Maximum altitude of Late Devensian glaciation on the Isle of Mull and Isle of Jura

Abstract

Synopsis Evidence provided by striae, ice-moulded rock, erratics and perched boulders indicates that the last (Late Devensian) ice-sheet reached an altitude exceeding 760 m on Mull and 660 m on Jura. The highest summits on both islands support periglacial blockfields, suggesting that they remained as nunataks above the ice surface. This interpretation is supported by analyses of clay-fraction mineralogy, which shows that gibbsite (a pre-Late Devensian weathering product) is widespread in blockfield samples but rare in samples below the inferred limit of glaciation, implying removal by Late Devensian glacial erosion. Maximum ice-sheet altitudes of 760–840 m and 660–700 m are inferred for the Ben More massif on Mull and Paps of Jura respectively. Reconstruction of ice-sheet configuration in the Inner Hebrides area suggests that the 900 m ice-surface contour followed the west coast of the mainland, but the altitude evidence is insufficient to constrain the westwards extent of the ice sheet. Inferred ice-surface altitudes and directions of ice movement are incompatible with most theoretical models, and even the ‘best fit’ model of ice dimensions in this area underestimates maximum ice thickness by ≥ 60 m.