The last ice sheet in North-West Scotland: reconstruction and implications

• Main Authors: Ballantyne, Colin Kerr, McCarroll, D, Nesje, A, Dahl, SO, Stone, JO
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1998
• Subjects: CONTINENTAL-SHELF WEST; SOUTHERN NORWAY; PERIGLACIAL PHENOMENA; DEVENSIAN GLACIATION; QUATERNARY DEPOSITS; MAXIMUM ALTITUDE; FORMER NUNATAKS; VERTICAL EXTENT; OUTER HEBRIDES; BRITISH-ISLES; Loch Lomond; Norway; Ice cap; Ice Sheet
• Online Access: https://research-portal.st-andrews.ac.uk/en/researchoutput/the-last-ice-sheet-in-northwest-scotland-reconstruction-and-implications(27ac381e-7261-4f52-9300-38402636eb30).html; http://www.scopus.com/inward/record.url?scp=0032423748&partnerID=8YFLogxK

Recent models of the last Scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation. This proposition has been investigated on 140 mountains over an area of 10,000 km(2) in NW Scotland. Outside the limits of the later Loch Lomond Readvance in this area there is evidence for a single high-level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris. This limit occurs at altitudes ranging from 425 to 450 m in the Outer Hebrides to > 950 m on the mainland, and is best developed on lithologies that resisted breakdown after ice-sheet downwastage. Interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by: (1) joint-depth and Schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit; (2) a much greater representation of gibbsite (a pre-late Devensian weathering product) in the clay fraction of soils above the limit; (3) cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit; (4) the sharpness of the limit at some sites and its regular decline along former ice flowlines; and (5) shear stress calculations based on the inferred altitude and gradient of the former ice surface. Reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from > 900 m to ca. 550 m at the north coast. Independent dispersion centres fed broad ice streams that occupied major troughs. On Skye an ice dome > 800 m deflected the northwestwards movement of mainland ice, but the mountains of Rum were over-ridden by mainland ice up to an altitude of ca. 700 m. The Outer Hebrides supported an independent ice cap that was confluent with mainland ice in the Minches. Extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too ...