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Monday, 6 December 2010

Isostatic rebound in the South-West

Top:  The ice cover during the Anglian Glaciation. (This model, kindly provided by Dr Alun Hubbard and his team from Aberystwyth university, strictly relates to the Devensian, but let's use it here as an Anglian Glaciation model....)
Middle:  Post-glacial and Holocene sea-level rise.-  Let's use this as a model of what might have happened at the end of each of the main glacial episodes to have affected the UK.
Bottom:  Approx bathymetry around the UK coasts.  Without any isostatic loading, the shoreline at the peak of the Anglian Glaciation would have been between the 100m and 200m contours.

Here is my comprehensive theory of almost everything!

In answer to the questions "At what time did the sea flood back into the English Channel and the Bristol Channel?" and "When might icebergs have carried those giant erratics onto the Channel coasts?" we have to work out how much glacio-isostatic loading there might have been.  I'm increasingly coming to the view that the whole of the SW of Britain acted as a relatively stable block during glacial episodes, with no hinge line and no forebulge.  Indeed, I'm not sure where the ideas of hinge line and forebulge came from in the first place, since they would imply an incredibly delicate response to loading variations just within a few tens of kilometres.  Many years ago Walcott showed that the elastic depression of the crust under an ice load, with an ice sheet centre 450 km distant, could be as much as 155m at the ice edge, decreasing to zero some 180 km beyond the ice edge.  Further away still, there might be a slight forebulge (+18m at a distance of 280 km from the ice edge.)    I'm not aware that his work has been discredited or disproved.

Note from the map at the top that at the peak of the glaciation there would have been glacier ice 800m thick in the centre of the Bristol Channel.  That in itself -- before adding elastic effects -- would account for more than 250m of isostatic depression of the crust.  With a thin ice cover over Dartmoor, Exmoor and the Somerset Levels (ice thicknesses of maybe 200 - 300m) and an ice edge somewhere near Salisbury Plain, it would be entirely reasonable to assume a sinking on the floor of the English Channel, and on the coasts of England and France, of over 100m.

So the edge of the Anglian Glacier in the Celtic Sea would have been floating and calving off icebergs which were rich in morainic debris and erratics; and these icebergs, driven by westerly winds, would have been driven up the Channel and onto a coastline which would not have looked all that different from that of today.  This is a reasonable analogy from Cape Murchison:

There were ideal conditions for the creation of periglacial slope deposits ("head") along the coastal cliffs, and for the dumping of the giant erratics.  So that's all sorted then.  

What we don't know is whether the subsequent sea-level rise during deglaciation was more or less matched by the isostatic recovery of the crust.  My guess is that for the most part, it was -- although there may have been a number of episodes when things were not in phase -- for example, eustatic surges associated with ice-sheet collapses in North America and other parts of the world, and maybe temporary cooling episodes which might have led to short-lived glacier expansion over the British Isles, with consequent "stillstands" in the process of isostatic recovery.

My current best shot at a timeline is that when the ice began to retreat northwards towards and through St George's Channel, isostatic recovery might have been faster than the eustatic rise, leading to an initial shoreline regression.  After 5,000 years or more, catastrophic ice sheet collapses might have led to sea-level surges, with transgressions.  Then there might have been a phase of equilibrium, with the shoreline stable some distance offshore, as sea-level rose from -60m to maybe -20m and as the land rose at a more or less equivalent rate.  Sea-level rise and isostatic recovery both slowed gradually, and then finally there was a coastal transgression similar to that which we have seen in the current interglacial, with the drowning of the coastal forests.  Following the Anglian glaciation, in the interglacial that followed, it is possible that relative sea-level was at one time at least 2m higher than that of today, leading to the creation of the raised beach platforms that we see all around the coasts of SW Britain.  But most of the platforms close to present sea-level are probably very old, having been formed during many interglacials -- and some of the giant erratics sit on them!

If anybody has a problem with any of this, let's see the colour of your evidence!


Anonymous said...

Hi Brian
What are your opinions on studies of river terraces along the River Severn and in the Thames Valley. That suggest that the main driver of river incision during the glacial interglacial warming period was isostasic uplift?
It has been suggested that the Mendip Hills have been uplifted by >90m since the Anglian glaciation and regional uplift in south central england has accelerated rapidly since 780ka.

BRIAN JOHN said...

Yes, I have seen various articles on this topic. I'm not entirely convinced -- over a very long period of time (going back to the Pliocene) we would probably all agree that there has been regional uplift in Southern Britain. That's the only way to explain all those coastal terraces. But isostatic adjustments in the Thames and Severn basins must have been VERY complex, with depression as well as intermittent uplift during a sequence of glacials and interglacials. At the moment there is supposed to be a structural tilting or downwarping going on in Southern England, but the evidence is very equivocal.