TORRIDONIAN  , in

geology, a series of pre-Cambrian arenaceous sediments extensively developed in the north-west high-lands of Scotland and particularly in the neighbourhood of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks by J . Nicol . The rocks are mainly red and chocolate sandstones, arkoses, flagstones 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 . Their outcrop extends in a belt of variable breadth from Cape Wrath to the Point of Sleet in Skye, running in a N.N.E.-S.S.W. direction through Ross-shire and Sutherlandshire . They form the isolated mountain peaks of Canisp, Quinag and Suilven in the neighbourhood 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 Rum, Raasay, Soay and the Crowlin Islands . The Torridonian rocks have been subdivided into three groups: an upper Aultbea group, 3000-5000 ft.; a middle or Applecross group, 6000-8000 ft.; and a lower or Diabeg group, 500 ft. in Gairloch but reaching a thickness of 7200 ft. in Skye . See " The Geological Structure of the North-West Highlands of Scotland," Mem . Geol . Survey (Glasgow, 1907) .

(J . A .