The Trotternish Landslides

Editor's Notes

1. Skye is famous for its stunning scenery at the Quirang and the Storr, with their post-glacial large-scale landslides. The landslide at the Quirang is the largest mass movement slide in Britain, extending over 2km in width. Geologically, the west and northern parts of Trotternish consist of Jurassic sedimentary rocks and Palaeogene lavas intruded by later Tertiary dolerite sills. Although Jurassic rocks are relatively rare elsewhere in Scotland, Skye offers the most complete sequence of Jurassic rocks in the country, with Trotternish being one of the main exposures.
2. The Cuillin Range is the most impressive igneous feature on Skye but the basalt plateau of the Trotternish peninsula isn't that far behind.
3. The rocks at Trotternish consist of Jurassic sedimentary sequences overlain by thick Palaeogene lava flows. Dolerite sills and dykes intrude the Jurassic rocks. All the rocks dip gently to the west, creating slopes rising gently across Trotternish peninsula from west to east, with steep scarp slopes on its eastern margin. N-S trending faults are also developed along the peninsula. The slides formed due to the overlying weight of the lava flows (a total of 24 flows, approximately 300m thick), weighing down on the weaker Jurassic sedimentary rocks. Under the pressure, the Jurassic rocks sheared along the N-S faults and huge blocks slid seawards along a rotational glide plane. (Scottish Geology)
4. Rotational landslides occur where more resistant rocks founder over underlying weaker rocks. In Scotland there are excellent examples at Trotternish in Skye and at Hallaig in Raasay, where multiple failures have produced spectacular whole mountainside collapse.
5. The basic relief pattern consists of ridges formed from down-faulted blocks separated by narrow valleys.
6. The Trotternish Peninsula is formed from the most extensive basaltic plateau in the British Isles.
7. Thick lava flows are supported on weaker underlying Jurassic sedimentary rocks. With the retreat of glacial ice, large sections of the edges of the lava flows became detached at fissures and underwent rotational slipping as the weaker underlying strata subsided.
8. The peninsula is a good example of a 'trap' landscape or stepped landscape.
9. The rotational landslides produced a series of ridges and valleys and also the Old Man of Storr, a basaltic pillar which stands proud of the confused area of slippage.
10. At the Quirang, five successive movements have been identified, extending over 2km from the scarp slope to the coast. Although features such as the Storr and the Quirang are currently stable, other areas remain active.The sedimentary sequences at Trotternish comprise Middle and Upper Jurassic clays, shales and sandstones. Bearreraig Bay offers an excellent opportunity to examine exposures of Middle Jurassic sedimentary rock sequences. (Scottish Geology)