The Torridon Group
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.
Figures & Tables
The Torridonian sandstones form one of the principal elements of British stratigraphy. They form the majestic mountains of NW Scotland and also extend westwards under the Minch basin. The sediments were deposited in a Proterozoic rift nearly contemporaneous with the Keweenawan Supergroup of North America.
This book contains the first complete field description of the rocks and the sedimentary environments in which they formed, together with a comprehensive examination of their tectonic and palaeoclimatic significance, geochemistry, palaeomagnetism and diagenesis. It includes the result of over forty years’ work by the author, most of it previously unpublished.