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 (see below)
Paper just published:
Gibson, S. M. & Gibbard, P. L.. 2024. Late Middle Pleistocene Wolstonian Stage (MIS 6) glaciation in lowland Britain and its North Sea regional equivalents – a review. Boreas (online).
The paper was submitted for publication on 19th April 2024.
ABSTRACT
ABSTRACT
Two major glaciations have been identified on land in England during the Middle Pleistocene. The earliest occurred during the Anglian Stage (= Elsterian, c. Marine Isotope Stage, MIS 12), evidence for which is best developed in lowland Britain, as well as offshore in the southern North Sea and Irish Sea basins. The second took place during the late Middle Pleistocene, with the most compelling evidence found in theWest Midlands, intermediate between the Hoxnian (= Holsteinian; broadly MIS 11) and Ipswichian (= Eemian; broadly MIS 5e) interglacial stages during the LateWolstonian Substage. Until recently this younger glacial episode was less clearly represented in the Pleistocene record and, as a result, had been little studied and weakly defined. Interpreted as the Moreton Stadial glaciation during the Late Wolstonian Substage (= Late Saalian Substage/Drenthe Stadial, c. MIS6), it was originally recognized in the English Midlands, subsequently being identified in Yorkshire, Lincolnshire and northern East Anglia, and potentially further SW as far as the Bristol Channel. Mapping, in particular by members of the British Geological Survey, however, resulted in the Wolstonian Stage glacial deposits being thought to pre-date the stage. This was particularly so in EastAnglia where there was considerable controversy concerning the number and relationships of glacial sequences, during the 1970–1980s. Yet to the west of East Anglia there remained unequivocal evidence for glaciation during the stage, particularly in Fenland and the eastern English Midlands. Recent radiometric dating across lowland Britain on glacial sediments long thought to belong to a glaciation event in the Wolstonian Stage have now placed a geochronological control on the established regional stratigraphy and confirmed that glaciation occurred in two phases between 199 and 147 ka during the Late Wolstonian Substage. The glacial events of the British Middle Pleistocene can clearly be correlated with the European continent.
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This is an interesting "review paper" which assembles a vast amount of information and presents it in a coherent form. In particular, it looks at the evidence for glaciation in the West Midlands and presents strong evidence for ice limits in the Cotswolds - Birmingham - Severn Valley region. I'll consider the strength of that evidence on another occasion. However, some of the other material in this paper is highly questionable, and appears to be based on suppositions and assumptions rather than evidence. I have particular concerns about the suggestions made by the authors for Wales and the Bristol Channel region.
On P 9 the authors consiuder the evidence from western Britain. Quote: A significant and important contribution to the debate on the Quaternary history of the Bristol Channel has been presented by John (1968, 2008), who argues that the Bristol Channel was glaciated from the west in the Devensian Stage but from the north during the late Middle Pleistocene. Do I really argue that in my little 1968 article and in the first edition of my Stonehenge book? I don't think so. I have never claimed that the Bristol Channel was glaciated from the north -- ie by "Welsh" ice. I have always argued (since 1965) that successive glaciations of the Bristol Channel were all dominated by Irish Sea ice moving in from the west and flowing broadly eastwards. Gibson and Gibbard should have been more careful, and should have cited my more recent publications including my 2018 book, my Isles of Scilly paper of 2018 and my South Pembrokeshire paper of 2023.
John, B.S.: The Stonehenge Bluestones, Greencroft Books, 256 pp, 2018.
John, B.S.: Evidence for extensive ice cover on the Isles of Scilly, Quaternary Newsletter 146, October 2018, 3-27.
John, B.S.: Was there a Late Devensian ice-free corridor in Pembrokeshire? Quaternary Newsletter 158, 2023, 5-16.
In their assessment of the evidence for glaciation on the Bristol Channel coasts of Devon and Cornwall, the authors get into a bit of a tangle (p 11), partly because of unreliable and contradictory earlier dating attempts. They refer to glacial deposits beneath Ipswichian raised beaches; but I am not aware of any such deposits being described in the literature.
