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Tuesday, 30 November 2010

Isostatic depression in Somerset and Wiltshire



Back in June, I put up a number of posts relating to isostatic and eustatic interactions in the SW of England, with particular reference to the "giant erratics" that have exercised geologists and geomorphologists for so long.  Here is one of the posts:
http://brian-mountainman.blogspot.com/2010/06/on-postglacial-rebound.html
Please have a look at the others too, for the period 6-10 June.  This is a fairly comprehensive statement of my position......

My current theory is that much of SW England was covered by glacier ice in the Anglian Glaciation, around 450,000 years ago.  At the peak of that glacial episode, global sea-level was around 140m lower than it is today.  If the giant erratics were emplaced at that time, either by sea ice or through the direct effects of glacial deposition, the land surface must have been depressed by more or less the same amount.  To achieve that amount of depression, we need about 420m of ice on the land surface.  Can we achieve that scale of loading from what we currently know about ice limits etc?

The top illustration is from a computer model created by Dr Alun Hubbard and colleagues at Aberystwyth University -- it should apply to any of the extensive UK glaciations.  It is predicted that the ice was about 500m thick along the north coasts of Cornwall and Devon and up to 800m thick in the Bristol Channel.  The ice would have been over 400m thick over the Somerset Levels and around the fringes of the Mendips, and 100 - 200m thick over the western fringes of Salisbury Plain.

So we have perfectly adequate ice thicknesses (if this model is correct) for the scale of isostatic depression that we need if we are to "explain" the giant erratics.  There are still lots of questions.  How sensitive was (and is) the crust in this area?  Would deep isostatic depression along the coasts of the Bristol Channel have been sufficient to push down the English Channel coasts as well by almost the same amount?  I suspect so, since the distances are not great -- some of those south coastal areas are less than 50 km away.  Was there a bulge beyond the area of deep isostatic depression, and if so, where was it?  So far as I know, there has been no attempt thus far to fix the position of a hinge line in Southern England separating depressed areas from uplifted areas.

The bottom photo (from the edge of the Greenland ice sheet) shows what the ice edge might have looked like in Southern England -- in the vicinity of the Mendips and the Chalk scarp -- at the time of maximum glaciation.  What did the glacier profile look like at the time?  Was it a classic equilibrium profile as established by Nye, Robin and all the other great glaciologists -- with the ice surface gradient steepest at the snout or ice edge, and gradually diminishing up-glacier?  Or was it something quite different, with a much flatter profile induced by sub-glacial ground conditions, terrain irregularities,  and the thermal characteristics of the glacier itself?

It would be great to get some input on this debate from geophysicists and glaciologists......

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