The myth of the shoreline erratics

I have published this map a number of times, assuming that it was a representation of the situation during the Anglian (MIS12) glaciation.  But maybe this was not the Anglian situation at all, but the  scenario that prevailed during the Wolstonian glacial episode (MIS6)??

One of the most frequently repeated myths about the erratics of the Bristol Channel is that they are all concentrated in the intertidal zone.  I have seen it in geomorphology and geology textbooks, conference proceedings and papers published in learned journals.  Over and again.  Let's get this straight.  This is simply untrue.

Of course, we all know about the famous "shoreline erratics" at Porthleven, Croyde, Saunton, Limeslade, Flat Holm and elsewhere, and the widespread assumption that they must have been emplaced by floating ice rather than by the ice of an active glacier.  As I have explained many times on this blog, I can see no realistic glaciological or isostatic scenario which would have allowed this "floating ice transport" to have happened.  In any case, the shore zone is special in that it is a "washed zone" in which wave action and tidal scour remove finer debris and leave large boulders behind.  Above it, inland, and below it, beneath the spring tide low water mark, sediments containing large foreign erratics survive, still holding their secrets.  So the "shore zone concentration" is more apparent than real, and it is extraordinary that one senior geologist or geomorphologist after another has failed to appreciate that fact.

The evidence of high-level erratics is hiding in plain sight, in the published records of Paul Berry, Peter Keene, Paul Madgett, Rosemary Inglis, Ann Inglis and others which are often ignored in the articles submitted to learned journals.   Here are some of the recorded  altitudes of erratics on or near the coasts of Cornwall, Devon and Somerset:

Lundy  138m

Shebbear  150m

Westonzoyland  10m

Baggy Point  80m, 60m and 45m

Ilfracombe  150m - 175m

Kenn  7m

Court Hill 68m  (ice surface was above 85m)

Nightingale Valley / Portishead Down  85m

These records leave us in no doubt that active glacier ice, on at least one occasion, crossed the Bristol Channel coasts of Cornwall, Devon and Somerset and pressed well inland.  

This brings us back to the recent paper by Gibson and Gibbard, which I find persuasive.  They argue that the most recent glaciation to affect the Bristol Channel coasts was the Wolstonian / Moreton Stadial / MIS-6 glaciation, which occurred around 150,000 years ago.  They argue that the Wolstonian ice rode over the limit of the Anglian ice in the Midlands and pushed far to the south in the Celtic Sea arena, making it the most extensive of all the glaciations in western Britain.  That is up for debate, since the green line shown on their map, based on Clark et al, 2018, appears to be very inaccurate and has already been questioned in a number of research articles.  

The suggested ice limits of Gibson and Gibbard, 2024. The suggested Devensian line across 

West Wales (green line) is unsupported by field evidence. The new Wolstonian line, in red, 

is more or less in the same position as the old "Anglian Glaciation" line, but it cannot be 

correct in the Somerset area.

An attempt to show where the Wolstonian limit of the Irish Sea Ice Stream might have been located -- based on a map by Gilbertson and Hawkins.