Box 95 Drumlins Formed By Subglacial Meltwater

Over the past 20 years John Shaw has championed the controversial idea that drumlins are formed by subglacial floods. The idea stems from the similarity in morphology of drumlins with erosional marks at the base of turbdites It rests on the premise that inverted erosional marks at the ice bed are subsequently infilled to form drumlins, as illustrated below. The recognition in the 1980s that many drumlins have a core of sand and gravel gave support to this hypothesis. Significantly, the theory implies that drumlins may not reflect ice-flow direction and that outbursts of subglacial meltwater are critical to landform evolution. Since its inception this theory has, however, proved extremely controversial, attracting fierce debate (Benn and Evans, 2006; Shaw and Munro-Stasiuk, 2006). Its opponents have focused on several key issues: (i) the mechanical implausibility of large areas of an ice sheet or ice margin 'being lifted off' the bed by flood waters and the absence of water sources sufficiently large enough beneath ice sheets to give the required floods; (ii) the

absence of evidence for massive outburst floods other than the drumlins themselves; (iii) the model is not supported by modern analogue observations and drumlins revealed by ice retreat since the Little Ice Age show no evidence of meltwater as a forming process; and (iv) large floods at modern glaciers such as in Iceland have not produced drumlins. Despite these problems some scientists still hold to the idea that subglacial floods provide a possible mechanism of drumlin formation. At its heart the idea challenges the concept of uniformitarinasm - the key to the past is the present - and to accept it one must invoke a different set of glacial processes in the past than operate now.

Source: Shaw, J. (1994) A qualitative view of sub-ice-sheet landscape evolution. Progress in Physical Geography, 18, 159-84. Shaw, J. (2002) The meltwater hypothesis for subglacial bedforms. Quaternary International, 90,5-22. Benn, D.I. and Evans, D.J.A. (2006) Subglacial megafloods: Outrageous hypothesis or just outrageous? in: Glacier Science and Environmental Change (ed. P. Knight), Blackwell, Oxford, pp. 42-6. Shaw, J. and Munro-Stasiuk, M. (2006) Subglacial megafloods: Outrageous hypothesis or just outrageous? Reply, in: Glacier Science and Environmental Change (ed. P. Knight), Blackwell, Oxford, pp. 46-50. [Diagram reproduced from: Shaw (2002) Quaternary International 90, figure 1, p. 6. Copyright © 2002, Elsevier Ltd].

0 0

Post a comment