From the new paper -- map showing suggested ice limits for MIS2 (Late Devensian) and MIS4 (Early Devensian). It's a pity that the author appears to have been unaware that the idea of the MIS2 Pembrokeshire ice-free corridor has been dumped. And the MIS4 ice limit? Even more dodgy.
There is a new and very strange article by James Scourse in the journal JQS. It purports to be on the subject of the giant erratics scattered around the coasts of SW England -- but of course it is really all about the bluestone monoliths of Stonehenge, without actually mentioning them at all.
It's common knowledge that one of the strongest arguments in favour of the glacial transport of the bluestones revolves around the presence of far-travelled giant erratics (and smaller ones) at Saunton, Croyde, Freshwater Gut, Porthleven and elsewhere which are deemed to have been emplaced by glacier ice in one or more pre-Devensian glacial episode. Like many other researchers, I believe that if glacier ice was powerful enough and extensive enough to reach the coast of southern Cornwall and the edge of the Celtic Sea shelf, it was also capable of reaching Salisbury Plain. That belief is strengthened by much of the computer modelling work on the BIIS. The thinking (articulated by Kellaway, Thorpe et al and many others including myself) is that the famous erratics cannot have been emplaced from floating ice because sea-level during sea ice episodes must have been around -100m, far too low to explain the presence of erratics on the present-day coastal platform. No no, says James Scourse in his latest paper, floating ice transport and stone dumping on the platform were perfectly possible, because of isostatic depression of the coastline during the early part of the Devensian. That involves thick ice -- and Scourse argues that there is some evidence for its existence, while admitting that there is work in progress.........
First, the paper details:
The timing and magnitude of the British–Irish Ice Sheet between Marine Isotope Stages 5d and 2: implications for glacio-isostatic adjustment, high relative sea levels and ‘giant erratic’ emplacement
J. D. Scourse
Journal of Quaternary Science
First published: 12 March 2024
https://doi.org/10.1002/jqs.3611
J. D. Scourse
Journal of Quaternary Science
First published: 12 March 2024
https://doi.org/10.1002/jqs.3611
The journal classifies the paper as an "invited review" -- and it seems to have been invited by the editor Neil Roberts. It does appear to have been peer reviewed, and it may be based largely on a 2022 lecture given by James Scourse at a QRA Discussion Meeting. Has the paper been a long time in the pipeline? Probably not, since the article incorporates references from 2023. The author cannot therefore claim ignorance of recent publications that might have been relevant to his discussion.
ABSTRACT
The extent, chronology and dynamics of the pre-Marine Isotope Stage (MIS) 2 last British–Irish Ice Sheet (BIIS) are not well known. Although the BRITICE-CHRONO Project has detailed the maximum extent and retreat phases of the last BIIS for the period after 30 ka and into the Last Glacial Maximum (LGM), the Project identified several pre-existing datasets and generated new data that implied glaciation pre-dating the LGM but which post-dated the Last Interglacial (Eemian; MIS5e); these data are reviewed here. There are no dated till units but are other indicators clearly indicative of glaciation: deep-sea ice-rafted detritus flux into the adjacent NE Atlantic, cosmogenic rock-exposure age dating from glaciated surfaces in Wales and the island of Lundy (Bristol Channel), and optically stimulated luminescence (OSL) ages of proximal glacifluvial sequences on the Isle of Lewis (Outer Hebrides) and in the Cheshire Basin. Taken together these indicate BIIS inception during MIS5d, growth into MIS4 and evidence for dynamic retreat–advance phases during MIS3. OSL evidence for high relative sea level indicated by raised beaches in southern Ireland during MIS4 and 3 at a time of lowered glacio-eustatic sea level indicates substantial glacial isostatic loading, explained by the early growth of the BIIS during the last cold stage. High relative sea level during MIS4 and 3 coincident with adjacent calving ice sheet margins provides an explanation for the rafted giant erratics found around the shores of southern Britain and Ireland.
