The extent of a large mountain icefield that existed in the western Grampians during the Loch Lomond stade has been mapped. The main types of evidence used in establishing the limits comprise moraines, thick drift, fluvioglacial landforms, erratics, ice‐smoothed bedrock, striae, friction cracks and relict periglacial forms. Trimlines on 198 spurs, and various forms of glacial and periglacial evidence on 73 cols and in c. 200 cirques, enable the upper limits and morphology of the icefield to be reconstructed. Abundant striae and friction cracks in many areas enable ice‐flow directions and the surface form of the icefield to be inferred in some detail. The icefield had ice‐shed altitudes of c. 750 m, a maximum width of 80 km and an area over 2,000 km2. At least 60 nunataks stood above the icefield, and on its western side outlet glaciers reached the sea and flowed for as much as 28 km along major tidal water lochs.

SYNOPSISTrimlines, indicated by marked contrasts between smooth ice-moulded and severely frost-affected bedrock surfaces, are used to map Loch Lomond Advance limits in an area extending from Rannoch Moor in the east, to North Ballachulish in the west. Supplementary evidence is provided by the distribution of hummocky moraines, thick till, erratics, fossil periglacial screes, lobes and terraces, and by abundant glacial markings. Reconstructed glacier surfaces indicate two large outlet glaciers flowing westwards towards Loch Linnhe from an ice cap on Rannoch Moor. The two glaciers were joined by ice flowing from numerous corries and tributary troughs.The primary aim of this paper is to outline the methodology, usefulness and problems associated with the mapping of trimlines in a highly dissected mountain area in the western Grampians. For the purpose of this paper a trimline is defined as a narrow zone (10-30 m) separating strongly ice-moulded, from frost-affected bedrock, below and above the trimline respectively.A comprehensive range of morphological evidence relating to the Loch Lomond Advance has been utilized to reconstruct former glacier limits and surfaces ( T h o m pson ). The main forms of evidence include the distribution of end, lateral, fluted and h u m m o c k y moraines, fluvioglacial landforms, boulder fields, till limits and their spatial relationships with fossil periglacial lobes, terraces, screes, blockfields and frost wedges.These attempts to map former glaciers have depended largely on the recognition of limits based on end, lateral and h u m m o c k y moraines, and on working up-glacier from the ablation zone to the accumulation zone where morphological evidence is generally less abundant. Such methods, however, are not suitable for the area under consideration as distinct unequivocal end and lateral moraines are absent (Charlesw o r t h 1955), except for one important inferred end moraine described on p. 61 Instead it is suggested that the upper limit of the glaciers can be most accurately defined in a mountainous area where nunataks were abundant, primarily by mapping trimlines and by working from the accumulation zone towards the ablation Scott.

Surface and basal long profiles are reconstructed for 13 outlet glaciers that drained ice from a large ice field (80 km by 120 km) that formed in the western Grampians of Scotland during part of the Late-glacial period (c, 14000–10000years BP). Basal shear stresses are calculated at 5 km intervals along the central flowlines of the reconstructed outlet glaciers. Individual basal shear stresses for the outlet glaciers range from 10 to 204 kPa. Variations in calculated basal shear stresses within and between the glaciers are mainly explained by differences in bedrock topography, extending and compressional flow, and by differences in basal boundary conditions. Low basal shear stresses (<53kPa) calculated for the terminal zones of Creran, Menteith and Lomond glaciers are partly explained by the overriding of glaciomarine clays with inferred high pore-water pressures and a low yield strength that may have led to rapid basal sliding and thinning of the ice lobes.

ABSTRACT. Surface and basal long profiles are reconstructed for 13 outlet glaciers that drained ice from a large ice field (80 km by 120 km) that formed in the western Grampians of Scotland during part of the Late-glacial period (c. 14 ODD-10000 years BP). Basal shear stresses are calculated at 5 km intervals along the cen tral fl owlines of the reconstructed outlet glaciers. Individual basal shear stresses for th e outlet glaciers range from 10 to 204kPa. Variations in calculated b asal shear stresses within a nd between the glaciers are mainly explained by differences in bedrock topography, extending and compressional flow, and by differences in b asal bound a ry conditions. Low basal shear stresses «53 kPa) calculated for the term inal zones of Creran, Menteith and Lomond glaciers are partly explained by the overriding of glaciomarine clays with inferred high pore-water pressures and a low yield streng th tha t may have led to rapid basal sliding and thinning of the ice lobes .