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Rocks of NW Scotland - Geological History

The geological history of NW Scotland stretches back more than halfway to the origin of the Earth and solar system. Here it is considered in five stages. See below for a timeline.

Lewisian Gneiss Complex:

Building the continental crust (2900 to 1750 million years ago)
The crust of NW Scotland, together with parts of Greenland and North America that make up the ancient continent of Laurentia, was built up mainly from igneous rocks that crystallized around 2900 to 2700 million years ago. At that time, the rocks we now see were deep in the Earth's crust. They were deformed and metamorphosed at very high temperatures, producing gneisses with a folded layering. Actually, there were two periods of deformation and metamorphism (named the Scourian and Laxfordian events), separated by a stable period when the crust fractured and allowed in basic magma that crystallized as a set of dykes.

To see the rocks, visit: Scourie; Achmelvich; Laxford

Folded gneisses at Achmelvich, formed deep in the continental crust

Torridonian red beds:

Rivers and lakes (1200 and 1000 million years ago)
By 1200 million years the old continent had been eroded down to a landscape of low rocky hills. A great thickness of red sandstone, brought in by rivers, buried the old hills. Lakes formed, and then dried out. This cycle of events happened twice: the older red sandstones (the Stoer Group) were tilted, eroded and overlain about 1000 million years ago by an even greater thickness of river-deposited red sandstone, the Torridon Group. At the present day the sandstones are being eroded away, so that in places the ancient landscape can be seen once more.

To see the rocks, visit: Clachtoll; Stoer; Loch Assynt

Dried-out lake sediments: polygonal cracks in mudstone filled with light-coloured sand

Cambrian sediments:

Shallow sea (540 to about 490 million years ago)
After another long interval, this part of Laurentia found itself next to the ocean. The deep ocean in the Cambrian period (about 540 million years ago) was away to the south-east, but here in NW Scotland the water was shallow and relatively quiet. A rather thin sequence of sandstones, siltstones and limestones formed in coastal sand-bars, tidal flats and in the shallow sea. Now there were animals active on the sea floor, burrowing and grazing: we can see the imprints they left. Also there were tiny shelled creatures, unlike anything known today. Limestone deposition continued until about 490 million years, into the Ordovician period.

To see the rocks, visit: Skiag Bridge; Glencoul

Hard white quartz sandstones laid down in the Cambrian sea

Igneous rocks of the Assynt area:

Intrusions of unusual magma (430 million years ago)
At about 430 million years ago this part of NW Scotland was intruded by magmas, which crystallized in the form of sills (sheets parallel to the bedding of the enclosing sediments), dykes (steep, cross-cutting sheets) and plutons (larger, rounded or irregular masses). These are unusual igneous rocks of alkaline composition (rich in potassium and sodium). The typical rock is a syenite, rich in potassium feldspar, rather than a granite or gabbro. The heat from the intrusions metamorphosed nearby Cambro-Ordovician limestones, converting them into marble.

To see the rocks, visit: Borralan; Ledmore

White-spotted borolanite, a variety of syenite

Moine Thrust zone:

Tectonics: collision of continents, and overthrust faulting
Meanwhile, a range of mountains, the Caledonian mountain belt, was forming to the south-east, as other continents began to collide with Laurentia during the Ordovician period. At about 430 to 420 million years ago a vast slab of metamorphic rocks, the Moine schists, was pushed out of the mountain belt, over the top of the rocks of NW Scotland, much as the Himalayas are today being pushed out over India. Along the base of the Moine rocks, the intense shearing deformation produced a fine-grained platy rock called mylonite. Here in NW Scotland about 100 years ago, pioneering geologists discovered one of the world's most impressive zones of overthrusting: they recognized the significance of the mylonite, and observed that older metamorphic rocks lay on top of younger sedimentary rocks.

To see the rocks, visit: Knockan; Glencoul

Platy mylonites, very strongly sheared rocks of the Moine Thrust zone


The localities that can be visited are in blue. Click on them to go there and see the rock images. C marks the Cambrian period.

Timeline for the geological evolution of NW Scotland

Many of the images can also be accessed through a map and cross-section that shows the rocks in their geographical and spatial relationships, and a stratigraphic column that shows them in vertical sequence, youngest at the top.

Scourie Achmelvich Laxford Clachtoll Stoer Assynt Skiag Bridge Glencoul Knockan Borralan Ledmore
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D.J. Waters, Department of Earth Sciences, May 2003