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Thursday, 10 June 2010

Isostatic loading and rebound in Southern England

Giant's Quoit, Porthleven, nr Helston, Cornwall

Following my recent posts, I have been chasing around the geological and geomorphological literature on this topic, with remarkably little success. All I can find is statements like "There has been isostatic rebound in Scotland, but not in Southern England, where there was no glaciation." That appears to be the fixed view. Then, in all of the comprehensive descriptions of the erratics of the Channel Coasts and the coasts of Devon and Cornwall, many authors (even the learned fellows who contributed to the Geological Conservation Review for SW England) say "One possibility is that the erratics were transported by floating ice -- but on the other hand sea-level would have been so low at the time that that becomes a considerable problem." Some authors seem to get themselves into very contorted positions in order to avoid mentioning the phrase "glacio-isostatic depression." OK -- come on, you guys. If you don't want full-blown glaciation of the Channel Coasts (as proposed by Kellaway and others) you cannot explain the emplacement of all these boulders without considerable crustal loading of the south-west quadrant of England. And how do you get this crustal loading without an ice load? Answers on a post-card please.......


Kostas said...

Good post Brian.

Just catching up with all your recent posts. Your point about coastal sarsens is the key. There are only two ways that these sarsens could have been placed along the coast. Either by sea, with a flotilla of ice-boats carrying sarsens, or over land through the agency of glacier ice. If the source of these sarsens can be scientifically established, then we could have our answer. Any geologists working on such important identification? Perhaps Ixer?


Brian said...

Actually there is no great problem relating to the sarsens. If you look at the article by Ulyott and Nash 2006 you'll see from their map that there are sarsens everywhere! And they imply that they were formed in a variety of different ways, depending on geological groundwater circumstances. They are differentiating different kinds of sarsen too -- so maybe the day is not far off when they and other researchers will be able to tie down particular sarsens (including those at Stonehenge) to particular geological locations.

Brian said...

What I'm suggesting there is that the sarsens on the South Coast and elswehere are not necessarily far-travelled. Most of those found today might be quite close to their places of origin. The real erratics of gabbro, granite, schist etc are much more of a problem -- that's where ice rafting and isostasty come into the frame.