevassed small ice mound in a oval-shaped lake

AndrÉ Cailleux

Laboratoire de Géologie du S.P.C.N., 12 rue Cuvier, Paris 5e. 27 September 1961