Baggy Point giant erratic (photo: Paul Berry). We just cannot find a scenario in which this boulder could have been ice rafted into this position..........
but it is also interesting in that it flags up the episodic nature of ice rafting on the edge of the Celtic Sea shelf edge, not just for the Devensian LGM but also for earlier glacial episodes.
Fabian, S.G., Gallagher, S. J.& De Vleeschouwer, D. 2023 (October): British–Irish Ice Sheet and polar front history of the Goban Spur, offshore southwest Ireland over the last 250 000 years. Boreas,Vol. 52, pp. 476–497.
Six distinct IRD peaks are associated with the middle to late Wolstonian BIIS, three of which correspond to the extreme glacial conditions at ∼267, ∼255 and ∼155 ka relating to the migration of the polar front south of Site 548. In contrast, the other three at ∼252, ∼221 and ∼132 ka correspond to Termination 3, 3A and 2, respectively. The IRD peak at ∼223 ka (MIS 7d) signifies increased BIIS activity during the extremely cold yet short-lived stadial period (Hughes et al. 2020), while the IRD peak at ∼132 ka signifies the last major iceberg calving from the Wolstonian BIIS.
Significant IRD deposition from the Devensian BIIS resumes from MIS 5d, with three significant peaks at 60, 15.5 and 14.6 ka. The two oldest peaks are coeval to HE6 and 1, whilst the youngest peak reflects one of the last significant icebergs calving from the Devensian BIIS before its demise.
Fabian, S.G., Gallagher, S. J.& De Vleeschouwer, D. 2023 (October): British–Irish Ice Sheet and polar front history of the Goban Spur, offshore southwest Ireland over the last 250 000 years. Boreas,Vol. 52, pp. 476–497.
https://doi.org/10.1111/bor.12631.
ISSN 0300-9483.
10.1111/bor.12631. ISSN 0300-9483.
10.1111/bor.12631. ISSN 0300-9483.
From the cited article -- a summary of the main glacial episodes after c 300 ka. The main IRD episodes were during the ice wastage phases of the larger glaciations.
The article is essentially an analysis of the sediment sequence on the shelf edge. The evidence of ice rafting in the sediments is very strong, including clast and grain shapes and the presence of dropstones.
Quote:
Six distinct IRD peaks are associated with the middle to late Wolstonian BIIS, three of which correspond to the extreme glacial conditions at ∼267, ∼255 and ∼155 ka relating to the migration of the polar front south of Site 548. In contrast, the other three at ∼252, ∼221 and ∼132 ka correspond to Termination 3, 3A and 2, respectively. The IRD peak at ∼223 ka (MIS 7d) signifies increased BIIS activity during the extremely cold yet short-lived stadial period (Hughes et al. 2020), while the IRD peak at ∼132 ka signifies the last major iceberg calving from the Wolstonian BIIS.
Significant IRD deposition from the Devensian BIIS resumes from MIS 5d, with three significant peaks at 60, 15.5 and 14.6 ka. The two oldest peaks are coeval to HE6 and 1, whilst the youngest peak reflects one of the last significant icebergs calving from the Devensian BIIS before its demise.
This is all relevant to the debate about the large erratic boulders that sit on the shore platforms of the coasts of Pembrokeshire, Gower, Devon and Cornwall. The ice rafting peaks seem to represent the episodes of maximum debris /erratic transport in the Celtic Sea, which in turn coincide with the wastage phases of the episodes of most intense glaciation, when there were floating ice fronts in the Irish Sea or St Georges Channel. These would have been times of rapid eustatic sea-level rise and also rapid isostatic recovery as the weight of the crustal ice load in the British Isles was rapidly reduced. It is probable that on each of these occasions global sea level was at least 100m lower than it is today.
As I have repeatedly pointed out, if the big erratic boulders were indeed ice rafted into position, this means that there must have been an equivalent degree of crustal depression -- and this means an ice cover about 300m thick. That means very intense glaciation not only in the area currently submerged in the waters of the Celtic Sea and the Bristol Channel but also on land as well -- and we just do not have the evidence for that.
I therefore suggest that the big erratic boulders on the coasts in question were emplaced by glacier ice along with other sediments including till during one or several of the "peak" glacial episodes of the Wolstonian or Anglian, some time BEFORE the IRD peaks recorded by the authors of this new paper.
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