Preliminary report on the Quaternary geology and palaeo-iceflow in the Bridge of Orchy : Glen Lyon area, SW Scottish Highlands : June 2003

This report describes the Quaternary deposits and geomorphology of an area of the south-west Scottish Highlands. Both the deposits and the landforms can be broadly divided into glacial, periglacial and recent in origin. As far as possible the nature of the deposits and landforms are described as seen in the field, and subsequent interpretations are made concerning their mode of genesis and their significance in the spatial context. The area has been repeatedly affected by glaciation throughout the Devensian, although most of the evidence remaining reflects only the last stage, the Loch Lomond Stadial glaciation. During this c. 1200 year period, an ice cap is thought to have developed on the mountains to the west of Rannoch Moor and at its maximum extent to have overwhelmed the valleys and many of the mountains of the field area. The extent of the former ice cap has been debated over the last 30 years, and an attempt is made here to assess the various models against the field evidence. A new model is consequently proposed, drawing on many of the findings of earlier workers as well as the new data. An ice cap with a maximum surface altitude of at least 850 m O.D. is suggested to have overwhelmed the area, with a generally east and south-east surface gradient. The ice cap most probably grew from rejuvenated ice that survived throughout the preceding Windermere Interstadial in the high corries and valleys. The ice cap became increasingly influenced by topography as deglaciation ensued, and eventually the valleys of the field area were probably occupied by ice that was being maintained by local accumulation areas.