I cannot attempt to describe the mountains. I can only say that I thought those… were the grandest I had ever seen… They were such forms as Milton might be supposed to have had in his mind when he applied to Satan that sublime expression— ‘His stature reached the sky.’
- Dorothy Wordsworth
Though more than 200 years have passed since Dorothy Wordsworth penned the above about her impressions of Glencoe, the words still powerfully capture the almost-intimidating beauty of the impressive landscape. Driving in along the A82 can be a little like slipping between the great gates of a fortress, bracketed as you are between lochs and towering craggy mountains on either side. There can be an overwhelming sense of permanence in these views, as if the peaks and valleys and rivers must have always been as they are today. But just as the community of the region has grown and changed over time, so too has the landscape. The iconic scenery of Glencoe is more than half a billion years in the making, with a geologic history that begins at the bottom of an ancient ocean…
Ancient Ocean Sediments and Drifting Continents
The geologic foundations of Glencoe can be traced back some 600 million years, to the end of a time geologists call the “Precambrian.” Like historians, scientists separate the Earth’s past into historical segments known as the “geologic time scale.” Some of these segments are more well-known than others. For example, Jurassic Park borrows its name from the Jurassic period – a segment of geologic time when dinosaurs roamed the Earth. The Precambrian Eon is the oldest of these segments, covering the longest overall period. It began at the formation of the planet some 4.5 billion years ago, and ended around 500 million years ago, with the emergence of multi-cellular life. If we could take a time machine back to the end of the Precambrian, Earth would be almost unrecognizable. There were different oceans and continents, the land was rocky and barren without any vegetation, and most life consisted of soft-bodied single-celled organisms like aquatic worms, algae, and jellyfish. Scotland began its life in the tropical shallows of the Iapetus Ocean, off the coast of the ancient continent Laurentia.
Most of the Highlands, including Glencoe, sit on the foundations of a group of metamorphosed sedimentary rocks collectively called “The Dalradian Rocks.” These rocks were originally made of layers of sediments, such as sand, mud, silt, clay, and lime, which eroded off Laurentia and gathered in the shallows of the Iapetus Ocean. Over time, these sediments became crushed under their own weight and transformed into rocks like sandstone, mudstone, and limestone. But how did these rocks go from the bottom of an ocean to the top of mountains? Well, about 500 million years ago, the continents were on the move!
Continents shift gradually over millions of years due to a process called “plate tectonics.” The earth is made up of many layers of metal, magma, and rock, sort of like the colourful bands of a Gobstopper. The uppermost of these layers is the “lithosphere,” which is a combination of the rocky crust (which we all live on) and the upper mantle (the semi-solid layer of super-hot rock many miles below our feet). Though from the surface the crust may appear to be a single unbroken slab of rock, it’s actually made up of many smaller slabs called “plates.” These plates are mobile and travel at different speeds relative to one another, from two to even 15 centimetres per year! The plates can move thanks to convection currents inside the mantle, like how a fan oven works. Essentially, waves of heat from the centre of the planet causes magma to rise towards the crust. But the further away from the hot core the magma gets, the cooler it becomes, making it sink back down again. This circular motion (convection) pushes the plates across a thin layer of plasticky semi-molten rock between the crust and the upper mantle called the “asthenosphere.” The jostling of the continents as they slide around and bump up against each other is what forms many of earth’s greatest geologic features, like mountains, oceans and even the continents.
The Iapetus Ocean closed at the end of the Precambrian when the continents Baltica and Avalonia (the home of ancient England and Wales!) ran into Laurentia. As plate tectonics brought the ancient continents together, the ocean was thrust upwards, like how the centre of a piece of paper will crumple when pushed inwards from both sides. The collision of Laurentia, Baltica, and Avalonia crinkled and uplifted the Dalradian rocks, creating mountains out of what was once the ocean floor. The modern mountains of Glencoe are the weathered remains of these first mountains, now known as the “Caledonian Mountain Range.” At their height, the Caledonians would have rivalled the modern-day Himalayas or Alps. When the continents broke apart again millions of years later, the Caledonians were torn apart. Today, the two major remnants of the range are separated by the Atlantic Ocean. In Europe, the Caledonians span from the British Isles to Scandinavia and up into the Arctic Circle. In North America, the remnants of the Caledonians are called the “Appalachian Mountains” and run along the eastern edge of the continent.
