The origin of the glaciotectonised lacustrine sediments in Gleann Ballach

Controversy has surrounded the origin of the sediments that underlie the generally flat–topped asymmetric ridge, up to 60 m in height, which extends from the western side of Gleann Ballach into the valley floor at (NH 6513 0106) (Fig. 60A; Boston and Trelea-Newton, this guide: 17). The most recent description of the deposits by Gheorghiu et al. (2012) is based principally on the sequence exposed in a river cliff of the Allt Ballach at the eastern end of the ridge at c. 490 m OD. The 6 to 7 m-high exposure is dominated by well-sorted, fine- to medium–grained sands with planar lamination and ripple cross bedding, with lenses of till, laminated clay, and sparse dropstone cobbles; all indicating deposition in a glaciolacustrine setting. Gheorghiu et al. (2012, p. 137) supplemented the information from the exposed sequence by augering to the west of the exposure, which they suggest confirms that similar lacustrine sediments underlie the main part of the ridge. This reaches an elevation of c. 560 m OD and coincides in altitude with a ‘horizontal notch cut into bedrock’ that they interpret as an ‘ice-marginal meltwater route’. Significantly, cosmogenic ages on boulders ‘on top of the lake deposit at 550 m OD and 528 m OD’ yielded exposure ages of 11.6 ± 1.1 ka and 10.9 ± 1.0 ka, with a mean age of 11.2 ± 1.1 ka. These ages have now been revised to 13.7 ± 0.8 ka and 12.8 ± 0.8 ka respectively, with a mean age of 13.2 ± 0.8 ka (Gheorghui and Fabel, this guide: 18).