BRITICE map showing the suggested exposure time since ice sheet inundation across the UK. Note that in Pembrokeshire, it is suggested that north Pembrokeshire was glaciated during the LGM, but south Pembrokeshire was not. That is the hypothesis that needs to be tested.
It's good to be a part of this research group. This is Alissa's short presentation for a recent geomorphology conference in New Zealand. Alissa, Prof John Hiemstra, Prof Iain Robertson and I collected numerous rock samples in early June which are currently being processed prior to analysis.
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Using multiple cosmogenic nuclides to constrain the British-Irish Ice Sheet in Pembrokeshire (Wales) during the Last Glacial Maximum (LGM)
Dr Alissa Flatley, Dr John Hiemstra, Dr David Fink, Dr Keith Fifield, Dr Brian John, Dr Reka Hajnalka Fülöp
Dr Alissa Flatley, February 5, 2026
There is considerable debate surrounding the southern limit of the British Irish Ice Sheet (BIIS) particularly in Pembrokeshire, Wales. This is partly due to differing interpretations of the sedimentary and geomorphic record, but a lack of detailed geochronology of regional LGM ice extent is also a major barrier. In Pembrokeshire, proposed LGM ice limits trace the northern flanks of the Preseli Hills suggesting that southern Pembrokeshire may have remained ice-free. However, significant uncertainty remains regarding the interaction between the Irish Sea Ice Stream and the Celtic Sea lobe, and the maximum extent of the Welsh Ice Cap. This project addresses these uncertainties by using multiple in-situ cosmogenic nuclides 10Be, 36Cl and 14C in lithologically varied samples across Pembrokeshire to further our understanding of the region's deglaciation history.
Dr Alissa Flatley, February 5, 2026
There is considerable debate surrounding the southern limit of the British Irish Ice Sheet (BIIS) particularly in Pembrokeshire, Wales. This is partly due to differing interpretations of the sedimentary and geomorphic record, but a lack of detailed geochronology of regional LGM ice extent is also a major barrier. In Pembrokeshire, proposed LGM ice limits trace the northern flanks of the Preseli Hills suggesting that southern Pembrokeshire may have remained ice-free. However, significant uncertainty remains regarding the interaction between the Irish Sea Ice Stream and the Celtic Sea lobe, and the maximum extent of the Welsh Ice Cap. This project addresses these uncertainties by using multiple in-situ cosmogenic nuclides 10Be, 36Cl and 14C in lithologically varied samples across Pembrokeshire to further our understanding of the region's deglaciation history.
Cosmogenic nuclides can determine the maximum time a bedrock surface or glacial erratic has been exposed. However, it is possible that exposure may have been intermittent over this time (e.g. complex exposure history) or that the previous exposure signal has been retained (i.e. it has not been removed through erosion under the most recent ice advance), an effect called ‘inheritance’. The issue of nuclide inheritance within samples can yield anomalously ‘old’ exposure ages common in more erosion resistant lithologies prevalent across the Pembrokeshire coastline and therefore it acts as a key scientific constraint surrounding LGM ice sheet reconstruction. Cosmogenic nuclides with a short half-life (e.g. in situ 14C) will be integrated into the project to date short timeframe changes in ice dynamics. For glaciations < ~ 20-30ka, in situ 14C, with a half-life of 5.7 kyr, can be used in conjunction with longer-lived nuclides such as 10Be to identify inherited signatures in samples. This project will provide a much-needed understanding of ice sheet dynamics across the region, advancing understanding of complex landscape processes.
Book of abstracts: 3ce3bf9f40984a6ab3bfc62b2a194cd5.
The proposed ice-free corridor at the time of the LGM -- as featured in numerous publications. This is what the new project seeks to test through a substantial sampling programme.
A more realistic proposal for the extent of LGM glaciation in the South Wales / Bristol Channel arena. Here the inner reaches of the Bristol Channel are essentially ice-free, with the depression occupied instead by a large pro-glacial lake..........
Above pics, thanks to Alissa. John and Iain at work at Maiden Castle, Trefgarn Gorge, in June 2025.
Book of abstracts: 3ce3bf9f40984a6ab3bfc62b2a194cd5.
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Iain, John and Alissa sampling bedrock at one of the north Pembrokeshire sites.
Dr Iain Robertson -- about to take a pXRF reading on the Abermawr erratic boulder
The other scenario. In this BRITICE model (from Clark et al, 2022) LGM ice fills the Bristol Channel and affects the coasts of Cornwall, Devon and Somerset. Note that this reconstruction applies to the Late Devensian only -- it does not preclude a more extensive ice cover during earlier glacial episodes.
The ice-free corridor as proposed in some of the early BRITICE work..........
For a detailed study of the problem, see this paper from 2023:
John, B.S. 2023. Was there a Late Devensian ice-free corridor in Pembrokeshire? Quaternary Newsletter 158, pp 5-16.
Above pics, thanks to Alissa. John and Iain at work at Maiden Castle, Trefgarn Gorge, in June 2025.
Me at the Roch Castle rhyolite crag. Thanks to Alissa for the pic. Don't worry --we had permission.......!!
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