This is my latest attempt to define the footprint of the Irish Sea Ice Stream as it might have appeared around 27,000 years ago. It's bound to be wildly incorrect, but I still think it's more reliable than any of the other maps that have appeared in print. I have tried to build into it the work by Scourse, Praeg, Lockhart and many others on the sedimentary record of the Celtic Sea and the increasing conviction on the part of many that the ice did indeed reach the shelf edge, around 500 km SW of St Georges Channel. I think this also fits the field evidence and most of the dating evidence -- but there are some seriously misleading (and incorrect) dates out there in the literature...... (The dates cannot all be right, because there are huge discrepancies.)
Note that there were two lobes which were parts of the Ice Stream -- one pushing into the Cheshire Plain to the east of the Welsh Ice Cap, and the other pushing through St George's Channel and extending (and spreading) southwards into the Celtic Sea. It should really be called a piedmont lobe -- but at the moment there is no agreement on terminology.
The white lines and arrows relate to the Irish Ice Cap and the Welsh Ice Cap. Both of those fed ice into the ice stream, and there must have been interesting dynamics along the contact zone, with fracturing and shearing, serious thrusting on sediments, and many shifts of position of the ice margins.
We can refer to the ISIS as marine terminating (unlike the Baltic Sea Ice Stream) but parts of the terminus were land-based because at the time sea-level was around 140m lower than it is now. i'm now rather convinced that both the Isles of Scilly and the summit of Lundy Island were nunataks projecting through rather thin glacier ice.
Work is needed on the differences between the "ice contact" margins, the "dry land" ice edges, and the marine terminating ice edges near the edge of the shelf.
One of the reasons why I have shown the ice piedmont spanning the whole extent between the south coast of Ireland and the coast of Cornwall is that the narrow lobe shown by other researchers does not make glaciological sense since they want it to flow across terrain in which there are no topographic constraints. There must have been much more lateral spreading than they have been prepared to admit.
The other issue which still confuses me is the placing of the ice surface contours. Ice in a vast ice stream / piedmont glacier like this can only flow forward all the way to the terminus if a constant surface gradient is maintained. I made suggestions on this many years ago, and am prepared to admit that my surface gradient was steeper than it needed to be, given factors like basal sediments, bed lubrication, basal ice temperatures and shear stress. This needs further thought......
Anyway, here is the hypothesis -- let's see what others think.
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PS. And this is interesting! this is a summary of a 2020 conference presentation. New data fixing maximum ISIS extent as around 26.5 ka, and catastrophic retreat between 25.3 ka and 20.6 ka. Clearly this does not fit all that well with other dating, but no doubt it will all be sorted out in due course.......Chiverrell, R., Thomas, G., Burke, M., Medialdea, A., Smedley, R., Bateman, M., Clark, C., Duller, G., Fabel, D., Jenkins, G., Ou, X., Roberts, H., and Scourse, J.: The evolution of the terrestrial-terminating Irish Sea glacier during the last glaciation,
Comprehensive mapping and the Briticechrono geochronology provides a reconstruction of the last advance and retreat of the only land-terminating ice lobe of the western British Irish Ice Sheet. The Irish Sea Glacier was fed by ice from Lake District, Irish Sea and Wales, and extended to maximum limits in the English Midlands. During ice retreat after 27 kyrs, a series of reverse bedrock slopes rendered proglacial lakes endemic in the land-system. Not resembling the more extensive definitions of the classical ‘Glacial Lake Lapworth’, these ice contact lakes were smaller time transgressive moraine- and bedrock-dammed basins that evolved with ice marginal retreat. Combining, for the first time on glacial sediments, OSL bleaching profiles for cobbles with single grain and small aliquot OSL measurements on sands, has produced a coherent chronology from these heterogeneously bleached samples, and constrained for the Irish Sea Glacier a post 30ka ice maximum advance, 26.5±1.8ka maximum extent, and 25.3±1.6 to 20.6±2.2ka retreat vacating the region. With retreat of the Irish Sea Glacier an opportunistic Welsh re-advance 19.7±2.5ka took advantage of the vacated space and rode over Irish Sea Glacier moraines. Our geomorphological chronosequence shows a glacial system forced by climate, but mediated by piracy of ice sources shared with the larger and marine terminating Irish Sea Ice Stream to the west. The Irish Sea Glacier underwent changes flow regime and fronting environments driven by stagnation and decline as the primary impetus to advance was diverted. Ultimately, the glacier of the English Midlands display complex uncoupling and realignment during deglaciation and ice margin retreat towards upland hinterlands ~17.8 kyrs (Lake District and Pennines) and asynchronous behaviour as individual adjacent ice lobes became increasingly important in driving the landform record.
The evolution of the terrestrial‐terminating Irish Sea glacier during the last glaciation
Richard Christopher Chiverrell , Geoff Stephen Powell Thomas, Matthew Burke et al
JQS Special edition, 07 July 2020
https://doi.org/10.1002/jqs.3229
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PS. There is another new article which looks rather relevant..... I'll take a look...
Bed erosion during fast ice streaming regulated the retreat dynamics of the Irish Sea Ice StreamKatrien J.J.Van Landeghem and Richard C. Chiverrell
Quaternary Science Reviews
Volume 245, 1 October 2020, 106526
https://doi.org/10.1016/j.quascirev.2020.106526
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