The Torridon Group

Abstract

The group unconformably overlies both the Lewisian gneiss complex and the Stoer Group, and is in turn unconformably overlain by the Lower Cambrian (Figs 2 & 22). The unconformity surface at the base is generally rugged, with relief reaching 600 m. The maximum thickness of the Torridon Group is about 7 km onshore and 6 km offshore in the Sea of the Hebrides basin (Stein 1988, fig. 11; 1992, fig. 2B), but albitization of the highest beds indicates that the original thickness was 3-4 km greater. Lake deposits at the bottom of the group occupying palaeovalleys in the gneiss are followed by kilometres of red sandstones, all deposited in a subsiding rift. Alluvial sands interfinger with lake sediments to form cyclothems at the top of the group. As mentioned in the Introduction the Torridon Group is by far the most extensive and voluminous part of the Torridonian (see Plate 1), but nevertheless poses fewer problems of interpretation than the Stoer and Sleat Groups.The formal stratigraphy established by the Geological Survey (Geikie 1894) has been retained even though it is in some respects unsatisfactory. The sediments filling the palaeovalleys at the base of the group form a well-defined lithostratigraphic unit, the Diabaig Formation, characterized by breccias and sandstones derived from the immediately adjacent basement (Fig. 23). The Cailleach Head Formation at the top of the Group is also a valid lithostratigraphic unit, formed of coarsening-upward cyclothems of grey shale and red sandstone. The bulk of the Torridon Group, however, is