More air-fall tuffs from the Southern Highland Group, Dalradian Supergroup, Scotland

Abstract

Following the discovery of ten ‘brown beds’ near Birnam, Perthshire, and their identification as air-fall tuffs (Batchelor 2004), further fieldwork along the regional strike has yielded more examples of these unusual beds. Each bed has a distinctively brown amorphous appearance in the field, with sharp upper and lower contacts with its host metasediment, and thicknesses range from 5 mm to 100 mm. The geographical distribution of the localities is shown in Figure 1. Description The first locality SW of Birnam which yielded ‘brown beds’ is at a small cliff on the west side of the Sma’ Glen, near Crieff [NN894305]. This cliff lies about 20 m above the road and is located opposite field gates, near to Clach Ossian. The cliff comprises vertical beds of psammite and grit with quartz segregations. Five ‘brown beds” were identified, four of which were sampled (GH/RB/52–55). Two of these beds formed an elliptical pod, suggesting tectonic squeezing of a soft material. The second ‘brown bed’ locality SW of Birnam is at the south end of the Glen Lednock road in Comrie. An isolated bluff of psammites and pelites crops out amid trees on the NE side of the road, above the point where a signpost directs walkers to the “Devil’s Cauldron”. Within the restricted outcrop [NN77002225], three ‘brown beds’ were found (GH/RB/61–63). The third ‘brown bed’ locality, and the richest in terms of individual beds, is on the east side of Loch Lomond, below the road between Balmaha and Rowardennan. A track leaves this . . . This work was funded by the Carnegie Trust for the Scottish Universities as part of a large collaborative project under the direction of Dr Grahame Oliver (University of St Andrews). Angus Calder provided the XRF and XRD analyses and my wife Rosalind Garton assisted with fieldwork.