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 and the North Atlantic Craton. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0 to 1.7 billion years (Ga). They form the basement on which the Stoer Group, Wester Ross Supergroup and probably the Loch Ness 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.
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👉 Lewisian complex in the context of Thrust fault
A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks.
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Lewisian complex in the context of Assynt
Assynt (Scottish Gaelic: Asainn [ˈas̪ɪɲ] or Asainte [ˈas̪ɪɲtʲə]) is a sparsely populated area in the south-west of Sutherland, lying north of Ullapool on the west coast of Scotland. Assynt is known for its landscape and its remarkable mountains, which have led to the area, along with neighbouring Coigach, being designated as the Assynt-Coigach National Scenic Area, one of 40 such areas in Scotland.
The western part of Assynt has many distinctively shaped mountains, including Quinag, Canisp, Suilven and Ben More Assynt, that rise steeply from the surrounding "cnoc and lochan" scenery. These can often appear higher than their actual height would indicate due to their steep sides and the contrast with the moorland from which they rise. Many of the most distinctive peaks such as Suilven were formed during the last Ice Age, when they were left exposed above the ice sheet as nunataks, and they now remain as inselbergs of highly eroded Torridonian sandstone sitting on a bedrock of much older Lewisian gneiss. The Moine Thrust runs through the area, and is most visible at Knockan Crag National Nature Reserve, which includes a visitor centre interpreting the geological features of the landscape. In the east of Assynt lies a region of limestone scenery surrounding Ben More Assynt. This part of Assynt hosts the longest cave in Scotland, Uamh an Claonaite, which lies five miles (eight kilometres) south of Inchnadamph. The geological importance of Assynt is reflected in its inclusion in the North West Highlands Geopark.