PERIGLACIAL TRIMLINES, FORMER NUNATAKS AND THE ALTITUDE OF THE LAST ICE-SHEET IN WESTER-ROSS, NORTHWEST SCOTLAND

High-level weathering limits separating ice-scoured topography from fr ost-weathered detritus were identified on 28 mountains in Wester Ross at altitudes of 700-960 m, and a further 22 peaks support evidence of ice scouring to summit level. Weathering limits are defined most clear ly on sandstone and gneiss, which have resisted frost shattering durin g the Late Devensian Lateglacial, but can also be distinguished on sch ists and quartzite. Schmidt hammer measurements and analyses of clay m ineral assemblages indicate significantly more advanced rock and soil weathering above the weathering limits. The persistence of gibbsite ab ove weathering limits indicates that they represent the upper limit of Late Devensian glacial erosion. The regular decline of weathering-lim it altitudes along former flowlines eliminates the possibility that th e weathering limits represent former thermal boundaries between protec tive cold-based and erosive warm-based ice. The weathering limits are therefore interpreted as periglacial trimlines that define the maximum surface altitude of the last ice sheet. Calculated basal shear stress es of 50-95 kPa are consistent with this interpretation. Reconstructio n of ice-sheet configuration indicates that the former ice-shed lay ab ove 900 m along the present watershed, and that the ice surface descen ded northwestwards, with broad depressions along major troughs and loc alised domes around independent centres of ice dispersal. Extrapolatio n of the ice surface gradient and altitude suggests that the ice sheet did not overrun the Outer Hebrides, but was confluent with the indepe ndent Outer Hebrides ice-cap in the North Minch basin. Erratics locate d up to 140 m above the reconstructed ice surface are inferred to have been emplaced by a pre-Late Devensian ice sheet (or ice sheets) of un known age. (C) 1997 by John Wiley & Sons, Ltd.