Lewisian gneiss

The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 Ga. They form the basement on which the Torridonian and Moine Supergroup sediments were deposited. The Lewisian consists mainly of granitic gneisses with a minor amount of supracrustal rocks. Rocks of the Lewisian complex were caught up in the Caledonian orogeny, appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event.

The main outcrops of the Lewisian complex are on the islands of the Outer Hebrides, including Lewis, from which the complex takes its name. It is also exposed on several islands of the Inner Hebrides, small islands north of the Scottish mainland and forms a coastal strip on the mainland from near Loch Torridon in the south to Cape Wrath in the north. Its presence at seabed and beneath Paleozoic and Mesozoic sediments west of Shetland and in the Minches and Sea of the Hebrides has been confirmed from the magnetic field, by shallow boreholes and hydrocarbon exploration wells. Basement rocks of similar type are found at the base of the Moine Supergroup, sometimes with well-preserved unconformable contacts, and these are generally accepted as forming part of the Lewisian, suggesting that the Lewisian complex extends at least as far southeast as the Great Glen Fault. Lewisian like granitic gneisses of Paleoproterozoic age of the 'Rhinns complex' are exposed on Islay and Colonsay in the southern part of the Inner Hebrides. Similar rocks also outcrop on Inishtrahull off the north coast of Donegal and in Mayo where they are known as the 'Annagh Gneiss complex'.

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