A geomorphological reconstruction of the deglaciation of Loch Etive during the Loch Lomond Stadial

Abstract

The Loch Lomond Stadial (LLS) (12.9 – 11.7 ka BP) was the last climatic episode in which Scotland possessed a significant ice mass. Loch and Glen Etive represent one of the best-preserved glacial landsystems in western Scotland, having formed a conduit for ice draining from high on Rannoch Moor. Loch Etive is a glacially over-deepened trough 6 km north of Oban, now a sea loch 30 km long and up to 1.5 km wide, extending from the Firth of Lorn NE towards Rannoch Moor. It is a key site in which to understand the glacial processes that shaped the Western Highlands during the LLS, leading to full glacial retreat by 11.4 ka BP. Based on a comprehensive high-resolution multibeam bathymetric sonar survey, the morphology of the seabed was analysed using ArcGIS to create a series of detailed geomorphological maps. Morphological interpretations, backscatter and seismic reflection data from the loch were combined to reconstruct the deglaciation of Loch Etive. The presence of transverse ridges interpreted as recessional moraines and submarine eskers indicates a dynamic glacial retreat from the maximum seaward extent west of the present-day coastline. At its maximum extent, the glacier formed a proglacial delta generating large volumes of meltwater that flowed over the rock sill of the present-day Falls of Lora. The confluence of glacial ice masses, flowing from the Pass of Brander and tributary corrie glaciers, created a complex glaciofluvial environment in the outer basin of Loch Etive. This is reflected in the formation of a large submarine esker. In the inner loch basin, the presence of recessional moraines at intervals suggests a dynamic glacier, responding to seasonal temperature fluctuations, possibly subsequently retreating northeastwards c. 50 m annually as the LLS ended.