Glacial erosion on soft-rock outcrops in Central Scotland

Abstract

In central Scotland, part of the Old Red Sandstone outcrop extends for 100 km, from Loch Lerling to Perth, and forms the bottom of a valley separating two higher-altitude regions composed of hard rock. To the north-west lie the Grampian Highlands (800–1000 m); to the south-east, a zone of hills formed of volcanic rock (Kilpatrick Hills, Campsie Fells, Ochil Hills, and Sidlaw Hills, 400–600 m). In both these regions, the effects of glacial erosion are clearly evident, and it could be assumed that erosion had also affected the softer rocks of the intermediate valley. Until now, however, no landforms created by glacial erosion had ever been observed there. On the contrary, the valley floor shapes are clearly due to glacial accumulation. At the highest points, Ogilvie (1928) noted two surfaces called "peneplains" of Tertiary erosion. Recent research, however, has revealed three distinct landforms in the valley, which have been called: “ice-moulded valley sides”, “tapered interfluves” and “bridge interfluves”. The relationships between these landforms and the ice flow lines (as deduced from the movement of transported stones) have demonstrated that these three forms are residual elements of the preglacial valley floor, which have survived because they are located in the most sheltered positions. Conversely, where the main glaciers crossed the Old Red Sandstone, all the Observations seem to indicate that there was a general displacement of 100 m of stones over an area of ​​more than 60 km² in Strath Earn and 100 km² in the Forth Valley. It is highly probable that these quantities far exceed the volumes of hard rock carried by the same glaciers, where the action of glacial erosion is more easily recognised.