The BRITICE model for 22 ka -- from Fig 6 of Clark et al 2022. At this time there was a calving ice front west of Llyn and Anglesey, withy debris laden icebergs capable of carrying erratics through the St Georges Channel and onto the Celtic Sea coasts. But note that the contemporaneous shoreline was c 50 km to the west of the current Devon and Cornwall coast.
Thanks to the BRITICE - Chrono team, we can now state rather definitively that there could have been no circumstances during the Late Devensian glacial episode which might have led to the ice rafting of large erratics onto the coasts of the Bristol Channel as we know them today. The modelled reconstructions all show that during episodes of high glacier discharge into tidewater calving fronts, in the Irish Sea, St Georges Channel and the Celtic Sea, the contemporary coastline was well to the west of the current cliffline of the South-West Peninsula -- sometimes between 30 km and 50 km away.
The details:
According to Clark et al (2022) and BRITICE modelling:
31 ka — ice cap over the Scottish Highlands. Ice from outlet glaciers beginning to feed into tidewater floating ice front near Glasgow. Narrow channel near E Irish coast. Most of Irish Sea, Cardigan Bay and Bristol Channel dry land. Coast 50 km or more W of Devon and Cornwall coast. Sea level falling gradually.
30 ka — coast and RSL similar — streaming ice carrying debris into tidewater channel in North Channel area. Ice front now near southern end of North Channel. Bristol Channel dry.
29 ka — similar — ice front now near Isle of Man.
28 ka — no change in RSL and coast position. Calving ice front now near Anglesey.
27 ka — RSL / coast position unchanged. Calving ice front now in St Georges Channel.
26 ka — floating ice front now in Carmarthen Bay and outer end of Bristol Channel. Large ice lobe with minimal surface gradient in Celtic Sea. Partly grounded — isostatic depression from Welsh Ice Cap and ISIS kicks in, and there is a coastal transgression. Coast approaches to within 30 km of present Devon and Cornwall coast. This is the Last Glacial Maximum position. Ice surface over N Pembs at c 1000m. Ice surface in Bristol Channel approx 500m; almost certainly the ice edge was against the cliffline rampart.
25 ka — retreat begins — shoreline static due to approx balance between isostatic depression and eustatic sea level position. (NB modelled ice free enclave in Pembs is an aberration, as recognized by BRITICE team — Pembs was ice- covered.)
24 ka — rapid ice wastage and calving ice front in St Georges Channel. RSL still static.
23 ka — wastage continues. Welsh ice cap shrinks but is still present. Calving ice front W of Anglesey.
22 ka — not much change, but local ice edge retreating in Wales and Ireland. Marine regression — coastline retreats westwards, partly because of isostatic rebound as ice load diminishes. Coast now c 40 km off coast of Devon and Cornwall. Lowest (LGM) sea level stand at -130m
21 ka — marine regression continues — coast c 45 km W of Devon and Cornwall. Welsh ice cap diminishes further, but still in contact with ISIS in N Wales.
20 ka — ice front now near Isle of Man —calving ice stream much diminished. And remnant of Welsh ice cap now isolated — S Wales free of glacier ice. More isostatic recovery — Bristol Channel coast now even further out to W — up to 50 km out from present cliffline. MSL at -125m — gradual rise until 15 ka……….
19 ka — coastal position hardly changed. Welsh ice cap now broken into several small detached ice masses.
18 ka — 16 ka — coastline unchanged. Eustatic rise and isostatic rebound rates approx equal? By 16 ka there is no longer any calving ice front in Scotland, so no feed of icebergs carrying debris.
15 ka — coastal position unchanged — Welsh ice cap gone — just a few small upland glaciers left. Central Valley of Scotland now ice free — Scottish Ice Cap much diminished. MSL at -110m — after 15 ka, rate of rise increases rapidly (meltwater pulse 1A). At 14 ka MSL stands at -80m.
8 ka — all ice now melted. Isostatic recovery slows down and eustatic rise leads to a major transgression — coasts of Devon and Cornwall now near present position of cliffline. MSL at -10m and still rising. At c 7 ka rate of rise slows dramatically — very gradual after that.
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It follows that during all earlier glacial episodes during which ice front calving might have occurred, the coastline in the Bristol Channel would have been similarly distant from the positions in which large erratics are now found on the Devon and Cornwall coasts. Isostatic / eustatic / coastal process relationships would have been broadly similar to those of the Devensian LGM. Therefore floating ice (icebergs and bergy bits) cannot have been responsible for large erratic boulder emplacement, whatever earlier researchers might have said in the published literature.
There is no escaping from the conclusion that the big erratics on the coastal platform -- in the current intertidal zone -- must be the remnants of destroyed glacial deposits laid down by active glacier ice during the LGM or one of the earlier glacial episodes.
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