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Sunday, 27 October 2013

New ice margin map gets it wrong....



I've been looking at the Abstract of a new article about the maximum of the Devensian (Last) Glaciation in the UK -- by some of the key members of a big new project that will hopefully nail down the actual Devensian ice margins on the basis of new fieldwork and comprehensive dating programmes, using a number of different techniques.  This project goes under the label BRITICE-CHRONO, and more detail is here:
http://briticechronoblog.wordpress.com/2013/02/18/94/comment-page-1/#comment-35

The paper is this one:
Bayesian modelling the retreat of the Irish Sea Ice Stream
Richard C. Chiverrell et al
Journal of Quaternary Science
Volume 28, Issue 2, pages 200–209, 25/26 February 2013Article first published online: 14 FEB 2013DOI: 10.1002/jqs.2616

Well, just in case faithful readers of this blog think I spend too much of my time slagging off archaeologists, now I'm going to slag off some geomorphologists as well.  I think they have got the ice margin all wrong in the Pembrokeshire - Bristol Channel area.  They show the maximum ice margin well offshore in the central and southern part of Pembrokeshire, and I am not aware of any evidence that supports that assumption.  I showed pretty conclusively in 1965 that Devensian ice impinged on the Pembrokeshire coast at least as far south as Milford Haven.  There are fresh till exposures at Druidston, West Angle, and West Dale,  and an extraordinary kame terrace at Mullock Bridge  on the north shore of Milford Haven -- all showing the presence of Devensian ice.  it's all published.  So far as I know, nobody has ever contradicted that evidence.  Just because it's old, it doesn't mean it's wrong!  So here is a call to all those authors of the new paper -- show us the colour of your evidence......... or put the ice margins in the right place.

As I have also argued on this Blog, there is fresh till which I assume to be Devensian on Caldey Island as well.  So here is another challenge to all these bright young researchers -- get over there and look at it, and let's see what you think.

And then there is the position of the ice edge in the Bristol Channel.  I do not see any justification for it to be placed where it is.  As I have agued before,  the strange ice lobe which is shown projecting far out beyond the Scilly Islands, far out into the Celtic Sea, is fine by me -- as long as there is a more realistic ice margin on its eastern edge.  This is all a bit technical -- to do with ice gradients and cross profiles.   I want the ice margin much closer to the coasts of Devon and Cornwall.  Maybe there are clues to be found on Lundy Island -- I hope the researchers are planning to go over there so that they can take a look.  Some cosmogenic dating may sort the problem out.

I hope the other Devensian ice margins shown on the map are based on rather more "ground truthing."

POSTSCRIPT

Thanks to Danny McCarroll I have now had a look at the full paper.  There's a lot of very interesting material in it.  But there is nothing to support the placing of the maximum Devensian ice margin in Pembs to the west of Skomer and Skokholm and 50 km to the west of Lundy.  The sedimentary record on the floor of the Bristol Channel is very messy indeed -- as of course it would be if there was a very short-lived advance akin to a surge which pushed a long lobe of ice south-westwards.  But ice streams do not move parallel to unconstrained ice edges -- in such circumstances ice movement is always perpendicular to the ice edge.  So I'll stick to my hypothesis that Devensian ice might well have reached Caldey and Lundy, unless somebody proves me wrong.


7 comments:

Anonymous said...

Brian,

Have the authors considered the Dartmoor Ice Cap during the Devensian? What sense does that pocket of ice-free area along Bristol Channel and Cornwall-Devon coastlines make? in light of this and of the other evidence you have discussed in your blog?

It seems even geologists gingerly tread-on-ice; keeping it out of the Bristol Channel and Cornwall-Devon areas. And so out of the “human transport / glacial transport” controversy. Making MPP and others happy by distorting the only truth that only matters.

I know Chris has been calling geologists to clarify this issue once and for all in your blog. Chris must be happy too! Especially now that he does not have me in your blog to confront his duplicity.

Kostas

BRIAN JOHN said...

That's a perfectly sound comment, Kostas -- I'll allow comments from you as long as you don't get back onto your hobby horse.....

The guys who are putting this map forward should have considered the Dartmoor evidence, and they should also have considered what was likely to have been happening at the time off the coasts of Cornwall and Devon.

chris johnson said...

Duplicity? Sound comment? I find this offensive, mischievous slander.

Brian's piece is as always informative. I look forward to hearing comments from somebody who knows something about the subject.

Anonymous said...

Chris,

”I find this offensive, mischievous slander.”
”I find [cranks] offensive, mischievous slander.”

You ask for ”...somebody who knows something about the subject.”. That would be Stephan Harrison who along with others researched the Dartmoor Ice Cap. Use the search tool in Brian's blog to research long discussions about the Dartmoor Ice Cap that included Stephan.

Kostas

BRIAN JOHN said...

Now then, chaps. Calm down please -- Politeness and respect please....

chris johnson said...

Sorry Brian, I don't buy this.

Accusing someone of being duplicit is highly offensive.

You do not make things better by saying it is a perfectly sound comment - although I think you meant something else.

Kostas is misrepresenting my words when he suggests I called him a crank, although If the cap fits ...

The suggestion that other geologists are in the business of distorting the truth is gratuitously offensive also. When the blog continues in this vein then I will be sadly disappointed.

BRIAN JOHN said...

Chris -- the "sound comment" I referred to related strictly to the bit about the Dartmoor Ice Cap.