4 January 2021
The Trotternish landslide complex on the Isle of Skye in Scotland
Posted by Dave Petley
The Trotternish landslide complex on the Isle of Skye in Scotland
A few years ago I posted briefly about the Quiraing landslide (also sometimes spelt Quirang) on the Isle of Skye in Scotland, suggesting that it is the most beautiful landslide complex in the world. I stand by my view (but would welcome alternative suggestions of course). It is hard to beat this in my views:-
The Quiraing, part of the Trotternish landslide complex on the Isle of Skye. Image by Stanley Chen Xi, landscape and architecture photographer.
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In fact the Quiraing is one part of a huge, ancient landslide complex located on the east side of the Trotternish peninsula in the northern part of the Isle of Skye. This landslide complex should be better known, but it is a long trip to visit it even from the major cities of Scotland. The complex, which is not active, dates from the period after the last glaciation, between about 13,000 and 5,000 years before present. The Google Earth image below shows the Trotternish peninsula and the location of the two best known landslides, the Quirang and the Storr:-
Googe Earth image of the Trotternish peninsula, showing the Storr and Quiraing landslides.
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As the image shows, the Trotternish peninsula is dominated by a large escarpment running apprixmately north-south, formed from Tertiary basalts. At the Storr this has an elevation of about 720 metres. This escarpment, which extends for 23 km, has ancient landslides along its whole length. This is the complex known as the Trotternish landslides – it is the largest landslide complex in the UK by far.
Colin Ballantyne of the University of St Andrews wrote an excellent summary of this complex (Ballantyne 2008), which was published in the Scottish Geographical Magazine. He included this very nice summary of the general structure of the landslides at Trotternish:
The general form of the landslides on the Trotternish peninsula. Diagram from Ballantyne (2008).
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As the diagram shows, these landslides consist of rotational failure through the basalt escarpment and the underlying Jurassic sediments, with the basal shear surface being defined by a resistant dolerite sill within the Jurassice rocks. There are multiple failures at the various sites, with the blocks buttressing those upslope. The rotated blocks become more degraded downslope.
This is a classic rotational landslide system, on a very large scale.
Reference
Dave Petley is the Vice-Chancellor of the University of Hull in the United Kingdom. His blog provides commentary and analysis of landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings.
The failure of a slope of the system “competent rock on an incompetent base” starts with yielding and squeezing out of the incompetent base material inducing tensile stresses in the competent rock near its edge. This leads to a disintegration into huge blocks formed by vertical tension cracks. These blocks may:
1. slide downhill translationally and upright,
2. cause a rotational slide in the base material (internal, backward rotation) or
3. topple downhill (external rotation; most dangerous case leading to sudden rock avalanches).
Thus, the rotational slide does not start in the competent rock (as shown in the diagram by Ballantyne (2008) but in the incompetent material underneath.
I’m sure there is serious deformation of the shales at the shore at Flodigarry?
I’m glad say I was there 20 years ago. The sheer scale of it is jaw-dropping. Looks like listric faulting to me, like the Hilina Pali south of Kileauea Volcano on Hawai’i.
The postglacial landslides on the Trotternish escarpment are more complex than the simple rotational model pictured in this blog. The Quiraing complex includes not only rotated blocks, but also laterally-translated blocks and evidence of toppling failure (Fenton et al. 2015). The Storr landslide is manifestly not rotational, as the displaced lavas dip away from the headwall. The Storr landslide may have been the first anywhere to have been dated using cosmogenic nuclide exposure dating: the (recalibrated) age is 6.1±0.5 ka, much later than most postglacial landslides in Scotland.
See Fenton, C., Martin, P., Cheng, F. and Murphy, B. 2015. Geomorphological analysis of large scale slope instability, Trotternish, Isle of Skye. In Lollino, G. et al. (eds.) Engineering Geology for Society and Territory, volume 2. Springer, Switzerland, 1037–1040.
I always wanted to tramp and cycle Skye – now I have another reason. Thanks Dave !
In a couple of years I might come and visit !
Does anyone know the origin/etymology of Quiraing? It sounds unlike Scots Gaelic or Norse. Any local lore?
Cuith-raing (pronounced coo-rang) = round fold, from Norse
The name Quiraing comes from Old Norse Kví Rand, which means ‘Round Fold’. Within the fold is The Table, an elevated plateau hidden amongst the pillars. It is said that the fold was used to conceal cattle from Viking raiders.