Precambrian Rocks in NW Scotland
(The following text is adapted from an essay on Assynt Geology hosted at earth.leeds.ac.uk and written by Rob Butler, University of Aberdeen.)
The Lewisian gneisses
The oldest rocks in the British Isles are the Lewisian gneisses. These ancient rocks show evidence for a long history of shearing, recrystallisation and igneous activity. All this points to episodes of ancient mountain building. Lewisian rocks are found in the Outer Hebrides and along parts of the NW Scottish mainland (Fig. 1). The oldest material is termed “Scourian” – named after the village of Scourie in Sutherland. Most of this material probably started as various types of igneous rock - most likely the roots of old volcanoes such as in modern Japan. The old magma chambers have been highly deformed and metamorphosed forming orthogneiss.
In the Lewisian the oldest crust formed about 3000 to 2700 million years ago. The shearing that reshaped the crust happened about 2450 million years ago, in an event called the “Badcallian”. After the Badcallian, the gneisses were split and intruded by sheets (dykes) of igneous rocks about 2400 million years ago and again at about 1900 million years ago. These intrusions, collectively known as the Scourie dykes, have a basic composition and must have come from melting part of the upper mantle – perhaps during a weak rifting episode. The gneisses and dykes were then sheared and metamorphosed again. The last serious activity in the Lewisian happened about 1800 million years ago, during the so-called Laxfordian orogeny. In places the shearing associated with this has smeared all the old Lewisian constituents (like dykes and gneissic banding) into parallel and is associated with local melting (migmatisation), with granitic dykes invading parts of the crust.
The Torridonian rocks
When the Lewisian gneisses got to the earth's surface - about 1000 million years ago - the uplift and erosion had worn away much of the older mountain ranges. However, out to the west of mainland Scotland there were mountains of Lewisian gneiss and these shed debris eastwards. The material piled up and as the Torridonian succession, which is mainly red (i.e., oxidised) sandstone. This material covered the Lewisian preserved as an “unconformity”, representing the Earth's surface at the time the Torridonian was deposited. The detritus probably piled up to over 8 km thick but much of this has since been eroded. The detritus that formed the Torridonian sandstone was carried by rivers and, the channels of which are still preserved in places.