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Wednesday, 16 February 2011

The Irish Sea Glacier -- fact or fantasy?

The classic picture of the Irish Sea Glacier -- described in the Wikipedia entry linked above -- is one of a great glacier or ice stream supplied from extensive icefields across the Irish Sea, Northern Ireland and the SW of Scotland, squeezed through St George's Channel and then expanding in a great lobe out into the Celtic Sea and the SW approaches.  I still think this picture is valid for the Anglian and possibly other early glaciations -- and indeed we need such a glacier if we are to explain all the evidence on the ground.

But it now looks as if the Devensian picture was very different.  As shown in the map above, if the Irish Sea Glacier had a "normal" long profile, the ice over St George's Channel must have had a surface altitude around 2,000m, with even higher ice domes to west and east -- this means ice surfaces at c 2,500m over Ireland and Wales.  These altitudes can be reduced somewhat if the passage of ice across the Celtic Sea was facilitated by a saturated substrate of marine muds and a highly deformable and unfrozen bed -- leading to rapid ice movement and surging or "purging" behaviour.  in those circumstances, forward glacier movement can be maintained -- at least for a short period of time -- by ice which has a much shallower surface gradient.  The extent to which we revise these ice surface contours downwards is something for the BRITICE and other modellers to work out!

As suggested in the last few posts, this model may only be applicable to the early glacial phases to have affected the British Isles.  In the Devensian something very strange seems to have happened -- in that the major areas of supply for this glacier seem to have been cut off and displaced by a large snow dome over the Midlands of Ireland.  Can we find an explanation for that in climate modelling?  Time will tell...

Perhaps we should not be too surprised if there were vastly different glacier dynamics and ice limits in the different glacial episodes on the west side of the country.  In eastern England (and to some extent in the Midlands) the pattern of glaciation between one glaciation and the next  did vary enormously, with the Devensian ice edge some 200 km further north than the Anglian ice limit, which was not far from London.

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