Periglacial features in part of the south-east Grampian highlands of Scotland
The study examines the nature and distribution of periglacial lobes, terraces, gliding boulders, rockfalls and avalanches in granite and metaroorphic terrains in 8, part of the South-East Grampian Mountains of Scotland. Areal variations in the weathering of granite boulders were also investigated. Massive granite boulder lobes are extensively developed upon slopes in the Lochnagar and Mount Keen granite areas. The values of 14 parameters, established to describe the main elements of a lobe, were measured on 300 granite lobes. Various techniques were used to analyse the resultant dato,. Granite lobes are rare upon north-facing slopes, and absent from ground believed to have been covered by Loch Lomond age glaciers. It is concluded that these lobes are fossil features formed during the severe conditions of the Loch Lomond Stadial. Detailed sampling of 50 metamorphic lobes revealed that they are smaller features, largely restricted to the upper slopes of quartzite hills. Terraces are rare in both types of terrain. Gliding boulders are common in the granite and metamorphic areas. The values of 15 parameters were measured on 200 metamorphic and 150 granite gliding boulders. The resultant data were analysed and compared. Individual boulder movements of up to 2cm/year were recorded between 1971 and 1975. Dendrochronological investigations of calluna and vaccinium plants growing in four furrows suggested that gliding boulder movements have decreased in the last 7 or 8 years. Rockfalls are frequently released from the backwalls of the three Lochnagar corries, contributing to the extensive postglacial screes. Nine rockfalls were observed between 13 June and 1 August 1972. The largest fall involved almost 4 tonnes of granite boulders. Avalanches are common on snow accumulation slopes. Their erosional activity is restricted, but they are efficient transporting agents. Four techniques were used to investigate possible differences in the extent of weathering of granite boulders in sites 'inside' and 'outside' the mapped limits of four presumed Loch Lomond age corrie glaciers. The tests assessed the amount of edge and corner rounding of the blocks, and also the degree of surface and subsurface granular disintegration. All tests indicated that boulders 'inside' the presumed limits are marginally but consistently less 'weathered' than those in 'outside' sites, independently supporting the view that corrie glaciers occupied these sites.