The slate found in Ballachulish is a lucrative by-product of this continental upheaval. As the continents came together, the pressure generated an incredible heat, hot enough to change or “metamorphose” some of the sedimentary rocks into metamorphic rocks. A group of mudstones in the Dalradians were metamorphosed into slate via this process. Millions of years later, these same rocks would be discovered by miners passing through Ballachulish, leading to the establishment of the Ballachulish Slate Quarry.
Quartzite is another metamorphic rock found throughout Glencoe thanks to the same geologic events. Usually pale in colour, either a light grey or white, large quartzite deposits can be seen on the mountains around Glencoe village. For example, the Pap of Glencoe and the peak of Sgòrr nam Fiannaidh are both made of this hard, weather-resistant rock. Notably, the eyes of the “Ballachulish Goddess,” a late bronze age statue discovered locally, are made of quartzite pebbles. While the original Goddess is currently on display at the National Museum of Scotland, a replica of the statue can be found in the Folk Museum.
A Valley of Fire
The Caledonian Mountains stopped growing around 400 million years ago, at the start of another geologic period called the ‘Devonian.’ During the Devonian, the planet was a little bit warmer than it is today. Plants finally covered the land, with the oldest known forests appearing during the earliest part of this period. The Earth was mostly covered by a single massive ocean filled with a diverse range of strange fish. In fact, the Devonian has been nicknamed the “Age of Fishes” due to this abundance of marine life. It was upon this backdrop that the second major forming event in Glencoe’s history took place. As the continents once again pulled away from one another, the mountains crumbled, the earth opened, and a time of volcanic activity began.
Volcanism is not all fiery explosions and fountains of lava, though Glencoe saw its fair share of both. The region’s volcanic period had three distinct phases, the earliest of which was gentler – at least, as far as volcanoes can be gentle! As the tectonic forces which brought the land together lessened, the crust cooled and began to fracture. This fracturing created deep cracks in the earth called fault lines, and in some cases these faults formed valleys, as was the case at Glencoe. Magma was forced upwards onto the surface along one of these fault lines, where it hit a river complex and cooled quickly in the wet sediment. This created horizontal layers of volcanic rock called “sills.” These sills hardened between layers of the Dalradians, as the magma which formed them lacked the explosive power necessary to burst through the much older rocks.
The second phase of volcanism was far more destructive, featuring explosions strong enough to collapse mountain sides. Glencoe sits on the remains of an ancient volcanic ‘Caldera.’ A caldera is a depression in the earth created after the eruption and collapse of a volcano. The Glencoe caldera is a large oval, 14 by 8 km in size. The volcano did not collapse after a single explosion but rather after many eruptions over the course of tens to hundreds of thousands of years. This is a process called “Caldera Collapse” or “Caldera Subsistence,” where eruptions over time collapse the ceiling of a volcano’s magma chamber. Glencoe is especially important to the study of these geologic phenomena because it was the first place in the world where Caldera Collapse was observed in ancient rock! These eruptions were triggered by the meeting of magma and large volumes of water deep underground. When the heated rock met liquid, it instantly vaporized the water, generating explosive gas that punched holes in the rock around it. These explosions were even violent enough to trigger pyroclastic flows.
Pyroclastic flows look a little bit like an avalanche made of clouds bursting from the eruption site and racing down the side of the volcano at break-neck speeds of more than 200 miles per hour (321 kmph). The ‘clouds’ are made of a mix of super-hot volcanic gases and particles like ash, pumice (a lightweight volcanic rock), and shattered rock fragments. The ancient Roman cities of Pompeii and Herculaneum were famously destroyed by a pyroclastic flow generated by the eruption of Mt. Vesuvius. Evidence of these ancient flows can be seen throughout Glencoe in the presence of ignimbrites rocks, which are made of pyroclastic deposits. For example, the tops of the Three Sisters and the highest cliffs of Aonach Dubh are made of ignimbrites.
The final phase of volcanic activity likely didn’t feature eruptions at all. Instead, the large pool of magma still trapped under the area slowly began to cool, and over time it created a large deposit of granite. Granite is a hard igneous rock that comes in a range of colours. The peaks of Beinn a' Bheithir, visible from Ballachulish, are made of this granite.
Carved By Ice
Glencoe finally began to take on its modern appearance about 2 million years ago, during the Quaternary period. The Quaternary is the youngest period on the geologic timescale and has been further split into two distinct phases: the Holocene Epoch (modern times) and the Pleistocene Epoch, better known as the Ice Age. The Pleistocene has been nicknamed the Ice Age or the “Great Ice Age” because of the high percentage of glaciers that covered the world at the time. Glaciers are large masses of compacted snow, ice, and sediment. At the height of the Pleistocene, the global world temperature was around 5°C cooler than today. While this may seem like a minor temperature change, the impact of even a single degree shift in either direction can be drastic.
