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Tuesday, 29 November 2016

Cwm Cerwyn



One doesn't often see Cwm Cerwyn in the right light conditions to appreciate its form and location.  It's the deep cwm or hollow on the lee (eastern) flank of Foelcwmcerwyn or Preseli Top.  In this photo the winter sun is very low, and the greater part of the cwm is in shadow.

There are slight morainic landforms on its floor, and till is exposed in some of the stream cuttings -- so I think that this is where the last trace of glacier ice in Pembrokeshire was located.  I reckon there might have been a Younger Dryas (Zone III) glacier here, at the same time as other small glaciers were being regenerated in other parts of the British uplands.  That's only about 10,500 years ago.........

The location is perfect for a small niche or corrie glacier.  East facing, plenty of shade in the winter part of the year, and on the downwind side of an upland plateau -- excellent for the accumulation of windblown snow and for its conversion into ice.

I also suspect that the whole of the area shown in this photo was covered by a Preseli Ice Cap during the Late Devensian glaciation about 22,000 years ago, when the ice of the Irish Sea Glacier was pressing against the northern flank of Preseli, just over the horizon to the right......

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