Tunnel valleys in the Celtic Sea

Tunnel valleys and other features showing a Late Devensian radiating ice flow in the Celtic Sea arena

This is another interesting article relating to the wastage phase of the Celtic Sea ice lobe -- or that part of the Irish Sea Ice Stream that occupied the Celtic Sea out as far as the shelf edge around 27,000 years ago.  This study of fluvioglacial features supports the concept of a catastrophic ice melt episode across the whole extent of the area shown on the above map in just a few thousand years.

I have considered the position of the eastern edge of the wasting "ice lobe" before:

https://brian-mountainman.blogspot.com/2020/04/late-devensian-glaciation-of-celtic-sea.html  

https://brian-mountainman.blogspot.com/2020/04/the-devensian-celtic-sea-piedmont-lobe.html


......but there still seems to be a reluctance on the part of those involved in the BRITICE Chrono project to accept that ice flows according to a rather basic set of glaciological principles, including the principle that ice lobes will always spread laterally unless they are constrained by topography or by an adjacent ice mass.  Do here, once again, we see a reluctance to accept that the Isles of Scilly were effectively a nunatak with ice flowing broadly southwards on both the western and eastern flanks -- in spite of rather clear evidence of seafloor streamlining between Scilly and the tip of Cornwall.  Why should there be a problem with this?

Associated with this is the assumption that the outer part of the Bristol Channel was not affected by LGM ice.  Where did this idea come from?  Well, it came from some very old articles purporting to show that only the north coast of Pembrokeshire was affected by Late Devensian ice, in spite of the fact that I showed many years ago that the ice of the Irish Sea Glacier affected the mouth of Milford Haven at the same time.  If it did that, the ice must also have pressed into the outer reaches of the Bristol Channel, if not into the area to the south of the Gower Peninsula.  This is of course confirmed by all the evidence which I have collected over the last few years relating to glacial sediments around the south Pembrokeshire coast, including the published evidence from Ballum's Bay on Caldey Island.

https://brian-mountainman.blogspot.com/2019/07/caldey-island-till-must-be-late.html

If you look at the alignments of the tunnel valleys and other channels on the map above, the pattern is indubitably one that shows a radiation pattern of ice flow -- exactly what one would expect, and exactly what I have been arguing for the last 50 years or more............

With regard to the black line on the map purporting to show the ice edge at 25,000 years ago, it is not supported by any field evidence, and I suspect that the eassetrn part of the line is in quite the wrong place -- it should be much further to the east.  The purported ice edges for 24.7 ka and 24.3 ka might be more accurate,  and are supported to some extent by cosmogenic dating work.  

--------------------------

A Late Pleistocene channelized subglacial meltwater system on the Atlantic continental shelf south of Ireland.

Giglio, C., Benetti, S., Sacchetti, F., Lockhart, E., Hughes Clarke, Giglio, C., Benetti, S., Sacchetti, F., Lockhart, E., Hughes Clarke, J., Plets, R., Van Landeghem, K., O Cofaigh, C., Scourse, J. & Dunlop, P.  

Boreas, 2022 (January), Vol. 51, pp. 118–135.

https://doi.org/10.1111/bor.12536

ABSTRACT

The studyof palaeo-glacial landforms and sediments can give insights into the nature and dynamics of ice sheets. This is particularly the case with regards to the subglacial record, which is challenging to observe in contemporary glaciated settings and hence remains only partially understood. The subglacial hydrological system is an essential component of ice dynamics, where increased water pressure enhances ice motion and sediment deformation, thus reducing ice-bed contact. Tunnel valleys are large, sinuous, steep-sided incisions that, together with smaller scale meltwater channels, indicate subglacial meltwater discharge beneath large ice sheets. Through the use of high-resolution marine geophysical data, a system of buried and exposed tunnel valleys, possible subglacial or proglacial meltwater channels and palaeo-fluvial valleys have been identified across the shelf of the Celtic Sea between Ireland and Britain. The presence of steep-sided and overdeepened tunnel valleys is indicative of a large channelized meltwater drainage system beneath the former Irish Sea Ice Stream, the most extensive ice stream to drain the last British–Irish Ice Sheet. After the rapid ice expansion across the Celtic Sea shelf around 28–26 ka, the tunnel valleys were carved into both bedrock and glacigenic sediments and are associated with rapid ice stream retreat northwards into the Irish Sea Basin between 25.6and 24.3 ka. The presence of a major subglacial meltwater system on the relatively shallow shelf suggests that significant erosive meltwater discharge occurred during the last deglaciation and highlights the important contribution of meltwater to the retreat of the British–Irish Ice Sheet on the continental shelf.