Home of the Scourie dykes and one of the best places to see (and touch!) Lewisian gneiss, the oldest rocks in Western Europe
Located in the North-West Highlands UNESCO Global Geopark, Scourie is known worldwide for its exceptional exposures of ancient metamorphic rock – the Lewisian gneiss – and as the home of the Scourie dykes, which are massive vertical black magmatic intrusions. At Laxford, you will be amazed by the tremendous rock formations at the road-side known as the ‘multi-coloured Rock Stop’.
The rocks at Scourie are mostly composed of three-billion-year-old Lewisian gneiss. Named after the Hebridean Isle of Lewis, these colourful, banded rocks are the oldest in Western Europe. Formed originally as igneous rocks in the Earth’s crust, the gneiss was later buried over 25km below the surface, where it was heated, squeezed and stretched. As the individual minerals which make up the rock were placed under pressure, they reformed and lined up. This helped create the banding effect you can see in the gneiss – it is partly due to the composition of the original rock types and partly due to this intense heating and squeezing. When the bands were squeezed by later tectonic activity, they sometimes folded – rather like a concertina – and formed some incredible swirling patterns.
After all this, the gneiss at Scourie Bay was split vertically and magma was pushed up through it, which crystallised quite quickly to form a dark igneous rock in sheets called ‘dykes’. These are visible in the outcrops at the pebble beach to the west of the graveyard. There is a large dyke (around 55 metres wide) which is being eroded to form the beach, and a thin 0.75-metre-wide dyke to the north. The dykes themselves have also been slightly metamorphosed; if you look closely you can see garnets, a pink crystal which ‘pops out’ under pressure.
The Lewisian gneiss at Scourie is part of extensive exposures in a narrow tract of ground stretching 120 km along the northwest coast, often covered by younger sedimentary rocks. The north and south ends of this tract were sheared and deformed by tectonic episodes during the Proterozoic (around 1700 million years ago – the “Laxfordian” event) and this obscures the early history. The Central Block preserves the older parts – best seen around the village of Scourie, on the peninsula of Scourie Mor and the north side of Eddrachillis Bay at Lower Badcall. Collectively these sites tell of the complex processes by which continental crust is formed and remolded progressively. It is a history of igneous activity recrystallized by metamorphism and reshaped by deformation. The history is termed “Scourian” in honour of the type locality at Scourie – but this is a history that spans an incredible 500 million years of geological time.
On the beach, you can find beautiful pebbles of the banded gneiss alongside wave-smoothed pieces of the Scourie dykes. The black sand comes from the dyke being broken up by wave action.
To the north of Scourie, you’ll reach the ‘Multi-coloured Rock Stop’ where you can see the Lewisian Gneiss spectacularly exposed in a road-side cutting. Here there are abundant sheets of pegmatite – an igneous rock incorporating exceptionally large crystals. At the road-side cutting, you can see three rocks of different ages. The original rock is pale grey gneiss, into which molten sheets of dark basaltic magma (the Scourie dykes) were later forced. The streaks of pink pegmatite must be the youngest of the three rock types since they can be seen cutting through both the older types. The outcrops here have been through the Laxford mill so that the old Scourian gneisses have been smeared out. So too have the Scourie dykes which here are metamorphosed into amphibolites. The shiny black minerals here are hornblende. Ant the cross-cutting pegmatites tell of yet more magma added to this crust – at a late stage in the Laxford shearing.
Text contributed by Laura Hamlet & Pete Harrison, North West Highlands UNESCO Global Geopark
Find out more
To begin an exploration of Scourie Bay, turn off the A894 at Scourie Village, heading toward the beach with a bird-hide and ranger hut. The hut contains a wealth of information on the area. Follow the road until it ends at the graveyard and head out on the footpath through the gate.
100 Great Geosites: https://www.geolsoc.org.uk/GeositesScourieMore
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