The paper concentrates on the question of whether there was a substantial BIIS prior to 31,000 yrs BP, ie during the Early and Middle Devensian. Scourse summarises much research dats from the North Atlantic on ice rafted debris in deep sea sediment cores -- and he suggests that the evidence shows the presence of a BIIS of limited extent after 70,000 yrs BP. This is consistent with evidence showing Laurentide and Fennoscandian ice sheet existence, waxing and waning, around the same time. There does seem to be substantial evidence for a quite expansive glaciation in MIS4, including the coalescence of the Fennoscandian and British and Irish Ice Sheet (BIIS). This enlarged ice sheet is called by some the European Ice Sheet or EIS. So far so good -- no problem with any of that, apart from the positioning of the MIS4 (around 60,000 yrs BP) ice edge to the south of Pembrokeshire by Batchelor et al, 2019. That is clearly incorrect, and goes against all the field evidence accumulated over the past 50 years. Just another example of a model that could have done with some more ground truthing!
Scourse gets into much trickier territory when he discusses the evidence from Lundy Island, including the claim made by Rolfe et al (on the basis of cosmogenic dating) that the island was affected by Early Devensian glacier ice but not LGM glacier ice. Scourse makes it clear that the cosmogenic exposure ages presented by Rolfe et al are hotly contested. He also refers to cosmogenic dating information from the high peaks of North Wales and OSL data from NW Scotland, the North Sea Basin and the Cheshire Basin, again suggesting BIIS growth in MIS3. But again there is contrary evidence, and it appears that the dating of rock surfaces and glacially derived sediments within the glaciated parts of the British Isles is currently in state of flux.........
Then the author gets into raised beaches (which or of course in some places not raised very much at all). He refers to the Courtmacsherry raised beach in southern Ireland, and the attempts at dating using the OSL method.
Quote:
"A total of 22 OSL ages from the Courtmacsherry Raised Beach therefore indicate deposition during the last glacial stage prior to MIS 2. Both Ó Cofaigh et al. (2012) and Gallagher et al. (2015) conclude that to generate relative sea level (RSL) at an elevation close to modern mean sea level at this time implies significant glacio-isostatic depression and hence ice loading. Ó Cofaigh et al. (2012) highlight the presence of erratics within the raised beach sequences, as previously reported by Wright and Muff (1904), and the lack of any diagnostic palaeoecological content that would enable a palaeoclimatic assessment."
On the other hand Scourse notes that all of the dates obtained from raised beach exposures around the Bristol Channel suggest that the deposits are of Last Interglacial (Ipswichian) age, as suggested by the great majority of field workers on these coastlines.
So there is a dilemma. Are the dating exercises on the Courtmacsherry raised beach all incorrect, or could there really have been a unique set of circumstances there, related to proximity to a large ice mass that caused isostatic depression equivalent to eustatic sea-level lowering in MIS3?
The author discusses this at the back end of his article, and gets into a frightful tangle. He discusses the possibility of multiple juxtaposed interglacial beaches in many coastal locations, where it might be difficult to separate one from another. I would agree with that, and would add that many raised beach exposures are associated with brecciated slope deposits which might suggest periglacial conditions immediately following an interglacial -- but this interpretation is fraught with difficulty, since rockfall deposits (commonplace on cliffed coasts) are notoriously difficult to separate from those representing a cold climate. Even churned "head deposits" can be interpreted in different ways. Then Scourse says: " it is likely that, outside areas of neotectonic uplift, interglacial raised beaches lie stratigraphically laterally juxtaposed with raised beaches of MIS4–3 age." I fundamentally disagree with that statement -- Scourse has provided no evidence whatsoever in support of it.
With regard to the Courtacsherry "anomaly", Scourse argues for a eustatic lowering of c 80 m during MIS4, and states that in order for raised beach formation to have operated at that time there must also have been c 80m of isostatic depression caused by a "proximal ice sheet", which did not progress across the coastline. To achieve that, there must have been an ice thickness of c 400m. What is the evidence for that? Scourse admits that the evidence in the field does not support the idea -- and information from all over the British Isles suggests that although there may have been a substantial ice sheet during MIS4, it was much less extensive than that of the LGM or MIS2. In order to explain a substantial crustal depression of c 80m, Scourse has to resort to some rather intriguing speculations involving an ice mass with a very steep gradient and with an accumulation centre further to the west than that of the LGM. He even strays into the field of glaciology, suggesting a cold-based ice sheet with limited basal sliding in MIS4 as distinct from a warm-based ice sheet with rapid sliding and a low surface gradient in MIS2. I can agree with that, and have played about with similar ideas myself! But I cannot agree that there is any evidence that might support an 80m isostatic depression around the coasts of the Bristol Channel which might have facilitated raised beach formation at approx the same level as the raised beaches as we see them today.
