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Tuesday, 4 May 2010

Stonehenge and the Pont Ceunant Moraine

You might wonder what on Earth the Pont Ceunant Moraine has to do with Stonehenge. Well, it has quite a lot to do with it. If the moraine relates to something like the Last Glacial (Devensian) maximum ice extent of the Irish Sea Glacier, this means that the ice surface in this area was probably not higher than 300m. This means that the ice will have pushed well up the Teifi Valley and probably pressed against the north face of Preseli and the north face of Carningli, without overriding the Carningli upland. The line isn't that different in North Pembrokeshire from that of the famous "South Wales End Moraine" drawn by Charlesworth in 1929. The Devensian lake deposits and glacial deposits of the Teifi Estuary are within the line, as are the abundant sands and gravels (including eskers and kames) of North Pembrokeshire, as are the fresh tills of the North Pembs coast, as are the rounded tors between Fishguard and Cardigan.

The map above is based on Bowen and various others -- in many publications. I have crossed swords with DQB many times in the past, but I think I agree with him here -- the evidence does stack up.

I'm pretty sure that the ice crossed the mouth of Milford Haven -- there are ice-contact deposits at West Angle -- and then there must have been a lobe pressing into the Bristol Channel. How far did that lobe extend? Maybe as far as the coasts of Devon and Cornwall, but maybe not as far as Somerset. It must have been a pretty big lobe, if the ice did indeed reach the Scilly Isles in the Devensian.

Now we have to look at ice surface profiles -- which must have been remarkably low..... and that's another issue to be examined.


Kostas said...

Brian, I have a question.
In your opinion, is it possible for an ice sheet to melt in place without ice flow and not leave behind any tale-tale traces of it being there?

Brian said...

This is one of the great questions of glacial geomorphology! See my entry on Jameson Land a while back......

Cold-based ice frozen to a glacier bed can have a protective rather than an erosive role -- so in theory quite complex sediments (including peats and other "soft" materials) might be frozen solid, effectively transformed into rock, and then left largely unmodified after the ice has come and gone.

So we might get areas where there are "free boulders" left as erratics with no other glacial traces in the neighbourhood -- or even quite fragile tors surviving where there has been no "streaming" of the ice. But to be sure of ice coming and going with virtually no effect upon the landscape, you would need to assemble info from many different disciplines -- and there would have to be some info somewhere -- eg in the pollen record -- to demonstrate a slide to glacial conditions and then a recovery afterwards.

I see where you are coming from and heading towards, Kostas -- but I would still be very wary of talking about ice on Salisbury plain 20,000 years ago!!

Kostas said...

Thanks Brian. And be sure you made your position about ice cover of Salisbury Plain 20,000 years ago very clear to me! But from what you have said in this and previous posts, the 'evidence' against NOT being an ice cover at Salisbury Plain during Stonehenge construction is VERY SOFT! Basically, the argument goes like this: “we don't have firm evidence that there was an ice cover at Salisbury Plain 20,000 ago, therefore there was no ice cover 20,000 years ago!” The softeness of the argument that there was no ice cover, in combination with the ability of this theory to explain ALL OTHER ENIGMAS OF STONEHENGE makes for a strong logical argument that there was ice when Stonehenge was made. The exact time may be unknown and cover a very broad period going back to early Paleolithic period. But as long as there were local people and there was ice cover my explanation works.

Brian, the many stone alignments and stone circles in the UK and in Brittany are incredible 'prehistoric writings' left behind by our ancestors to tell us geological conditions back then. This story should fascinate scientists and common people more than the fabricated mysteries build up over Stonehenge! I shiver just thinking about this! (maybe it's the ice!)

I think you should re-read my paper, “The un-Henging of Stonehenge”

PS Where can I find your entry on Jameson Land that you mentioned in your post?

Constantinos Ragazas

Brian said...

Jameson Land -- 6th Dec 2009:

Scroll through the list on the right.

I don't like using the word "impossible" -- but sadly, your theory does NOT explain all the other enigmas of Stonehenge. There is a mountain of evidence militating against ice on Salisbury Plain 20,000 years ago. I think I accept most of it. Please go off and read the Geological Conservation Review for SW England -- it's a big book -- and then come back to this issue. The selective plucking of evidence -- and the wilful neglect of large areas of "inconvenient" evidence -- is unscientific. What I am trying to do is examine the pros and cons as honestly as I can, with a hope of eventually getting to the truth.