ABSTRACT: Fresh arguments against the Shaw megaflood hypothesis. A reply to comments by David Sharpe on “Paleohydraulics of the last outburst flood from glacial Lake Agassiz and the 8200 BP cold event”.

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Abstract

We disagree with the premise underlying most of David Sharpe's comments, namely that the Shaw subglacial megaflood hypothesis enjoys sufficient mainstream acceptance that we were negligent in failing to cite it. Although the literature on Shavian megafloods has grown over the past decade, it is less clear that the ideas have gained ground. As a recent datum, Benn and Evans (2005) assert that “most Quaternary scientists give little or no credence to the [Shaw] megaflood interpretation, and it conflicts with an overwhelming body of modern research on past and present ice sheet beds” adding that “the idea of floods of such unimaginable dimensions is the outcome of taking flawed assumptions to their logical conclusion, a form of reductio ad absurdum in which the final absurdity is taken not as evidence of false premises but as fact.” Whatever one makes of these barbed statements, they do not suggest that Quaternarists must accept the megaflood hypothesis as part of their interpretative framework. Others who have examined the hypothesis have drawn attention to the pitfalls of using form analogies (e.g., Benn and Evans, 1998, p. 447) or questioned the flood hydraulics (Walder, 1994). We suspect that these lines of criticism will not deliver a conclusive outcome. We conclude that a fatal problem of the meltwater hypothesis for drumlin formation is the requirement that a huge volume water be subglacially or supraglacially stored, then suddenly released. Efforts to salvage the core idea, that drumlins have a subglacial fluvial origin, should focus on ways to reduce the water volume requirements. Meanwhile, other lines of investigation hold promise. Contemporary West Antarctic ice streams have width scales (Raymond et al., 2001, Fig. 3) that are comparable to that of the Livingstone Lake drumlin field, and submarine deglaciated surfaces beyond the grounding lines of currently active ice streams show drumlin-like features (Shipp et al., 1999). Acoustic imaging of the sea floor off the Antarctic Peninsula also reveals spectacular drumlin-like bedforms and megalineations that are attributed to a paleo-ice stream (Ó Cofaigh et al., 2002). Although these features cannot be linked to a currently active ice stream, the geotechnical properties (Dowdeswell et al., 2004) of the sediments are strikingly similar to those encountered beneath active West Antarctic ice streams. Finally, the subglacial environment is remarkably complex and involves the interplay of thermal, mechanical, hydrological and soil-mechanical processes (e.g., Clarke, 2005). It is conceivable that the problem of drumlin genesis cannot be solved until we gain a better understanding of subglacial process interactions.

Bibliography of Canadian Geomorphology