A battleground in the nineteenth century’s Highlands Controversy with wonderful tectonic and glacial landscapes and some of the oldest preserved landforms in Europe
Beinn Eighe, one of the most spectacular mountain ridges in Scotland, is Britain’s oldest National Nature Reserve. The mountain massif has been sculpted by ice and the moraine debris from the most recent glaciers form the “Valley of a Hundred Hills” in Glen Torridon below. The range of geology is particularly striking. Not only does it provide a natural transect through the complex structure on the fringes of the ancient Caledionian mountain belt, but also offers vantage points onto a billion-year-old landscape. The area became important in the mid-nineteenth century in a debate about the geological structure of northern Scotland, later termed the Highlands Controversy, that pitted James Nicol (professor at the University of Aberdeen) against Sir Roderick Murchison (director of the Geological Survey). It includes some of the clearest examples of thrust structures in the British Isles.
The oldest rocks lie on the NE shores of Loch Maree, and are part of the Lewisian basement – the crustal foundations of NW Scotland. The last rock-forming events recorded in these rocks are more than 1650 million years old. The rocks above are horizontally-layered Torridonian sandstone, deposited around a billion years ago, and form the mountain massif of Slioch. The contact between these two rocks is an ancient erosion surface – an “unconformity” that illustrates part of the billion-year-old landscape of valleys and hills across which ancient rivers flowed. These rivers eventually buried the landscape in the sand that became the Torridonian sandstone. This is the best example of a Precambrian unconformity in the British Isles.
However, the Precambrian geology is only the start of the story. The Torridonian sandstones are overlain by sandstones of Cambrian age (deposited about 550 million years ago) – these form a prominent cliff line overlooking the head of Loch Maree. These sandstones, which weather grey on cliffs but form bright white scree slopes, are made almost entirely of quartz grains; this is what gives them their bright colour. In contrast, the Torridonian strata form brown-red crags because they have more diverse composition (including grains of feldspar). These differences in colour means the two formations are clearly distinguishable in the mountainsides – and this is what made the area important in the mid-nineteenth century.
The ridge of Beinn Eighe is made up of the contrasting Torridonian and Cambrian sandstones thrown together in a complex structure; red-brown Torridonian rocks are sliced into the grey-white Cambrian sandstones. Modern accounts interpret these as thrust slices that have stacked up and telescoped the strata – indeed, they represent some of the finest examples of such structures in the British Isles.
Faulting on Beinn Eighe was originally recognised by James Nicol in the 1850s, and he used this as evidence that the NW Highlands contained major tectonic faults. His views were famously (and acrimoniously) contested by Sir Roderick Murchison. In late summer 1860, Murchison, then aged 68, visited Kinlochewe with the aim of disproving Nicol once and for all. He was accompanied by Archibald Geikie, aged 24, who did most of the fieldwork. Geikie studied this ground, discounted the importance of faulting and then headed north on a one-day walk to Ullapool. The first part of his walk was up Glen Bianasdail from Loch Maree and over remote ground to An Teallach. He failed to recognise the significance of major geological features that he walked past – most critically that Lewisian gneiss had been carried up onto the Cambrian sandstones. The Geikie-Murchison interpretation was that the eastward inclination of strata across northern Scotland meant a simple passage of rocks from the oldest rocks in the west to the youngest in the east. Murchison and Geikie were also misled by the outcrops at Knockan Crag where fossiliferous rocks are overlain by Moine metamorphic rocks.
It wasn’t until the early 1880s that Thomas Bonney recognised the significance of the gneisses around Kinlochewe – he showed that the rocks on top of the Cambrian grey-white sandstones were Lewisian gneiss, locally crushed by the tectonic movements that had moved them upwards. Soon after, the Murchison-Geikie hypothesis was abandoned.
The remarkable Coire a’ Cheud-chnoic, or Valley of a Hundred Hills, is one of the best examples of hummocky moraine in Scotland. On the ground, this is a chaotic array of moraine mounds and ridges. When viewed from above, however, the moraines are often aligned in parallel rows, at an angle to the valley sides. These alignments indicate the former ice-margin positions as the glaciers retreated. Behind Beinn Eighe, Coire Mhic Fhearchair is one of Scotland’s most spectacular corries. At the lip of the corrie there are good examples of ice-moulded bedrock with glacial striations and rock surfaces abraded by the ice. The scale of the corrie, with a rock basin and loch on its floor, and the giant rock buttresses of its headwall formed in Torridonian sandstone overlain by Cambrian quartzite, are particularly impressive.
The higher parts of the mountain trail on the Beinn Eighe National Nature Reserve provide fine views of Beinn Eighe and its extensive quartzite scree slopes to the south-west and the glacial trough occupied by Loch Maree to the north. Good examples of ice-scoured quartzite bedrock with scattered Torridonian sandstone erratic blocks can be found on the plateau.
Text contributed by Rob Butler
Find out more
Beinn Eighe National Nature Reserve nature.scot/Beinn-Eighe
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