A Former Ice-Dammed Lake and Associated Glacier Limits in the Achnasheen Area, Central Ross-Shire

Lake shorelines, delta terraces and fine lake-floor sediments show that an ice-dammed lake existed around the site of Achnasheen in Strath Bran in northern Scotland. Associated with the lake were overflow channels, areas of water-swept bedrock and cross-valley moraines. The lake existed between four glaciers that converged on the site of Achnasheen but failed to meet each other. The ice-dam was a major glacier that flowed into and across Strath Bran, where it produced a major end moraine. The sequence of lake levels and associated geomorphic events is elucidated. Particular attention is given to the conspicuous terraces of the area: these include a major delta terrace, steeply sloping terraces associated with sudden lowering of lake level, outwash terraces and a terrace that represents the infill of a drift-dammed lake. Similar terraces have been described by the writer elsewhere in Scotland and are included with the Achnasheen evidence in a general consideration of terraces with particular reference to ice-dammed lakes. The hamlet of Achnasheen is located in the heart of the Northern Highlands of Scotland close to the divide between eastward- and westward-flowing streams (Figs 1 and 6). At Achnasheen two major through valleys, one from the west and the other from the south-west, join together to become Strath Bran. Around Achnasheen the floors of these valleys are largely occupied by conspicuous terraces, which are the principal features of the area designated as a Site of Special Scientific Interest by the Nature Conservancy Council. Since Achnasheen lies at the junction of two main roads and is served by a railway line, the terraces have been visited by innumerable geomorphologists/geologists during the last hundred years. Yet the last published consideration of them is in a Geological Survey memoir dated 1913. Here Peach et al. interpreted the highest terrace as a delta deposited in an ice-dammed lake. They referred to lateral moraines crossing the River Bran 3 km east of Achnasheen, interpreting them as recording the western margin of a lobe of ice from the north that blocked Strath Bran. However they did not elaborate the conse- quences of this interpretation or present the abundant evidence that supports it

Archaeology · Geography · Geomorphology · Glacier · Physical geography · Shire · Cryospheric studies and observations · Geology · Geology and Paleoclimatology Research · Soil erosion and sediment transport

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