NUNATAKS OF THE LAST ICE-SHEET IN NORTHWEST SCOTLAND

High-level weathering limits separating ice-scoured topography from an upper zone of frost-weathered detritus were identified on 17 mountain s in NW Scotland at altitudes of < 600 m to > 900 m, and a further 6 p eaks were found to support evidence of ice scouring to summit level. W eathering limits are most clearly defined on Torridon Sandstone, which is resistant to frost shattering, but can also be mapped on Cambrian Quartzite, Lewisian Gneiss and Moine Schist. Contrasts in degree of ro ck surface weathering above and below the weathering limits were evalu ated using measurements of joint depth and rock surface hardness, and through X-ray diffraction analyses of clay mineral assemblages. The re sults indicate significantly more advanced rock and soil weathering ab ove the weathering limits. Widespread persistence of gibbsite above th e weathering limits suggests that they represent the upper limit of La te Devensian glacial erosion, and the regularity of the decline in wea thering limit altitude along former flowlines eliminates the possibili ty that it represents a former thermal boundary between protective col d-based and erosive warm-based ice. The weathering limits are therefor e interpreted as periglacial trimlines defining the maximum surface al titude of the last ice sheet around former nunataks. Calculated basal shear stresses of 50-78 kPa are consistent with this interpretation. T he altitude of the trimlines implies that the former ice shed lay at 9 00-930 m in the Fannich Mountains and descended gently northwards, and that the ice surface descended NW from ice shed to < 500 m over the e xtreme NW tip of Scotland and to 700-730 m at the head of Little Loch Broom.