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Wednesday 9 February 2011

The Irish Sea Ice Stream

 Ice directions around 23,000 years ago -- note the Irish Sea Ice Stream or Glacier pushing as far south as -- and even beyond -- the Scilly Islands.

I'm grateful to Prof Chris Clark for copies of two recent papers which deal comprehensively with the growth and wastage of the British-Irish Ice Sheet during the Devensian glaciation.  The illustrations above are acknowledged as from these papers.

The relevance of these papers for our understanding of glacial effects in SW and S England is obvious -- although we may not be talking of the same glacial episode, there is no reason to think that earlier glaciations will have been hugely different from the Devensian with respect to the overall history of the glacial event, the positioning of major glaciers, ice domes and ice streams, or glacier dynamics.

So what are we to make of the Irish Sea ice stream as portrayed?  The first thing to notice is that it is very similar in its dimensions to that suggested by Prof James Scourse with reference to the glacial deposits and radiocarbon dates obtained from the Scilly Islands.  The latest thinking is that the ice stream pushed southwards in a massive surge around 23,000 BP,  pushing southwards from its earlier position by around 300 km, before then retreating at a rate of over 140m per year towards St George's Channel, leaving a temporary lake behind, which was still there around 18,000 BP.  The latest generation of glacial geomorphologists will no doubt discuss this at some length -- but my reaction is exactly the same as when I saw James Scourse's reconstructed ice lobe pushing far out into the Celtic Sea and the SW Approaches.  That is, I don't like the look of it.  I don't believe it is glaciologically feasible to have an ice stream (even a surging one) pushing this far out from the main body of the ice sheet and flanked only to the west by ice-covered or glaciated terrain.  Ice streams, as I understand them, move through the outer parts of an ice sheet, constrained by either terrain or by pressure of flanking ice, and more or less perpendicular to the position of the ice edge or the transition into shelf ice.  There is no deep trough to channel or direct the glacier southwards and to prevent its lateral spread. 

The Bristol Channel cannot have remained ice-free with such a glacier pushing southward through St George's Channel, and I think it quite possible that the ice reached Lundy Island and the coasts of Devon and Cornwall at the same time as it reached the Scilly Islands.  This makes the interpretation of the glacial deposits on the Bristol Channel coasts even more crucial to our understanding of what went on in this part of England.

Clark, C.D., et al., Pattern and timing of retreat of the last British-Irish Ice Sheet, Quaternary Science Reviews (2010), doi:10.1016/j.quascirev.2010.07.019

Reconstructing the last Irish Ice Sheet 2: a geomorphologically-driven model
of ice sheet growth, retreat and dynamics
Sarah L. Greenwood, Chris D. Clark
Quaternary Science Reviews 28 (2009) 3101–3123

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