Glaciation and deglaciation of the SW Lake District, England: implications of cosmogenic 36Cl exposure dating

• Published in: Proceedings of the Geologists' Association
• Main Authors: Ballantyne, Colin K., Stone, John O, Fifield, L Keith
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier
• Subjects: Keywords: andesite; bedrock; chlorine isotope; cosmogenic radionuclide; dating method; deglaciation; geochronology; glacial erosion; glaciation; Last Glacial Maximum; last ice sheet; Loch Lomond Stadial; periglacial process; rhyolite; rockfall; Cumbria; England; Eu Cosmogenic 36Cl; Deglaciation chronology; Periglacial trimlines; Surface exposure dating; Loch Lomond; Ice Sheet
• Online Access: http://hdl.handle.net/1885/55609; https://doi.org/10.1016/j.pgeola.2009.08.003; https://openresearch-repository.anu.edu.au/bitstream/1885/55609/5/ANUP_1910.pdf.jpg; https://openresearch-repository.anu.edu.au/bitstream/1885/55609/7/01_Ballantyne_Glaciation_and_deglaciation_of_2009.pdf.jpg

Exposure dating using cosmogenic 36Cl demonstrates that the summit plateau of Scafell Pike (978 m) in the SW Lake District escaped erosion by glacier ice during the last glacial maximum (LGM; c. 26-21 kyr) and probably throughout the Devensian Glacial Stage (MIS 5d-2). Exposure ages obtained for ice-moulded bedrock on an adjacent col at 750-765 m confirm over-riding and erosion of bedrock by warm-based glacier ice during the LGM. The contrast between the two sites is interpreted in terms of preservation of tors, frost-shattered outcrops and blockfields on terrain above 840-870 m under cold-based ice. An exposure age of 17.3 ± 1.1 kyr for the col at 750-765 m suggests that substantial downwastage of the last ice sheet had occurred by c. 17 kyr, consistent with deglacial exposure ages obtained for other high-level sites in the British Isles. An exposure age of 12.5 ± 0.8 kyr obtained for a glacially transported rockfall boulder within the limits of later corrie glaciation confirms that the final episode of local glaciation in the Lake District occurred during the Loch Lomond Stade (c. 12.9-11.7 kyr). This research also demonstrated the difficulties of obtaining reliable exposure ages from rhyolite and andesite bedrock that has proved resistant to glacial abrasion.