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Wednesday, 15 February 2017

Wolstonian glaciation of west Wales: an hypothesis



In a previous post, I suggested that there is now so much evidence of strange goings-on in the glacial record of Pembrokeshire that it may well be time to reinstate the Wolstonian as a serious and extensive glacial event.  Here is my earlier post from a month ago:

https://brian-mountainman.blogspot.co.uk/2017/01/time-to-bring-wolstonian-in-from-cold.html

As I have suggested on many occasions, the old Wolstonian limit, as proposed by assorted researchers including Gibbard and Clark, does not make any sense, at least in Wales, so anything has to be an improvement. To repeat -- my reasoning is as follows:

1.  The orientation and scale of the Gwaun-Jordanston meltwater channel system cannot be explained as a product of Irish Sea ice coming in from the N and NW, either during the Anglian or Devensian glaciations.   The channels must have been eroded at a time when there was a strong hydrostatic gradient from E towards W -- that is the undoubted direction of meltwater flow.  So there must have been Welsh ice flowing from east to west.

2.  There are anomalous striae on the north Pembrokeshire coast, which have always puzzled me since they run approx E-W.  All other striae match the expected directions of Irish Sea ice across the north Pembrokeshire coast.  I have not been able to work out the relative ages of the assorted striae, but some of the anomalous ones might well be of Wolstonian age.

3.  The glacial and fluvioglacial deposits that scatter the Devensian "ice free area" south of Mynydd Preseli have to be explained somehow.........  some are very rotten, as at Llangolman, suggesting a pre-Devensian age.

4.  The ancient till deposits at West Angle, Lydstep and maybe New Quay may well be of Wolstonian age.

5.  The frequency of Ceredigion grits and coarse sandstones in the glacial deposits of the Nevern Estuary in Newport suggests to me that they were deposited far out into Cardigan Bay during a Wolstonian glaciation, and then picked up and redeposited by Irish Sea ice during the Late Devensian.

6.  The deep downcutting of Milford Haven may also be partly a result of large quantities of glacial meltwater flowing westwards at the end of a Wolstonian glacial episode on the lines shown on the above map.

What do others think?  Comments very welcome.


Suggested Wolstonian limits of the Welsh ice cap.  That ice cap must have been similar to the modelled Devensian ice cap, but it was probably thicker, with more active ice streams in all of the major radiating valleys.  It was also substantially more extensive. It is assumed that the Irish Sea Glacier was at the time less powerful than during the Anglian and Devensian glaciations -- but there must have been contacts between these two ice masses in the NW, N and NE sectors. 

1 comment:

BRIAN JOHN said...

My comment above about an ancient till at West Angle is subject to revision! Having re-examined the evidence, I am convinced that there is Devensian till at West Angle, but I am not at all convinced that there is an older till as well. Until somebody published detailed evidence supporting the idea of an "ancient till" I prefer to think that there has simply been a mis-interpretation of the stratigraphic sequence at the site.