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Thursday 5 November 2009

More thoughts on ice movement

(This map has been tweaked since the original posting. I had the ice-shed over Mid Wales too far to the east. I have now shifted it westwards, closer to the Cardigan Bay coast -- which of course wasn't there at the time.......)

I'm still waiting to see how the models of the Greatest British Glaciation (GBG) turn out, but I'm more and more convinced that the contact zone between the Welsh Ice and the Irish Sea ice must have had "waves" or wobbles in it. As the Irish Sea Ice was thinning on its way up the Bristol Channel, I think it would have been pressurized by the sheer weight of ice coming off the Welsh Uplands -- the mid Wales plateau and the Brecon Beacons. The ice would have been flowing fastest in the main troughs -- the valleys of the Tywi, Tawe, Neath, Rhondda, Cynon, Taff, Usk and Wye. A vast amount of ice must also have poured over the interfluves. This must have pushed the contact zone out from the present coastline of South Wales.......

This is a suggestion, but may explain the relative paucity of Irish Sea Till on the coasts of Gower and in the vale of Glamorgan. On the map the black line is the reconstructed ice limit for the last (Devensian Glaciation) which is not really relevant for the GBG except insofar as it shows where the main outlet glaciers were. The red arrows show Welsh Ice movement directions; the orange arrows show the movement of Irish Sea Ice; and the suggested contact zone is shown in blue.

At various stages of the glaciation the pressure of Welsh ice may have pushed the Irish Sea ice even further to the south.

The entrainment zone which I have postulated for the Dinas Head - East Preseli area may just have operated for a short time -- maybe that was displaced also at the peak of glaciation, with or without any entrainment going on.

Comments, anybody?


Chris R said...

A short lived entrainment zone as you suggest would presumably mean fewer erratics being transported. Could that therefore explain why we don't see much evidence of large boulder erratics now?

What I don't understand is if the stonehenge bluestones are a representative sample of the transported erratics why are there apparently more from sources in the Preselis than from say the Brecon Beacons. Would a different entrainment process operate in the 2 different regions which favoured extraction from the Preselis?

BRIAN JOHN said...

Yes, this is an interesting problem. Short-lived entrainment is the norm rather than the exception -- because you need quite exceptional glaciological conditions for entrainment to happen on a glacier bed (or by rockfalls ONTO a glacier, as in the case of the Foothills Erratic Train in the USA). Glacier beds beneath ice sheets are often "protected" with nothing much going on; in some cases there can be streaming and areal scouring, but if the bed is frozen there will be no erosion. So a cluster or short stream of erratics would not be a problem for a glaciologist.....

On the matter of the Altar Stone and the transport of erratics from the Brecon Beacons, see my late October posts. The Altar Stone erratic must have been transported southwards initially by Welsh ice, and must then have been transported eastwards along the contact zone. My current theory is that the ice carrying this mixed material (Welsh and Irish Sea erratics) came in on the north side of the Mendips -- and we know that there are some interesting erratics at Stanton Drew.

Another p[ossibility is that the Altar Stone was carried southwards during one ancient glaciation, dumped somewhere near the South Wales coast, and then picked up and carried eastwards by the Irish Sea glacier in a later glaciation. "re-entrainment" of this type occurs wherever glaciers and ice sheets operate, since the sequence of events in a glaciation is never precisely duplicated.