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Wednesday, 9 February 2011

The Carningli Nunatak

Most of us who are thinking about where the edges of the Devensian Irish Sea Glacier might have been are now convinced that on the north Pembrokeshire coast the ice pressed inland as far as the upland ridge of Carningli - Dinas mountain.  Danny McCarroll (see previous post) thinks that the ice did not override the summit of Carningli, and has the cosmogenic dates to prove it.  But I still have not managed to find out PRECISELY where the relevant rock samples were collected, and I am still convinced that there are fresh glaciated slabs very near the summit.  The question is "how fresh?"  Are they fresher than the glaciated slabs at Carn Meini and on other tors on Mynydd Preseli?  More work needs to be done to sort this out.

Because there are hummocky areas of sands and gravels on the south side of Carningli, near the entrance to Cwm Gwaun, I think that the ice overrode at least a part of Carningli, and that it might have looked like the above photos around 20,000 years ago at the height of the Devensian glaciation.  I also think that the south flank of the mountain was the site of a large windscoop, within which a substantial scree bank was able to accumulate.  How long it took for the whole scree bank to form is another question -- it's possible that it was forming throughout the cold parts of the Devensian when the "main head" was accumulating on the coast and on hillsides everywhere.  But why is there no thick scree bank on the NORTH side of Carningli?  I suspect that the ice might have "cleaned out" whatever scree or rockfall debris there was.

Not far away, at Pont Ceunant, we have the "Pont Ceunant moraine" which may represent the maximum position of the Devensian ice on the north flank of the mountain, or it may simply represent a "stadial" or stillstand position where the ice edge remained in the same place for decades or maybe even centuries.  See the post for 3 May 2010.

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