Periglacial trimlines in the Scottish Highlands

Two periglacial weathering limits have been identified on mountains in the Highlands of Scotland. The lower has been interpreted as a periglacial trimline marking the upper limits of Loch Lomond (Younger Dryas) Stade glaciers, and has been employed to reconstruct the dimensions of parts of the main West Highland icefield and certain smaller glaciers and icefields. Interpretation of the upper weathering limit has been more contentious, but recent research has demonstrated: (1) significant contrasts in the degree of bedrock weathering above and below this limit; (2) that gibbsite, a pre-Devensian weathering product, is abundant above the limit but rare in soils farther downslope; (3) that the isotopic (Al-26) age of bedrock exposures above the limit greatly exceeds that at lower altitudes; and (4) that the altitude of the weathering limit descends regularly along former ice-sheet flowlines. Collectively, these attributes suggest that the upper weathering limit is a periglacial trimline defining the maximum altitude of the last ice sheet. Calculated basal shear stress values are consistent with this interpretation, which implies that the mainland ice-shed lay at ca. 900 m in Wester Ross and descended greatly northwards, whilst Harris and Lewis in the Outer Hebrides supported an independent ice-cap that achieved a maximum altitude of ca. 700 m. Wider implications of this interpretation and the future potential for ice-sheet reconstruction are explored. Copyright (C) 1996 INQUA/Elsevier Science Ltd.