Hebridean terrane

Similar rocks are also thought to be present in Shetland and have been proved west and north of the Outer Hebrides by BGS shallow boreholes and hydrocarbon exploration wells.

[1] The Lewisian complex consists of mainly granitic gneisses, subject to a series of metamorphic and tectonic events, interrupted by the intrusion of a major dyke swarm.

At the beginning of the Proterozoic, the Scourian gneisses were locally affected by deformation and retrogression to amphibolite facies in the Inverian event, which overlapped in time with the emplacement of the Scourie dykes.

[1] The Torridonian is a sequence of Neoproterozoic sediments, mainly sandstones that rest unconformably on an old land surface, with up to 300 m of relief locally.

It is metamorphosed to greenschist facies and sits within the Kishorn Nappe, part of the Caledonian thrust belt, making its exact relationship to the other outcrops difficult to assess.

The lowest part of this group consists of a basal breccia passing up into sandstones and subordinate shales deposited as alluvial fans building out into ephemeral lakes.

[3] The An-t-Sron Formation consists of calcareous siltstones of the Fucoid Beds[4] and overlying sandstone of the Salterella Grit.

It consists of several hundred metres of mainly dolomites with some limestones and cherts, although the effects of later faulting make precise estimates of thickness difficult.

Blocks within the Lewisian complex were then juxtaposed by Inverian and Laxfordian deformation from about 2.4–1.7 Ga. At about 1200 Ma, the terrane was affected by extensional tectonics causing rifting and the deposition of the thick coarse clastic sequence of the Torridonian.