10Be and 26Al exposure history of the highest mountains in Wales: evidence from Snowdon and the Glyders
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Date
2019-07-28
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International Union for Quaternary Research (INQUA)
Abstract
Cosmogenic 10Be and 26Al exposure ages from samples collected from the mountains of Glyders (1001 m) and from Snowdon (1085 m) in Wales provide new insights into the glacial history of the highest mountains in the British Isles outside of Scotland. Sample elevations range from 598 to 1013 m and taken from centimetre-thick quartz veins on bedrock and also one boulder. The summits of the Glyders are characterised by intensely modified frost-shattered surfaces and have long been recognised as exemplars of mountain summit periglacial activity in the British Isles. However, numerous glacially-transported boulders on the highest ground indicate that ice overran the summits. Bedrock samples at c. 960 m yielded 10Be exposure ages of 72 ka and 153 ka. In addition, a glacially-transported boulder at 985 m gave a 10Be age of 57 ka. This boulder sample is important because it negates issues of inheritance that are possible with bedrock samples and it provides the closest estimate of the timing of ice thinning and the true exposure age of the Glyders summits. All 26Al ages were consistent indicating non-complex histories. These results clearly confirm the Glyder summits were overtopped by the Welsh Ice Cap during marine isotope stage (MIS) 4, when ice in this area was thicker than at the global last glacial maximum (LGM) in MIS 2. The summits were revealed as ice thinned during the transition from MIS 4 to 3. Both the geomorphological evidence and our new cosmogenic ages support recent ice cap modelling predictions that the summits then stood as nunataks above the LGM ice sheet surface in MIS 2. The oldest 10Be (and 26Al) age of ~150 ka from the frost-shattered summit tor displays significant nuclide inheritance and previous work has demonstrated evidence of gibbsite in the summit soils. The wide range in 10Be apparent exposure ages and the evidence of glacially-transported boulders on intensely frost-shattered bedrock suggests erosion of the Glyder summit surfaces largely proceeded by plucking/quarrying rather than abrasion. This would have occurred under cold-based conditions with ice flow dominated by internal deformation rather than sliding. In contrast, at altitudes below 900 m ice-scoured rock surfaces on both the Glyders and neighbouring Snowdon yield exposure ages consistent with deglaciation after the global LGM in MIS 2. Based on these ages and similar results from other summits in North Wales the Welsh Ice Cap rapidly thinned after c. 20 ka leading to a phase of alpine-style glaciation. However, on Snowdon, arête crests yielded very young apparent ages of ~ 5 ka. These young ages reflect continual stripping of the arête rock surfaces through the current interglacial.
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Keywords
United Kingdom, Mountains, Beryllium 10, Aluminium 26, Quartz, Frost, Ice, Rocks, Soils
Citation
Hughes, P., Glasser, N., & Fink, D. (2019). 10Be and 26Al exposure history of the highest mountains in Wales: evidence from Snowdon and the Glyders. Poster presented to the 20th INQUA Congress 25th - 31st July 2019, Dublin, Ireland. Retrieved from: https://virtual.oxfordabstracts.com/#/event/public/574/submission/1886