But then the author argues for millenium-scale or rapid oscillations in the MIS4 ice sheet, as distinct from the longer-term and slower oscillations of the LGM ice sheet. That is is conflict with most of the conclusions coming out of the BRITICE-CHRONO research -- namely that there was a sort of surging behaviour with catastrophic melting during the final dissolution of the ice sheet.
Winding up, Scourse mentions the past debate about the giant erratics on the coasts of Devon and Cornwall, and says: "It has been assumed that the timing of ice rafting involved was during the Anglian glaciation (MIS12) or earlier (Kidson et al., 1977; Bone, 2022) but, given the evidence reviewed here, it is likely that at least some were emplaced during MIS4–3."
With regard to the Courtacsherry "anomaly", Scourse argues for a eustatic lowering of c 80 m during MIS4, and states that in order for raised beach formation to have operated at that time there must also have been c 80m of isostatic depression caused by a "proximal ice sheet", which did not progress across the coastline. To achieve that, there must have been an ice thickness of c 400m. What is the evidence for that? Scourse admits that the evidence in the field does not support the idea -- and information from all over the British Isles suggests that although there may have been a substantial ice sheet during MIS4, it was much less extensive than that of the LGM or MIS2. In order to explain a substantial crustal depression of c 80m, Scourse has to resort to some rather intriguing speculations involving an ice mass with a very steep gradient and with an accumulation centre further to the west than that of the LGM. He even strays into the field of glaciology, suggesting a cold-based ice sheet with limited basal sliding in MIS4 as distinct from a warm-based ice sheet with rapid sliding and a low surface gradient in MIS2. I can agree with that, and have played about with similar ideas myself! But I cannot agree that there is any evidence that might support an 80m isostatic depression around the coasts of the Bristol Channel which might have facilitated raised beach formation at approx the same level as the raised beaches as we see them today.
But then the author argues for millenium-scale or rapid oscillations in the MIS4 ice sheet, as distinct from the longer-term and slower oscillations of the LGM ice sheet. That is is conflict with most of the conclusions coming out of the BRITICE-CHRONO research -- namely that there was a sort of surging behaviour with catastrophic melting during the final dissolution of the ice sheet.
Winding up, Scourse mentions the past debate about the giant erratics on the coasts of Devon and Cornwall, and says: "It has been assumed that the timing of ice rafting involved was during the Anglian glaciation (MIS12) or earlier (Kidson et al., 1977; Bone, 2022) but, given the evidence reviewed here, it is likely that at least some were emplaced during MIS4–3."
Sorry James, but the evidence does NOT support the contention.
As for the relevance of all this for the "giant erratic" debate, the following points come to mind:
1. Scourse fails to mention that the erratics on the coasts of Devon and Cornwall are not all at or near present sea-level. Some of them are more than 60m above sea level, and I have mentioned them many times on this blog. The Ramson Cliff erratic is at c 80m asl, and the Shebbear erratic lies at c 150m asl. There is no way that these erratics can be explained away by reference to cold-climate isostatic depression and ice rafting, and Scourse should have admitted this. Glacial transport and dumping in a past extensive glaciation is the only explanation that makes any sense.
2. Scourse fails to mention that the erratics are in places embedded in apparently interglacial raised beaches and associated sandrock and slope breccia deposits. It is clear that they have nothing whatsoever to do with the Devensian climate oscillations; they pre-date the Ipswichian Interglacial.
3. Scourse fails to mention that the Quaternary stratigraphic evidence from all around the coasts of West Wales suggests that prior to the LGM glacial episode there was a long period of slope breccia deposition, with no sign at all of the proximity of glacier ice. There are signs of periglacial / permafrost conditions at times (including churned deposits and fossil ice wedges), but if there was a MIS4 ice edge anywhere, it was not to the south of the Pembrokeshire peninsula. It might have been somewhere to the north, in Cardigan Bay.
