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Wednesday, 25 January 2017

Devensian ice edge in Pembrokeshire

I have been working on several new sections and maps for the forthcoming Pembrokeshire Historical Atlas, and in thinking about the Devensian or LGM glacial limits in Pembrokeshire this is the best I can come up with.  The line is partly based on BGS mapping (which seems to me to be pretty accurate) and partly on my own observations.  Note the following:

1.  The ice edge along the north face of Mynydd Preseli seems to me to be fine, as mapped by BGS, and I have written many times on this blog of an apparent trimline and marginal meltwater channels in the vicinity of Carn Goedog and Carn Alw.  But I now think that in places there was an ice contact here, between Irish Sea ice pressing in from the north (and completely covering Carningli and Dinas Mountain) and rather thin and stagnant ice belonging to a small Preseli ice cap.  That accords with the modelling by Henry Patton and others.

2.  I think that parts of eastern Pembrokeshire were affected by Welsh ice coming down from the north and north-east.  Some of it was related to discharge from the Teifi and Tywi Glaciers.  More work needs to be done on this.

3.  I have deliberately left out of the equation the most spectacular Ice Age features in Pembrokeshire -- namely the Gwaun-Jordanston meltwater channel system -- in the area south of Newport, Fishguard and Mathry.  That's because I am convinced the meltwater channels are not Devensian, having been formed in either the Anglian, or more likely the Wolstonian, glaciation.  See other posts on this.  The channels were used in the Devensian, but probably not altered very much.

4.  The map shows just a few of the till locations, moraines and flubvio-glacial accumulations in the county.  There are many more that could have been shown.

5.  The underlying thesis here is that although the Irish Sea glacier (or ice stream) affected western Pembrokeshire, the driving force came not so much from the north as from the west.  That means that there was very thick ice over Ireland during the LGN,  pushing eastwards.  Ice must have pressed far into the Bristol Channel and against the coasts of Devon and Cornwall at this time and also into the Scilly Islands archipelago.  Fresh till was deposited on Caldey Island.  So most of southern Pembrokeshire must have been ice covered -- although the evidence is scanty.

6.  The big question mark in all of this is the apparent ice-free enclave in SE Pembrokeshire.  I don't really like the look of it, but will go with it for the moment.  Of course, there are glacial and fluvioglacial deposits here, as identified by the BGS officers.  I have examined many of them myself in the past.  They are subdued, and look rather weathered, so for the moment I'll assume they are most likely to be of Anglian or Wolstonian age.  But I am aware of the opinion of Prof Danny McCarroll that areas like this, apparently beyond a recognisable ice edge or trimline, could well have been affected by thin cold-based ice.........

All up for discussion.  Opinions please.....

Some of the glacial (blue) and fluvioglacial (pink) deposits identified by BGS fieldworkers in SE Pembs -- are they Devensian, or are they older?

This is the limit of the Devensian as shown on the BRITICE map of the British Isles.  The cream-coloured area was supposedly ice free at the LGM.  The brown areas show the distribution of some -- but not all -- patches of fluvioglacial materials beyond the supposed ice limit.


MoA said...

Is there an importance to the term stagnant ice?

No hope for local northward moving ice on the northern slopes of the Preselis?

BRIAN JOHN said...

Should have been more specific. Stagnant ice can be either ice that is shut off from its supply, so that it just sits there, quietly melting away. Or it could be warm-based ice that simply can't go anywhere, because it is blocked up against an upland ridge transverse to ice flow or because it is stranded between two ice streams and separated from them by shear zones. yet another possibility is that the ice is too thin to reach the pressure melting point on the bed -- so it remains cold-based, moving just a little by internal deformation but with no basal sliding.

Re northward flow from a Presely ice cap, yes, that is a distinct possibility, although the glacier modelling boys would probably say the ice cap was so small that it remained cold-based and therefore incapable of much work. I have a gut feeling that there might have been northward flow at some stages, and if that happened then dolerite and other erratics might have been carried northwards and dumped closer to the coast. There could certainly have been substantial fluvioglacial meltwater flow during the wastage of the ice cap -- this might account for the orientation and depth of the meltwater gorge at Rhosyfelin. An interesting little problem........