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Sunday, 26 August 2012

Where was the Somerset GBG Limit?

 A computer-generated model of the area affected by the British-Irish Ice Sheet -- one of many models coming from the glaciology team at Aberystwyth university.  Note that according to this model Stonehenge lies within the ice margin.  But local ice caps are not shown -- and the modellers have not shown the Dartmoor Ice Cap....

In the light of the Dartmoor evidence which was recently published -- showing pretty unequivocally that Dartmoor and the other uplands of SW England were glaciated during the Devensian or LGM (Last Glacial Maximum) -- we have to do some serious revision relating to ice limits.  Although the evidence for the Devensian is easier to pick up than the evidence for older glaciations (including the Anglian) things are complicated by the fact that ice edges have overlapped in quite complicated ways -- with the "greatest extent of ice" being of one age here, and of quite a different age over there.  Each glacial episode and each growth of the British-Irish Ice Sheet has its own dynamics ...... and that makes life complicated for glacial geomorphologists.

That having been said, one must try to make some sense of the situation in the south-western counties of England.  As I have pointed out many times on this blog, there is physical evidence on the ground for a glacial advance by the Irish Sea Glacier across the Somerset coast and into the Bristol area.  There is further evidence of glacial material in the Bath - Bathampton Down area, and as far as the Greylake No 2 Quarry shown on this map:

 Relief map showing main uplands and lowlands, rivers and key Quaternary sites in part of the SW quadrant of England. (Source: GCR tome on SW England)

 Further inland, the evidence is more equivocal, and we still do not have clear evidence that the ice reached  Street, Glastonbury, and the SE end of the Mendips.   The glacial deposits so far recorded in this area appear to be very old -- they are conventionally interpreted as Anglian in age.  On the other hand the Dartmoor evidence appears to be supported by the glacier modelling experiments done at Aberystwyth University, showing that (on the basis of glaciological principles) the Irish Sea Glacier COULD have covered the whole of the Somerset Lowlands and reached as far east as Stonehenge and Salisbury Plain.  So where is the ground evidence to support that contention?  Maybe a mottley collection of erratics at Stonehenge is the evidence we need -- but I quite accept that not everybody is inclined to accept that!

But there is a real dilemma here --  why are the glacial materials already described in the literature low down in the stratigraphic sequence, with no fresher deposits above them?  But it defies logic to say that Devensian ice affected the SW uplands of Devon and Cornwall and reached the Scilly Isles, but did not extend as far as Salisbury Plain.  Even if the ice sheet had an extremely low surface gradient when it pressed in from the W and NW, it must surely have pressed as far inland as Yeovil, Shepton Mallet and Frome?    The laws of physics must apply -- although sometimes a simple law needs to be replaced by something more complex if explanations are to make sense.....

I still think it's probable that the Stonehenge erratics are genuine GLACIAL erratics, transported initially in the Anglian glacial episode.  But I also think that in the Devensian the area shown in the lower map was affected by a very complex set of processes in which low temperatures, periglacial action, snowfields, icefields, small local ice caps and the glacier ice of the Irish Sea glacier all interacted with one another -- with an emphasis on landscape protection rather than alteration.  Prof David Evans and his colleagues have pointed out that the ice on Dartmoor must have been thin, very cold, and very slow-moving -- so that there are very few traces of either erosional or depositional features in the landscape of today.  Also, most of the "erratic" material carried by the ice was not very erratic at all, having been moved maybe no more than a few tens of metres from its source areas. 
Ice thicknesses on the Dartmoor Ice cap at the time of its greatest Devensian extent.  (After David Evans, Stephan Harrison and colleagues.)  Note that in some places the ice was c 180m thick.

To synthesise. I think that during the Devensian maximum the whole of this landscape was covered by perennial snow and ice. Maybe for short periods in the warmer summers, the snow blanket was broken up, allowing solifluxion and some redistribution of materials to occur. How long did this snow cover last for? Centuries or millennia -- it's difficult to say, without hard evidence. Maybe it waxed and waned several times. Beneath most of the snow blanket, the ground must have been permanently frozen or locked into permafrost, but maybe some of the best-known periglacial processes (frost cracking, polygon formation etc) were inhibited because ground temperatures were not low enough. In hollows and valleys there must have been thick accumulations of snow, and there must have been many other snow-banks against escarpments and hillsides. The landscape might have looked like this from above:
Winter landscape, West Greenland, well beyond the ice sheet edge.  Some of these snowbanks are perennial -- but most of them melt away every summer.  The distance from the left edge of the photo to the right edge is less than 2 km.

Over the following uplands there were probably small, thin ice caps such as those I have often illustrated on this blog: Bodmin Moor, Exmoor, Dartmoor, the Blackdown and Brendon Hills, the Quantocks, the Polden Hills and the Mendips. As for the other smaller hill masses and upland plateau areas (including Salisbury Plain) -- they too might have supported thin ice caps at the same time as the Dartmoor Ice Cap was in existence. I also think that a lobe of ice from the Irish Sea Glacier must have pressed in from the Bristol Channel and Bridgwater Bay -- pressing inland for maybe 40 km or more. The ice also reached the Scilly Isles and pressed against the coasts of Devon and Cornwall, sometimes unable to push inland much beyond the cliffline and in other places making small incursions into old estuaries and embayments of lower land.

So what we have here is an extremely messy situation.  If you had flown over or walked about in the area at the time, you would have been hard pressed to define the junctions between Irish Sea glacier ice, local ice created on the upland ice caps, and firn and snowfields which were quite independent and more liable to seasonal melting and regeneration.  I will shortly post a map showing where I think the ice edge might have been.....


BRIAN JOHN said...

I should explain that GBG means "Greatest British Glaciation"......

Myris of Alexandria said...

"landscape protection" rather than alteration.
What does that mean? No overall erosion/deposition?

BRIAN JOHN said...

Yes -- it's been known for a long time that in some circumstances glacier ice can freeze onto the bed and effectively protect a pre-existing landscape. Normally this means relatively thin ice and a situation in which pressure melting cannot occur on the bed. There may be some movement higher up within the ice mass, by shearing and creep, but ice movement on the bed might be so slow as to be virtually unmeasurable. On the other hand, where we have thicker ice, as in valleys or depressions, pressure melting will be possible at sub-zero temperatures, and bingo -- there can be much more rapid ice movement and substantial erosion on the glacier bed. This is why you get ice streams within large ice sheets and ice caps, and why trough depths get exaggerated while the flanks of troughs may survive even if the rock there is rotten or fractured. It's all in "Glaciers and Landscape"!!