Revisiting the GBG (Greatest British Glaciation) of western Britain

Suddenly everybody seems to be interested in the erratics on the shores of the Bristol Channel, which is great.  The above map, which I think I first published in 2017, becomes very relevant indeed.  There are of course many similarities between what I refer to as the GBG and the LGM (Last Glacial Maximum -- Late Devensian):

I'm dithering on the precise dating of the GBG  it might have been the Anglian (MIS 12) or it might have been one of the phases of the Wolstonian (MIS 6).  Then again, the maximum ice extent might  not be represented by a single ice edge -- if such a thing might indeed have been discernible on the ground surface.  The outer limit of this glaciation might have occured in different sectors at different times -- in other words there might have been asynchronous oscillations.

In any case I am increasingly convinced that the ice reached the Fremington / Croyde / Saunton area on at least two occasions.

Interestingly enough, in my map for the GBG I show local icecaps on Dartmoor, Exmoor and Bodmin Moor as being incorporated into a landscape dominated by glacier ice, permanent snowfields and intermittent snow-covered terrain.  And on my model glacier ice could have carried the Ramson Cliff erratic broadly northwards from a "Cornubian outcrop" as suggested by Daw, Madgett and Ixer......

The main problem with that is that it implies a dynamic northward flow of ice from the Dartmoor ice cap as envisaged by Prof David Evans and others at a time when there was no competing ice flow from the N or NW.  See this:

https://brian-mountainman.blogspot.com/2012/06/dartmoor-ice-cap.html

The glaciation of Dartmoor: the southernmost independent Pleistocene ice cap in the British Isles
David J.A. Evans, Stephan Harrison, Andreas Vieli, Ed Anderson
Quaternary Science Reviews 45 (2012) 31-53

https://www.researchgate.net/publication/258707211_The_glaciation_of_Dartmoor_The_southernmost_independent_Pleistocene_ice_cap_in_the_British_Isles

https://www.torsofdartmoor.co.uk/tor-page.php?tor=slipper-stones

David and his colleagues suggest that the ice cap was thin and sluggish, and probably incapable (even at the time of its maximum extent) of deep erosion and significant erratic transport.  The northward known extent of the ice cap was at c 460m asl, near the Slipper Stones in the West Okement Valley --  roughly halfway between Meldon Reservoir and the high peaks of Yes Tor and High Willhays.  That's almost 50 km from the Croyde-Saunton area.  And therein lies the problem.........  at the time of the GBG, the intervening area would have supported extensive connected snowfields, some of them permanent (in that they survived during the summer months) and some melting intermittently.  But we can see no glaciological scenario suitable for the transport of big epidiorite boulders.......  On the other hand David and his colleagues were referring to a Late Devensian ice cap on Dartmoor.  Might there have been a much larger, earlier glacier about which we currently know virtually nothing?  We have to admit that this is quite possible......

The other question which deserves attention is that of the parallel ice streams.  Geoffrey Kellaway suggested many years ago that there were several confluent  ice streams flowing into the Bristol Channel, including one carrying Scottish erratics and one carrying Welsh erratics.  I also explored this idea with Lionel Jackson in a 2009 journal article:

https://brian-mountainman.blogspot.com/2020/09/stonehenges-mysterious-stones.html

Nowadays I think I prefer the idea of a zig-zag movement of erratics over several glaciations. but we have not heard the last of this!