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Sunday, 9 January 2011

Those Little Ice Caps



What did the little ice caps of Dartmoor, Exmoor and the other uplands of SW England look like?  Well, when they were reasonably healthy, they probably looked like Drangajokull, a small ice cap in Vestrirdir, NW Iceland -- shown in the lower of the two photos above.  The current ice cap is about 20 km long and 6-7 km wide , and it was at one time much larger.  Look at the lower right quadrant of the photo -- the barren area with lots of lakes has clearly been deglaciated during the last 100 years or so.

And how long did they survive for? Little ice caps like this can be created, and can then disappear, very quickly indeed.  It would be quite normal for ice caps like this to develop over a period of 500 years or so, and then to disappear again in maybe 200 years.  Because they sit on upland plateaux, a cooling of climate, accompanied by a fall in the firn line or equilibrium line to a level below the edge of the plateau can trigger rapid accumulation and growth, with very little loss of mass through melting.  On the other hand, if the firn line rises maybe 50m because of climate warming, suddenly the whole of the ice cap becomes vulnerable, and there can be catastrohic melting.

The Glama Plateau (top photo) is a case in point.  It's not very far from Drangajokull, but it is very vulnerable.  In the Middle Ages there was no ice cap here.  Then came the Little Ice Age, and a plateau ice cap started to develop in the 1500s.  By the time of the coldest part of the Little Ice Age (c 1750 - 1850) it may well have looked like Drangajokull.  Then the little Ice Age came to an end, and catastrophic melting occurred.  By 1900 it was mostly gone, and now there is just an assortment of snow-patches, with a rocky barren terrain and lots of lakes.

So what about the Exmoor and Dartmoor ice caps?  Because they were effectively beyond the southern limit of ice sheet glaciation, they will have experienced glacial conditions only for a very short period of time. In western Scotland, at the centre of the British - Irish ice sheet, it's reasonable to assume that ice covered the landscape in the Devensian for more than 20,000 years; but the ice that reached the coasts of Devon and Cornwall may only have been there for around 1,000 years or less.  I would guess that the little ice caps of the South-West were in existence for 2,000 years at the most, around 21,000 - 19,000 BP.  They just sat there, as Glamajokull did in the Little Ice Age, and then melted away.  The effects on the landscape were minimal and even protective, since periglacial processes under the ice would have been temporarily stopped.  Maybe, beyond the ice edges, there was a phase of very intense cryoturbation and ice wedge formation.  And maybe, when the ice melted, there would have been another phase of  periglacial slope development, with a good deal of summer melt and even resumed water flow in the river valleys.

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