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Monday, 1 November 2010

Dewatering of warm-based ice

I've been looking at James Scourse's 1997 chapter in the big Stonehenge book, called "Transport of the Stonehenge Bluestones: testing the glacial hypothesis."  It's an interesting chapter designed to systematically demolish the theories of Kellaway and Thorpe et al about glaciual transport.  It's particularly interesting because the archaeological establishment has decided to accept it as THE glaciological and geomorphological assessment of the situation -- because of course it all sounds very learned, and because it suits them very well to be able to say "The best glaciological advice we have is that glacial transport of the bluestones to Stonehenge was impossible."    James Scourse himself, in the last sentence of the chapter, claims that he has "eliminated the impossible" from the debate about bluestone transport, thereby agreeing that the human transport of the stones -- however improbable -- actually happened.

As I have said before, scientists should not really use the word "impossible" because it may come back like a boomerang and hit them between the eyes.  I will return in due course to some to James's glaciology, which I think is pretty dodgy or selective (I think he puts up a number of Aunt Sallies just in order to knock them down), but for the moment I just want to think about the implications of ice close to an ice sheet margin flowing across highly permeable bedrock -- namely chalk.  I came across some discussions in the North American literature to the "dewatering" of warm-based ice when it is flowing across limestone or chalk.  This would presumably slow down or stop basal sliding and would then only permit movement by internal deformation, shearing etc.  What would the implications of this be for till emplacement and for the transport and dumping of erratics?

I'm of course aware that in Eastern England there is a lot of chalky till around, which implies that chalk was easily eroded by overriding ice and that the ice that did the eroding had plenty of basal lubrication ..........

Not sure who reads this blog, but if anybody has any thoughts on this, all comments gratefully received!

4 comments:

PeteG said...

how would a receding glacier create a landscape like this?
http://i296.photobucket.com/albums/mm200/TJJackson66/005-17.jpg

Do you know of any similar landscapes?
Pete

BRIAN JOHN said...

No problem at all, Pete. Not sure where the photo was taken -- but it is classic dead-ice topography, formed when a mixture of tills of all sorts, sands and gravels and even lake deposits is dumped into an area where masses of detached ice lie beyond a retreating ice edge. The situation is pretty chaotic -- and can be very dangerous when you walk across an area like this when large masses of ice are still melting out. Because the debris can be very thick, some ice masses can take centuries to melt out. Slopes are very unstable too -- and masses of debris can flow or slide catastrophically down into lakes or into fast-flowing glacial streams. A guy was killed near the Cook Glacier in NZ when an ice-cored slope that he thought was safe suddenly gave way. Had a few close shaves myself, over the years......

If you look at my posts about the outer edge of the Roslin Glacier in Greenland, you can see the sort of environment we are talking about.

BRIAN JOHN said...

Type Ivar Baardson into the search and it will come up. Click on the Google image to enlarge it -- at a guess I'd say that the chaotic mess beyond the glacier snout would have been pretty similar to the origins of that nice bucolic pic to which you gave that link. Allow 15,000 years for the grass to grow.......

BRIAN JOHN said...

Sorry -- spelt that wrong This is the link:

http://brian-mountainman.blogspot.com/2010/09/ivar-bardarsons-glacier-then-and-now.html