Deglaciation and neotectonics in SE Raasay, Scottish Inner Hebrides

Abstract

Changes in the physical landscape of SE Raasay at the end of the last Quaternary glaciation are examined. The area is marked by a major fault system defining the Beinn na Leac Fault Block, and field survey shows this to comprise a rollover anticline in the SW, with extensional movement towards the NE along an oblique transfer fault, the Main Beinn na Leac Fault. The fault system was reactivated after the Last Glacial Maximum. Survey of a distinctive ridge of detached scree along the Main Beinn na Leac fault shows it to have involved a single movement of at least 7.12 m vertical displacement, arguably the greatest fault movement since before the Younger Dryas in Scotland. The present work confirms that the scree became detached during the Younger Dryas, but finds that it overlies a lacustrine deposit of at least 5.6 m of laminated sediments from a lake which had begun to accumulate earlier. Radiocarbon dating of peat overlying the lake sediments gave 10 176–10 315 cal years BP, but morphological and stratigraphical evidence indicates that drainage of the lake occurred earlier and only shortly before movement of the scree. Possible causes of displacement at the fault system are briefly discussed.