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Wednesday 14 December 2022

The LGM Welsh ice / Irish Sea ice contact line


As faithful readers will know, I have changed my mind many times over the position of the contact zone between Welsh ice and Irish Sea ice during the LGM. Here is my latest effort, drawn onto a "base map" derived from th BRITICE-CHRONO map which is available to all via a Creative Commons license. So grateful thanks to the team for that.

The symbols on the map show designated moraines, lakes and other features which are mostly assigned to the Late Devensian. The yellow area is the area assumed to have been ice-free during the LGM. However, that is still a matter of debate, and I am now convinced that the designation of the ice-free enclave or corridor in Pembrokeshire is based on unreliable evidence. There are other questions about Gower and the Vale of Glamorgan.

Note that there was no synchroneity between the maximum LGM extent of ice on different parts of the contact zone. The BRITICE-CHRONO team thinks that the Irish Sea ice might have begun its retreat earlier on the western flank of the Welsh uplands than on the eastern flank.

Anyway, this will do for now -- let's assume that the line may be reasonably reliable to +/- 10 km or so..........

So what went on along the contact line between the Irish Sea Ice stream and the Welsh ice forced southwards in Cardigan Bay and across West Wales? There are analogies in the literature, relating to the ice streams as reconstructed for the Laurentide Ice Sheet. See this:

Ice streams in the Laurentide Ice Sheet: Identification, characteristics and comparison to modern ice sheets
Earth-Science Reviews
Volume 143, April 2015, Pages 117-146
Martin Margold, Chris R. Stokes and Chris D.Clark

Ice stream activity scaled to ice sheet volume during Laurentide Ice Sheet deglaciation
C. R. Stokes, M. Margold, C. D. Clark & L. Tarasov
Nature 530,322–326(18 February 2016)
And I have also drawn attention in the past to the analogy of the Foothills Erratic Train which appears to have been carried along the contact zone between Cordilleran ice and Laurentide ice in western Canada:

This was of course in relation to the transport of erratics from West Wales to Salisbury Plain, but there may well be lessons for us in our attempts to understand what happened on the western edge of the Welsh Ice Cap.
Utting, D., Atkinson, N., Pawley, S. et al. (1 more author) (2016) Reconstructing the confluence zone between Laurentide and Cordilleran ice sheets along the Rocky Mountain Foothills, southwest Alberta. Journal of Quaternary Science, 31 (7). pp. 769-787. ISSN 0267-8179

Of course, much of this contact zone is now lost beneath the murky waters of Cardigan Bay........

Currently there is not much agreement as to what can be expected along a contact zone between two parallel ice streams.  In some articles there is an assumption of a "shear zone" up above, in the ice, and a discontinuous and elongated morainic ridge down below.   In other articles there is an assumption that there would be a "saddle" or depressed zone between the ice streams, with meltwater concentration and possibly subglacial lake development.  This is an interesting idea, which may tie in with the evidence relating to Glacial Lake Teifi and the abundant glaciofluvial deposits in the vicinity of Cardigan and the the lower Teifi Valley.

More thought needed......

Shear zone between two moving ice masses -- Recovery Glacier, Antarctica

Crossing (old and new) lineations, Transition Bay palaeo-ice stream, Canada

Shear margin moraine identified by arrows. M’Clintock Channel palaeo-ice stream in Arctic Canada.  The moraine is rather insignificant, but marks the junction between fast-flow features including drumlins and mega-lineations on the right, and ice stagnation features on the left, inherited from an area of sluggish or stagnant ice flow.


Palaeo-ice stream landsystem
By Jacob Bendle

Lower Skelton Glacier, Antarctica, looking downglacier.  A smaller ice stream comes in from the left and converges with the larger one, with streamlining / flowline features disappearing into the distance.  Note the area of relatively stagnant ice on the up-glacier (near) side of the confluence.  

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