Torridonian

  In geology, Torridonian describes a series of proterozoic arenaceous sedimentary rocks, extensively developed in the Northwest Highlands of Scotland, and particularly in the district of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks by James Nicol.

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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 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.

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.

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.