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:
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.