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Tuesday, 24 May 2016

Isles of Scilly -- where are the meltwater deposits?


These are interesting images of the Isles of Scilly.  The first is a NASA image, and the lower one shows the position of the coastline about 5,000 years ago.  The area shown green on the map was not inundated by the rising sea level until after this date -- and maybe this is where all the meltwater deposits are located.  No matter where the actual maximum extent of the Devensian ice might have been, the ice must have come in from the NW (ice flow is almost always broadly perpendicular to the broad alignment of the ice front).  Some of the other geomorphologists writing about the Scillies seem to have assumed that the ice must have flowed from the NE towards the SW, but that defies all the glaciological rules!

The actual ice edge, when seen in detail from a satellite, must have been crenellated or fingered, with lobes of ice flowing into the straits between the islands.  I have reconstructed this ice limit on an earlier post.  Because the ice was rather thin, when it wasted away almost all of the meltwater flow must have been within these lobes, and largely at the base of the ice.  So we have to assume that the meltwater deposits which must have been laid down as the ice melted away are all beneath present sea level, in the area shown green on the map.  They must also have been reworked and redeposited by wave action as the sea rose, and some of the material must have been incorporated into the modern sandy and gravelly beaches that fringe the islands.  I am not aware that any work has been done on this yet -- but some coring of the sediments within this area might yield interesting results.


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