Driving north from Fort William on the busy A82 trunk road towards Inverness, you enter a very long valley, bounded on both sides by steep, wooded hillsides and with a series of lochs along its base, including the world-famous Loch Ness. This valley is known as the Great Glen (or Glen Albyn – the Glen of Scotland) and marks the line of one of the most significant geological fault lines in the UK, the Great Glen Fault. In this article, we’ll go on a journey, north and west from this fault line, through the hills and glens to unique landscapes like Torridon and Assynt. I’ll give an introduction to the most significant aspects of the area’s geology and explain how this wild and rugged mountain area came to exist as we see it today.
THE GREAT GLEN
We’ll start with the Great Glen Fault. The fault is a strike-slip fault, where rocks either side of the glen have moved horizontally in different directions to one another, and dates to the Caledonian mountain building period 430 – 490 million years ago. During the last ice age (11000-20000 years ago), large glaciers carved out the deep U-shaped valley we see today. More recently, the Great Glen has become a main route through the Highlands, with the main road from Fort William to Inverness, and the Caledonian Canal, now a popular tourist destination, but originally built as an inland route for the transportation of commercial goods from the east coast to the west. It even has its own long distance walking route, the Great Glen Way!
The Highlands north and west of the great glen have an altogether different feel to those in the south. Human habitation is sparse and the hills are rugged and rocky. This area is home to some of the last remnants of wilderness in western Europe and also some of its most interesting and diverse geology. Anyone walking the Cape Wrath Trail (CWT) will get to experience the full variety of geology on display as they travel from Fort William to the North Coast.
THE MOINE SUPERGROUP
Much of the rock immediately to the northwest of the Great Glen is metamorphic (i.e. rocks that have been transformed within the earth under intense temperature and pressure conditions) and belong to a formation known as the Moine Supergroup. This name applies to the majority of the rocks found in a broad stripe running north from Glenfinnan (site of the monument to remember the Highlanders who fought in the 1745 Jacobite Rebellion and the Glenfinnan viaduct recognisable to many Harry Potter fans) all the way to the north coast. The Moine Supergroup takes its name from the A’Mhoine peninsula, a large area of peatland on the north coast of Sutherland, near to the end of the Cape Wrath Trail and soon to become home to the UK’s first spaceport.
Some of the rocks in the Moine group were formed 2.5 billion years ago. Because they are so old, these rocks do not contain fossils and as a result, it is very difficult to confirm their exact age, but they are certainly ancient! Many of the rocks are schists – a class of metamorphic rock which is easily split into horizontal sheets – and in many places it is possible to see where the rocks have been folded and tilted by great forces within the earth. This band of rock is also home to the UK’s only known meteorite impact crater. The meteorite hit the earth 1.2 billion years ago but the crater was only discovered in 2015. It is centered on the village of Lairg in central Sutherland and is approximately 40km in diameter. Although it is not immediately apparent on the ground that you are standing in a crater, from many of the surrounding peaks, the view over the gentle landscape around Lairg, surrounded by high mountain summits, gives a sense of the site.
THE MOINE THRUST
Continuing westward from the Great Glen, through the area of the Moine Supergroup, there are a series of other fault lines which mark the boundary between various subdivisions of the Moine rocks. The most significant of these, marking the western boundary of the Moine Supergroup, are a tightly packed grouping of thrust faults (a fault where older rocks have been pushed up and over younger rocks), known as the Moine Thrust Zone. The line of this thrust zone stretches for nearly 200km in a southwest-northeast line from the Sleat Peninsula at the southern tip of the Isle of Skye, over to the mainland and all the way to the north coast.
The Moine Thrust Zone is a band containing several thrust faults where old metamorphic rocks described above were pushed westwards by as much as 80km and now sit above much younger rocks. The zone is typically about 1km across but in Assynt in northwest Sutherland, it is as much as 10km wide. The study of rocks within this feature during the late nineteenth century was fundamental to our understanding of geology and plate tectonics and is still an important site of study for geology students today.
TO THE NORTHWEST AND BEYOND
West of the Moine Thrust Zone, there is a stark change in the landscape. Here we enter what is known as the ‘Hebridean Terrane’ as there are many similarities with the geology of the Hebrides, the islands sitting off Scotland’s west coast. This area contains some of Scotland’s most iconic mountain landscapes including the magnificent individual peaks of Torridon and Assynt. This part of the landscape has a characteristic succession of rocks that create the stunning scenery we see today – distinct layers of Lewisian Gneiss, Torridonian Sandstone, and Cambrian Quartzite.
The metamorphic Lewisian Gneiss at the base of this succession forms the ground around the bottom of the mountains. Formed around 3-billion year ago, this rock has weathered into a distinctive, rugged landscape known as a ‘Cnoc and Lochan’ landscape and is amongst the oldest rock on earth.
Sitting on top of the Gneiss are Torridonian Sandstones. These are sedimentary rocks formed from deposits of sediment that were subsequently buried and turned into rock by a process called lithification, a compaction under pressure. These rocks were formed 1.2 billion – 800 million years ago from river sediments laid down during the erosion of a far larger mountain range that once lay to the east. It is unusual to find sedimentary rocks of this age and they are highly significant for that reason. A characteristic feature of sandstone mountains is terraced rock bands marking different periods of deposition. You can see these particularly clearly in the mountains around Torridon.
Finally, on top of the sandstone, many of the mountains are capped by a white rock that can make the mountains appear as if they are snow-capped, even in summer. This rock is Cambrian Quartzite, another sedimentary rock formed from beach deposits when Scotland was located much further south around 500 million years ago. These rocks form the upper reaches of many of the hills and often there is a clearly visible boundary between the quartzite and the sandstone. This boundary is known as a geological unconformity and as you walk over it, you literally step across several hundred million years of the earth’s history!
These layers of gneiss, sandstone and quartzite have been moved and tilted over time by movements in the earth’s crust so that now the unconformities between them are not always horizontal. On some mountains, it is possible to see where the line of the unconformity between the rock types dips in one direction and the thicknesses of the bands of different rocks vary across a relatively short distance. All of these rocks have then been eroded during the last ice age to form the large mountain ridges of the far northwest that are visible today, including the likes of Liathach, An Teallach and Quinag.
I hope this article has given a bit of a flavour of the amazing and special landscape of the northwest Highlands. There is so much more to discover and getting out into the landscape is a great way to learn more about the fascinating geology of the Scottish Highlands. Why not plan an adventure and go and check it out for yourself!
North West Highlands Geopark – https://www.nwhgeopark.com/
Background on the Lairg Asteroid Crater – https://toriherridge.com/2016/09/23/walking-through-time-scotlands-lost-asteroid-the-backstory/
Geology of Britain viewer – http://mapapps.bgs.ac.uk/geologyofbritain/home.html
ABOUT THE AUTHOR
John King is a qualified ML, passionate about passing on his knowledge and skills, and sharing his enthusiasm for the hills with others. He first developed his love of the outdoors in the Scottish Highlands and has explored many of the wilder and more remote corners of the Highlands, in all seasons. He has built up an intimate knowledge of the Scottish hills and glens as well as a keen interest in the ecology, geology and history of the natural environment. He also enjoys getting his hands on rock and has completed many of the UK’s classic scrambles and is a keen rock climber.