Loch Glencoul: Continental collisions laid bare

Loch Glencoul, situated in the North West Highlands UNESCO Global Geopark, gives visitors a dramatic insight into what happens when great sheets of rock slide on thrust planes

1. Beinn ard da loch – Across the loch there are layers in the hillside. The layers tilt downward to the right (east). Image: Rob Butler.

Glencoul is nothing short of breath-taking. A two-pronged, glacier-carved fjord which wouldn’t look out of place in Norway is split by one of the most photographed hills in British geology; Beinn ard da loch. Every year hundreds of geology students come to visit this locality and it has been inspiring generations of geologists for more than a century. So, what is all the fuss about? It is here that you can see the landscape-scale results of continental collisions that drove mountain-sized slabs of rock westwards on thrust planes.

Beinn ard da loch is made up of layers of rock, tilting downwards to the east. The bottom layer is Lewisian gneiss; this is the foundation of the stratigraphy in the North-West Highlands Geopark, and is among the oldest rocks found in the world. The next layer up is 500-million-year-old Cambrian quartzite. Normally, we’d expect to find older Torridonian sandstone on top of the gneiss, but here the sandstone was eroded away more than 500 million years ago. This ‘re-excavated’ the Lewisian gneiss that was then buried directly under the Cambrian quartzite when it was deposited in a shallow sea on the shores of the continent of Laurentia.

Around 430 million years ago, the closure of the Iapetus Ocean brought continents together to form the Caledonian mountains. The force of this collision forced rocks hundreds of kilometres away to be thrust over one another – imagine a bulldozer ramming into rock layers in slow motion, and the distortion that would cause in the layers both at the collision site and the layers underneath.

At Loch Glencoul, the Lewisan gneiss basement was fractured and broken up and some sections of it were thrust westwards and over the top of the Cambrian quartzite. This layer of Lewisian Gneiss is out of sequence, and is there because of massive thrust movement related to the continental collision. The collision was a very slow process (just 2.5 centimetres per year), and multiple thrust events can be observed. The main thrust that can be seen across the loch is the Glencoul Thrust, first recognized by Charles Callaway in the early 1880s. The significance of this thrust for Highland geology was recognised by Peach and Horne following the exceptionally detailed mapping by their colleague Charles Clough in the late 1880s. Recent calculations suggest the rocks above this thrust plane have moved about 30km.

The most important thrust plane in this area, the Moine Thrust, is located just underneath the Stac of Glencoul. You can go and touch this famous geological boundary at Knockan Crag.

Text contributed by Laura Hamlet & Pete Harrison, North West Highlands UNESCO Global Geopark

Find out more
Visit the North West Highlands UNESCO Global Geopark Rock Stop visitor centre: http://www.nwhgeopark.com/plan-your-visit/rockstop/

100 Great Geosites: https://www.geolsoc.org.uk/GeositesGlencoul

2. Unconformity between the Lewisian Gneiss and the Cambrian Quartzite, with the Glencoul Thrust above. Image: North West Highlands UNESCO Global Geopark

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