1911 Encyclopædia Britannica/Torridonian

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TORRIDONIAN, in geology, a series of pre-Cambrian arenaceous sediments extensively developed in the north-west highlands 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 Torridania 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 of 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. H.)