In geology, the term Torridonian is the informal name for the Torridonian Group, a series of Mesoproterozoic to Neoproterozoic arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. The strata of the Torridonian Group are particularly well exposed in the district of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks by James Nicol. Stratigraphically, they lie unconformably on gneisses of the Lewisian complex and their outcrop extent is restricted to the Hebridean Terrane. Rock type The rocks are mainly red and brown sandstones, arkoses and shales with coarse conglomerates locally at the base. Some of the materials of these rocks were derived from the underlying Lewisian gneiss, upon the uneven surface of which they rest, but the bulk of the material was obtained from rocks that are nowhere now exposed. Upon this ancient denuded land surface the Torridonian strata rest horizontally or with gentle inclination. Some of the peaks, such as Beinn Eighe, are capped with white Cambrian quartzite, giving them a distinctive appearance when seen from afar. Some of the quartzite contains fossilized worm burrows and is known as pipe rock. It is circa 500 million years old. The Torridon landscape is itself highly denuded by glacial and alluvial action, and represents the remnants of an ancient peneplain. Occurrence Their outcrop extends in a belt of variable breadth from Cape Wrath to the Point of the peninsula of Sleat in Skye, running in a N.N.E.-S.S.W. direction through Caithness, Sutherland, Ross and Cromarty, and Skye and Lochalsh. They form the isolated mountain peaks of Canisp, Quinag and Suilven in the area of Loch Assynt, of Slioch near Loch Maree, and other hills. They attain their maximum development in the Applecross, Gairloch and Torridon districts, form the greater part of Scalpay, and occur also in Rùm, Raasay, Soay and the Crowlin Islands. They are also found beneath much of the Sea of the Hebrides overlying the Lewisian gneiss. Sub-divisions The Torridonian is divided into the older Stoer Group and the younger Sleat and Torridon Groups separated by an angular unconformity. Paleomagnetic data suggest that this unconformity represents a major time break. These sediments are interpreted to have been deposited during a period of rifting. Stoer Group The Stoer Group outcrops on the peninsula of Stoer, near Assynt, Sutherland. It is subdivided into three formations. Clachtoll Formation A basal