Take for example the environment of Pleistocene Scotland. Two million years ago the land had roughly settled into its current position on the globe, yet the climate was radically cooler. Scotland looked more like the arctic with temperatures closer to modern day Greenland and was covered in ice sheets and glaciers. Ice Age ‘megafauna’ like the woolly mammoth and rhino, giant deer, reindeer, and bison would have dominated the icy landscape, stalked by lions, bears, wolves, and even spotted hyaenas. Humans also first set foot in Scotland at the very end of the Pleistocene, though didn’t settle on the land for another few thousand years (for more on Glencoe’s ancient inhabitants, check out The Mystery of the Ballachulish Goddess blog).
Though on average the world was much colder, the Ice Age saw cyclical periods of warming and cooling. Glaciers grew during the cold periods, carving away rock as they moved across the landscape. During warmer periods, they receded and deposited ground rock and sediments along their path. 20,000 years ago, during one of the cooler periods, Rannoch Moor was buried under a kilometre of ice. This ice sheet covered more than 50,000 square kilometres, with glaciers spreading away towards the west. It was these glaciers, through their expansion and retraction, that scoured out the land of Glencoe. The glaciers tore away earth and rock, exposing and mixing and re-depositing the layers underneath.
U-shaped valleys like Lairig Eilde (between Buachaille Etive Beag and Bidean nam Bian) offer striking visual evidence of this glaciation, with the crinkled-looking slopes a product of the glaciers’ movement through the area. Today, granite boulders, called ‘erratics,’ dot the landscape, including at the tops of some of the highest peaks. These erratics were carried inside the glacial ice as it moved and were then left behind as the glaciers melted away during warm periods.
The Henderson Stone, found in a croft in Glencoe Village, is a famous example of one of these granite erratics. Oral histories say that before the Massacre at Glencoe, a soldier billeted with one of the local families attempted to warn his hosts using the stone. While on a walk with a villager, the soldier supposedly stopped at the granite boulder and said something along the lines of “Great stone of the Glen, though you have every right to be here, if you knew what was to happen tonight you would not stay here on any account.” According to tradition, the person who overheard this ominous warning managed to escape the massacre.
Coire Gabhail or ‘The Lost Valley,’ also importantly connected to the Massacre at Glencoe, is another product of Ice Age glaciation. On the night of the massacre in 1692, many MacDonalds fled into the valley to escape their attackers and managed to survive thanks to its hidden location. Some 12,000 years before that night, the freeze and thaw of the receding glaciers triggered rock falls across Glencoe. The large boulders at the end of the Lost Valley were deposited there during one of these rock falls, which was perhaps the largest rock fall in the history of the British Isles. These rocks blocked the river flowing through the valley and created the gravelly scenery seen today.
More on Discovering Glencoe Geology
The best way to experience Glencoe’s incredible geology is to go hiking in Glencoe! Our blog “Small Walks, Big Views: The Best Short(ish) Walks in Glencoe” details several short, beginner level walks that show off some of Glencoe’s iconic views. In particular, the “Glencoe Lochan” walk offers a great view of the Pap of Glencoe with its Dalradian quartzites. A bit more experienced hikers with some time on their hands can even access a trail to the summit of the Pap from Lochan.
But for those seeking a bit more structure in their geologic journey, the Lochaber Geopark offers amazing interpretation all over the Glencoe area. The Lochaber Geopark Visitor Centre can be found on the High Street in Fort William and features museum displays and information all about the geology of the region. The organization has also installed 26 interpretation panels all around Lochaber on “Rock Routes,” which allow visitors to learn about the landscape while taking in the views. Visiting each of the panels is a fun and informative addition to any Glencoe visit!
Additonally, the Glencoe Visitor Centre managed by the National Trust for Scotland offers some information on local geology and hiking, as well as exhibits on the mountaineering history of Glencoe. And, of course, the Folk Museum offers insight into the community’s connection to the land, and features further interpretation on many of the topics covered in this blog!
Additional Reading
This blog was written and researched by Anika Martin, who completed a work placement with Glencoe Folk Museum between Jan-May 2023 as part of her MSc Museum Studies course with University of Glasgow.
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