In western Scotland, late Precambrian sedimentary rocks lie unconformably on the Lewisian basement complex on the foreland and in the marginal nappe zone of the Caledonian orogen. The name "Torridonian" has been applied indiscriminately to all of these rocks. The extensive, 7,000-m-thick wedge arkoses of the predominantly red Torridon Group include all the sandstone of the original type area about Loch Torridon. Locally, they unconformably overlie similar red sandstone (Stoer Group) which is more than 2,000 m thick.

In the Caledonian Kishorn nappe of Skye, red Torridon Group sandstone is underlain, apparently conformably, by 3,000 m of cross-bedded gray sandstone and siltstone (Sleat Group). These sandstone and siltstone units are thought to indicate the margin of an important sedimentary basin. Correlation of the Sleat and Torridon Groups with the Lower Dalradian is suggested.

In Islay and Colonsay, a turbidite succession--succeeded by cross-bedded arkose, quartzose graywacke, mudstone, dolomite, and quartzite--probably lies unconformably on the Lewisian basement. The thickness of the rocks overlying the basement is more than 5,000 m; formerly, they were referred to the Torridonian. The rocks have undergone low-grade Caledonian metamorphism in the marginal nappe zone. They are Precambrian but otherwise uncorrelated.

Sandstone and conglomerate in central Islay (Bowmore Group), Iona (Iona Group), and near Stornoway (Stornoway Formation) commonly have been assigned to the Torridonian. The Stornoway Formation may be Triassic. The other units are of uncertain Precambrian age.