The authors then get into another tangle, with regard to the "giant erratic blocks" that litter the Bristol Channel coasts. I agree with their conclusion that many of the blocks lie beneath Ipswichian littoral sands or "sandrock" -- this is supported in Pembrokeshire at a number of coastal locations. That means that the boulders were probably emplaced in the Late Wolsoonian (MIS 6) glaciation, or maybe even earlier. They say: "These erratics (containing porphyry, dolerite and spilite) were certainly transported by an ice sheet sourced in western Scotland." I agree with that. But then they continue: "The most likely explanation for the occurrence of these erratic boulders is that they were ice rafted, reflecting their widespread distribution....... " With that statement I profoundly disagree, for reasons enunciated in my recent paper (2024) on the Limeslade Boulder. The arguments about tectonic uplift and isostatic readjustment as an explanation for the high-level erratics on the Devon coast are not at all convincing, and the citation of Scourse (2024) relating to a supposed Early Devensian "high relative sea level" adds nothing to the argument.
That having been said, it seems that Bennett et al (2024 - forthcoming) are on the right track, arguing that most of the large coastal erratics date from the MIS 6 glacial episode but that some might have been emplaced during the Devensian. I have suggested this in a number of posts.
With respect to the lines drawn on a map, I'm glad to see that the old "Wolstonian glacial limit" line across Wales has now been abandoned, having been reproduced far too often in assorted publications.
The old Wolstonian limit proposed by Gibbard, Clark and others. The section drawn across Wales is based on no evidence whatsoever, and defies glaciological principles........ so thank the Lord it has now been dumped. The Devensian line across West Wales is similarly unreliable, as shown in a 2023 QN article.
The new Wolstonian line, more or less coinciding with the "Anglian" line on earlier maps, is interesting but also inaccurate:
Map of the Moreton Stadial (Late Wolstonian / MIS 6 glaciation) -- annotated. Note that this line occupies more or less the same position as the old presumed Anglian glaciation limit.
The reason for my scepticisn regarding the line between Bristol and the Quantocks is that it follows the current coastline, disregarding the topographic / relief features of the landscape. If active ice pressed against the north coast of Devon, carried erratics to altitudes in excess of 175m (according to Prof Nick Stephens), and occupied the inner section of the Bristol Channel, it must also have pressed into the extensive natural depression now occupied by the Somerset Levels. Kellaway appreciated this in 1970, and so did Gilbertson and Hawkins in 1978.
In drawing a map of ice limits you cannot afford to ignore field evidence or glaciological principles. We know that there are glacial deposits under the peat, in the Somerset Levels depression, and also further inland from the coast in the Bristol area. I therefore suggest that the Moreton Stadial line is pushed a little further inland everywhere on the Devon coast of the Bristol Channel, particularly in the Fremington area, and MUCH further inland in Somerset. Then let's check if that line is falsified (or not) by the field evidence.
One other thing. This is really interesting:
The nature of the glaciation during the Late Wolstonian Substage, compared with that during the preceding Anglian Stage, implies a different set of immediately pre-existing ground conditions. For example, the Anglian glaciation deposits characteristically comprise thick sheets of till. The former suggests that an abundant supply of weathered regolith material was available for recycling during the Anglian ice advance. Likewise, the occurrence of deeply eroded and infilled tunnel valleys implies that large volumes of meltwater were drained from the ice sheet in these substantial channel systems during this phase. In contrast, the Late Wolstonian Substage ice lobes appear generally to have deposited notably thinner till sheets, potentially suggesting a more limited regolith stock than that in the preceding glaciation (the first to have advanced into lowland Britain). This, combined with the recessional landforms that characteristically occur as ice-push marginal landform complexes, like those in the Fenland, beneath the adjacent North Sea and in the central Netherlands marginal zones, again contrasts with a lack of such features apparently formed during the Anglian deglaciation phase(s). Furthermore, the apparent absence of tunnel valleys on land, at least in lowland marginal areas, suggests that meltwater discharge during the Moreton Stadial glaciation, rather than being restricted to and focused on substantial tunnel valley networks, was generally through widely distributed subglacial meltwater channels. (p 15)
I look forward to more information on this in the future. But it's interesting that in my piece for the Pembrokeshire Historical Atlas, somewhat tongue-in-cheek, I suggested that the Gwaun-Jordanston meltwater channel complex might have been created or modified by huge volumes of metwater flowing broadly westwards following a Wolstonian glacial episode of expanded Welsh ice:
All in all, a very interesting contribution. I shall revisit it. I agree with some parts, and not others. But hey, that's the way with science.........
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We await this publication with interest:
Bennett, J. A., Gibbard, P. L., Hughes, P. D., Murton, J. B. & Cullingford, R. A. 2024: The Quaternary geology of Devon. Geoscience in South-West England. Proceedings of the Ussher Society, in press.
.... and we hope it does not repeat the same mistakes as the Boreas article examined above!
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