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Wednesday 12 September 2018

Devensian ice extent in the Celtic Sea arena




This is my latest map of suggested Devensian ice extent, maybe around 27,000 years ago or 21,000 years ago, depending on which paper you are reading.......

Give or take a few kilometres here and there, I reckon that the ice edge shown fits with the field evidence of fresh glacial deposits.   What is much more contentious is the flow pattern shown.

As I have explained on this blog many times, I cannot see any logic in the portrayal of a Celtic Sea ice lobe, pushing southwards through the constriction of St George's Channel and then southwards towards the south-western shelf edge with very little lateral spreading.  Most reconstructions show constraining ice (from Southern Ireland) on the western flank, and that is perfectly sensible, but nothing at all on the eastern flank.  I don't think there is any sedimentological or other evidence from the Celtic Sea or from the outer reaches of the Bristol Channel to support this contention.

The BRITICE map published this year from Clark et al still shows this strange lobe:


The idea is that there was a short-lived surge, lasting a few thousand years, during which the ice pushed southwards from St George's Channel and then retreated back again. I don't have a problem with that, but I do have a problem with the eastern ice edge and the flowline as shown.  I would also like to see some realistic attempts made to show ice surface altitudes and gradients, as I tried to do years ago.  I'm sure that rather crude map has been overtaken by much more sophisticated work since!


This is an interesting map from Google images -- but the paper from which it came is behind a paywall, so I can't even cite a proper reference.  The Devensian retreat stages are particularly interesting, as is the mapped extent of the "Melville Till" on the bed of the Celtic Sea.


Reference:
Clark, C. D., Ely, J. C., Greenwood, S. L., Hughes, A. L. C., Meehan, R., Barr, I. D., Bateman, M. D., Bradwell, T., Doole, J., Evans, D. J. A., Jordan, C. J., Monteys, X., Pellicer, X. M. & Sheehy, M. 2018 (January): BRITICE Glacial Map, version 2: a map and GIS database of glacial landforms of the last British–Irish Ice Sheet. Boreas, Vol. 47, pp. 11–27. 
https://doi.org/10.1111/bor.12273. 
ISSN 0300-9483.



These ice surface contours are suggested for the Anglian Glaciation, and they show thick ice overtopping Preseli and an ice surface over 2500m over North Wales.  How different might things have been in the Devensian?  This map is now rather old, and I would dearly like to see an update from some of the more recent modelling experiments.




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