As for the relevance of all this for the "giant erratic" debate, the following points come to mind:
1. Scourse fails to mention that the erratics on the coasts of Devon and Cornwall are not all at or near present sea-level. Some of them are more than 60m above sea level, and I have mentioned them many times on this blog. The Ramson Cliff erratic is at c 80m asl, and the Shebbear erratic lies at c 150m asl. There is no way that these erratics can be explained away by reference to cold-climate isostatic depression and ice rafting, and Scourse should have admitted this. Glacial transport and dumping in a past extensive glaciation is the only explanation that makes any sense.
2. Scourse fails to mention that the erratics are in places embedded in apparently interglacial raised beaches and associated sandrock and slope breccia deposits. It is clear that they have nothing whatsoever to do with the Devensian climate oscillations; they pre-date the Ipswichian Interglacial.
3. Scourse fails to mention that the Quaternary stratigraphic evidence from all around the coasts of West Wales suggests that prior to the LGM glacial episode there was a long period of slope breccia deposition, with no sign at all of the proximity of glacier ice. There are signs of periglacial / permafrost conditions at times (including churned deposits and fossil ice wedges), but if there was a MIS4 ice edge anywhere, it was not to the south of the Pembrokeshire peninsula. It might have been somewhere to the north, in Cardigan Bay.
4. In the glacial and glaciofluvial deposits of West Wales there are organic remains including marine shell fragments. Although radiocarbon dating was not very reliable in the 1960's, studies suggested that the majority of shell fragments dredged up by the advancing ISIS as it came in from Cardigan Bay indicated a cold interstadial environment around 30,000 - 40,000 years ago. Scourse should have mentioned these studies.
Pleistocene stratigraphy on the southern coast of Cardigan Bay -- spanning the whole of the Devensian glacial cycle. From my 1973 paper.
Climatic oscillations, stages and Pleistocene stratigraphy of West Wales. This was the situation in 1973, and in spite of some changes in terminology it remains reliable, after 50 years......
5. That being the case, I can see no case at all for ice loading in the Bristol Channel on a scale that might have depressed the crust by 80m or more. Having worked in this field many years ago in Greenland and the Antarctic, I am a firm believer in rather sensitive isostatic adjustments to changes in ice loading, and if anything I would expect that during the MIS4 "glacial phase" there would not have been isostatic depression in the Bristol Channel but isostatic UPLIFT as a result of the forebulge effect.
=========================
The debate will continue, but one just wishes that Scourse had done some more extensive reading of the literature. Suggested bedtime reading:
John, B.S. 1965. A Possible Main Wurm Glaciation in West Pembrokeshire, July 1965, Nature 207(4997):622-623
John, B.S. 1970. The Pleistocene drift succession at Porth-clais, Pembrokeshire, Geological Magazine 107 (5): pp 4239-457.
John, B.S. 1970. Pembrokeshire. In Lewis, C.A. (ed.) The Glaciations of Wales and Adjoining Regions. Longman, London, pp 229-265.
John, B.S. & Elis-Gruffydd, I.D., 1970. Weichselian stratigraphy and Radiocarbon Dating in South Wales. Geol. en Mijnbouw 49, pp 285-296.
John, B.S. 1973. Vistulian Periglacial phenomena in South-west Wales, Biul. Periglacialny, pp 195-212
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.
John, B.S., Elis-Gruffydd, D. and Downes, J.: Observations on the supposed “Neolithic Bluestone Quarry” at Craig Rhosyfelin, Pembrokeshire. Archaeology in Wales 54, 2015, 139-148.
John, B.S., Elis-Gruffydd, D. and Downes, J.: Quaternary Events at Craig Rhosyfelin, Pembrokeshire. Quaternary Newsletter, October 2015 (No 137), 16-32.
etc etc.......
========================
PS. Just in case you don't get the irony, the above list of refs is my gentle joke at the expense of those who indulge themselves in this silly thing called "cancel culture." I have apparently been cancelled. Of course I'm not obsessed with promoting my own work -- but I will stand up and defend it in the face of outright hostility. There are scores of excellent papers written by many researchers which might be cited here and elsewhere on the matter of giant erratics and glaciation. But for certain individuals, who refuse point blank to cite anything I have written, no matter how relevant my papers might be, I'll just say that I am alive and well, and that I wish them a very happy Easter, full of fluffy bunnies and chocolate eggs.
No comments:
Post a Comment
Please leave your message here