Sir, Ice action on lakes

As a contribution to the documentation of the occurrence of lake features due to lake ice action, the subject of interesting observations by L. Goldthwait reported in the last issue of the Journal, Vol. 3, No.22, 1957, p. 99–103, may I draw attention to certain Tasmanian examples which are unlikely to be known to many of your readers.

The examples come from the Central Plateau between 2000 and 4000 ft. (610–1220 m.) in elevation, where there are many lakes, the smaller ones of the west being certainly the product of Pleistocene ice action, the larger ones of the centre and east more probably the result of tectonic movement (J. N. Jennings, The legacy of an icecap, Australian Geographer, Vol. 7, 1957, p. 62. The Great Lake falls into the latter category, and it is from this lake that there is an early description of phenomena of the type under discussion by W. V. Legge (A contribution to the physiography of Tasmania, Proceedings of the Royal Society of Tasmania, 1902, p. 138; A physiographical account of the Great Lake, Tasmania, Report, 10th Meeting of the Australian Association for the Advancement of Science, 1904, p. 348) . Nearly uniformly sloping 35° ramparts of dolerite boulders, which rise 3–7 ft. (0.9–2.1 m.) above lake level, are attributed to the pressure of ice floes, even though he recognized that some of these ramparts faced the north-east, from which direction strong winds are infrequent. Ice expansion pressure seems a more Iikely genetic process. Similar features have been seen by the writer in various other lakes of the Central Plateau.

There are in addition at least four instances of lakes divided or very nearly divided into two by ice-built ramparts of boulders—Double Lagoon, First Bar Lake, Second Bar Lake and Lake Ina. The names of the first three do of course draw attention to their special character of twin lakes. These are briefly referred to in the paper of mine cited above but had previously been observed and interpreted by Professor S. W. Carey of the University of Tasmania. The first three are shallow lakes in glacial drift, the last is partially rock-rimmed and partially moraine-dammed. The drift derived from dolerite consists of large dolerite boulders set in a fine matrix as a rule, with usually comparatively little material of intermediate size. These lake-dividing ramparts must be due to ice push from different centres of expansion, operating on the boulders projecting from the shallow drift floors.

As in New England, ice expansion seems more important than the work of ice-floes, though the latter do play some part on the larger lakes of the Central Plateau.

There are in addition at least four instances of lakes divided or very nearly divided into two by ice-built ramparts of boulders—Double Lagoon, First Bar Lake, Second Bar Lake and Lake Ina. The names of the first three do of course draw attention to their special character 0f twin lakes. These are briefly referred to in the paper of mine cited above but had previously been observed and interpreted by Professor S. W. Carey of the University of Tasmania. The first three are shallow lakes in glacial drift, the last is partially rock-rimmed and partially moraine-dammed. The drift derived from dolerite consists of large dolerite boulders set in a fine matrix as a rule, with usually comparatively little material of intermediate size. These lake-dividing ramparts must be due to ice push from different centres of expansion, operating on the boulders projecting from the shallow drift fl0ors.

As in New England, ice expansion seems more important than the work of ice-floes, though the latter do play some part on the larger lakes of the Central Plateau.

J. N. Jennings

Geography Department, Research School of Pacific Studies, Australian National University, Canberra 12 November 1957