Lochaber Geopark – Glencoe

Glencoe – a landscape fashioned by Geology

Glen Coe is a strikingly dramatic glen which contrasts starkly with the flat and desolate Rannoch Moor which lies to its east. It is deep glen with high mountain peaks and ridges on either side which shows clear evidence of sculpting by glaciers. What is perhaps less well known is that the road A82, the major communication corridor between the South and the West Highlands cuts through the glen, also cuts through an ancient volcano which hints at Glen Coe’s fiery past.

Rannoch Moor seen below is a flat expanse, and is underlain below by a body of granite.

Lochaber Geopark

See below are two of the “Three Sisters” of Glen Coe. Gearr Aonach (short ridge) and in the distance Aonach Dubh. Out of shot on the left is Beinn Fhada (long hill). Between Beinn Fhada and Gearr Aonach is the entrance to Coire Gabhail (corrie of the booty) a hanging valley carved by a glacier.

Lochaber Geopark

The Scottish Highlands are a small part of an ancient mountain range, comparable to the present day Alps or Himalya, that extended for over five thousand kilometres. Its eroded remnants are found in eastern North America, east Greenland, Ireland, Scotland and Norway, separated by the much younger North Atlantic Ocean. These are known as the Caledonian Mountains (Dalradian rocks) and the foundations on which the volcanoes of Glencoe were built. Sediments deeply bufried at the bottom of the ocean were then changed or metamorphosed by great heat and pressure during the formation of the Caledonian mountain chain some 460 million years ago. These metamorphic rocks which include quartzites, limestones, slates and schists which were later exposed by erosion at the Earth’s surface became the land surface upon which the Glen Coe volcano erupted some 400 million years ago.

Below left, Gear Aonach is in the middle, on the right peaks of Coire nan Lochan. Below right, Loch Achtrioctan with the peak of Bidean nam Bian in Stob Coire nam Beith.

Lochaber Geopark

Lochaber Geopark







The eruption of the Glen Coe volcano began when hot molten rock, or magma, was injected sideways into wet layers of sandy sediment to form horizontal layers of rock known as sills. Later the same magma poured out onto the surface to form lava flows. Lying on top of the andesites is a great pile of other volcanic rocks reaching to the tops of the highest mountains. These later volcanic rocks include rhyolite lava which erupted as a slow moving sticky flow, volcanic ash known as tuff and a rock known as ignimbrite which formed from the debris produced by explosive “glowing cloud” eruptions of extremely hot gas and rock. As the eruptions continued the rocks forming the roof above a huge chamber of magma started to collapse in a piecemeal fashion thus forming a huge surface crater or caldera. Glen Coe was the first place in the world where “cauldron subsidence” was recognised in ancient volcanic rocks. Caldera is Spanish for cauldron. Calderas are common in modern volcanoes but Glen Coe is world famous because it was the first ancient caldera recognized in the geological record. A huge number of deep fissures developed and molten material was injected into them from below. This created a great number of dykes trending north east to south west. These dykes are made up of a different type of igneous rock called basalt. It is darker in colour than andesite and rhyolite and are often softer than the surrounding lavas. They erode to form gullies and deep clefts on the mountain sides.

Climbing on the East Face of Anoach Dubh with the backdrop of the A82 in the distance. Glen Coe is a magnet for climbers and mountaineers.

Locaber Geopark

Look up from the carpark at An Tor you get a great view of Clachaig Gully Sgurr nam Fiannaidh and at the western end of the Aonach Eagach ridge. Situated on the outside of the Glencoe caldera.

The gully marks the line of fault where rocks on either side once slid past each other. You can also see folded metamorphic rocks (quartzites and limestones) on the neighbouring hillside. These rocks were once sands and limy silts at the bottom of an ancient sea.

Lochaber Geopark

From the long layby on the north side of the road just beyond Loch Achtriochtan you can see the dark imposing walls of Aonach Dubh to the south, you are now well inside the caldera and the dark mass of Aonach Dubh.

Lavas from the volcanic eruptions, composed of a rock type called andesite (named after the American Andes) can be clearly seen on the steep western slopes of Aonach Dubh.

Lochaber Geopark

West Face Aonach Dubh 8







The lower part of Aonach Dubh is made up of dark andesite sills and lavas with an obvious ledge running across the hillside. Below the lower grassy terrace marks the start of a different group of volcanic rocks. These are lighter coloured rhyolites, tuffs and ignimbites associated with more explosive eruptions.

Below (left) walkers moving along the lower ledge which traverses across the West Face of Aonach Dubh (black height) and brings you out just below Ossian’s Cave. A vertical gash in the in the rock where a huge block has fallen out of a dyke. Below (right) is the ridge line of the Aonach Eagach which follows the line of caldera margin. The ridge is andesite on one side and granite and rhyolite on the other.

Lochaber Geopark

Lochaber Geopark







About 400 million years ago the volcanic activity had ceased and the high ground of the Highlands was gradually eroded away by wind and water. The final sculpting occurred during the Ice Age. During the last two million years the climate of Scotland has fluctuated between cold glacial periods with ice sheets and glaciers and warmer interglacial periods when the ice disappeared. At times a huge ice sheet built up, centred on Rannoch Moor and glaciers flowed down through Glen Coe and Glen Etive. Side valley glaciers fed ice down from the high corries, especially on the south side of Glen Coe, leaving the sharp ridges and deep glens so beloved by climbers and visitors today.

Coire nan Lochan (seen below) would have held a glacier around 12,000 years ago. 18,000 years ago the glaciers began to retreat and may have disappeared completely. However, another cold period began about 13,000 years ago and the glaciers reformed. Once again an ice cap formed on Rannoch Moor with glaciers flowing down the glens.

Lochaber Geopark

As the glaciers melted away, freeze thaw processes shattered the bedrock of the mountains, which became very unstable. These loose blocks tumbled down the steep slopes to form screes. In places whole cliffs collapsed to produce rockfalls. By far the most spectacular rockfall occurred close to Coire Gabhail where boulders the size of houses lie scatter across the narrow glen. This was a huge event which formed the largest single rockfall feature in the whole of Great Britain.

The fallen boulders blocked the mouth of the valley, so that the stream that flowed through it could no longer flow down to the River Coe. Behind this natural dam, the “Lost Valley” has a completely flat floor where sand and pebbles have built up as they were deposited from the blocked stream. Glacial meltwater carrying large amounts of loose rock, led to rapid erosion long the fault lines and dykes, forming deep gullies. These can be seen around the “Meeting of the Three Waters” where gorges eroded along two separate dykes meet (see photo below).

Lochaber Geopark

Intensive study of Lochaber’s igneous rocks since the 19th Century has led to the development of geological principles that have applied world wide.

For more information visit the Lochaber Geopark website www.lochabergeopark.org.uk or visitour Visitor Centre on Fort William High Street.


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