Glen Nevis is a beautiful place and a Geologist’s idea of heaven! There are 6 main stopping points along a ‘geo trail’ that help to explain the forces that have shaped the glen we see today: glacial features that will fascinate even the novice geologist.
1) Roaring Mill – This lies on the left hand side of the road, about 5km into the glen. You will see some steep inclined slabs of metamorphosed sandstone. Nicknamed by locals the ‘Roaring Mill’ in account of the noise it makes, these were once sandy sediments on the edge of a great ocean and were created during the continual collisions that formed the Caledonian mountains. At about 700 million years old, these are the oldest rocks in the glen.
2) Lower Falls – These are situated 7.5km up the glen. These twin falls have been formed by the river eroding along the lines of two separate dykes, cutting through the granite of Mullach nan Coirean. The dykes were formed when magma was injected into fractures in the granite after it had cooled and crystallised.
3) Polldubh Crags – These crags are scattered across the hillside on the left hand side of the glen. Particularly popular with climbers, the slabby, South faces of the crags show clear evidence of scraping by ice. The crags are made of Mica Schist rock that began as muddy sediment that was metamorphosed during the formation of the Caledonian mountains. If you look closely you can see the twists of this process in the rock.
4) Rock Sheep (Roches Moutonnees) – Continue up the Glen to see several good examples of Rock Sheep (which is the literal translation of the French term ‘Roches Moutonnees’). These rocks have a distinctive smooth back where ice has scraped up and over the rock, smoothing the edge, and a steeper face where the rock has been pulled out by the ice on the downstream side.
5) The Waterslide – You have now reached the end of the road in Glen Nevis, and as you park up, the view all around is magnificent; but if you look to the left you will have a spectacular view of the waterslide that rushes down the Southern slope of Ben Nevis and down the granite slabs for more than 350m. If you take a careful walk up the hillside about 60m you will reach a small pool. The lip of this pool is made of schist, but the slabs just above are made of the inner granite of Ben Nevis. This contact marks the edge of an ancient granite magma chamber.
6) An Steall Falls and Hanging Valley – Leaving the car park, you’ll have to take a walk to this last one. The gorge to get there is a spectacular feature in itself but is overshadowed by the main reward of this walk. Once you get through the gorge, the Glen opens out dramatically and you see ‘An Steall’ (the spout) waterfall cascading down a high wall of white quartzite rock. It spills out of Coire a Mhail, which is a superb example of a glacially carved hanging valley. It stands at a massive height of 120m (390ft) in a single drop. This makes it the second highest waterfall in Scotland.
These are just some of the main geological points along the Glen. There are many more, from the massive erratic that is the Wishing Stone to the Mamore Stob Ban (White Peak when translated to English) named so because its schist peak, in certain light, looks snow-covered all year round. So keep your eyes peeled and see what features you can find.
A book about the unique geology of Ben Nevis can be purchased from the Lochaber Geopark Website.