Geomorphological analysis of large-scale slope instability, Trotternish, Isle of Skye, Scotland

Abstract:

The Trotternish Peninsula, Isle of Skye, has experienced Late Glacial and on-going slope instability. The area of landsliding extends over an area of 45 km by 6.5 km along the eastern side of the peninsula making this one of the largest landslide complexes in Europe. A series of Tertiary lava flows and concordant intrusions resting on and within a sequence of Upper Jurassic marine sediments form a west-tilted plateau. The eastern scarp face has failed as a series of interlinked failure blocks. Previous investigations have interpreted the mode of failure as progressive rotation, with the slip plane located at the contact between the Upper Jurassic sediments and the transgressive Little Minch Sill. The driver for failure is gravitational collapse. The eastern coast of Skye was subject to extensive glacial erosion during the Late Devensian leading to (possibly structurally-controlled) over-deepening of the Inner Sound between the island and the Scottish Mainland. The decay of the ice mass at the end of the last glaciation left an area of over-steepened slopes along the east coast of Skye and the neighbouring island of Raasay.  Geomorphological and structural mapping has identified a more complex mode of failure than previously recognised, involving rotation, toppling, planar sliding, rockfalls, mudflows and ‘sackung-like’ gravitational spreading. The boundaries between adjacent landslide blocks and the zones of different modes of failure areoften demarked by pre-existing structure, including fault zones, dyke intrusions and major joint sets. The basalslip plane, at least in part, appears to have reactivated syn-depositional listric faults within the Jurassic marine sequence. Overall, the landslide complex can be simplified into: an inner zone of postglacial failure close to the present-day headscarp, where toppling, translation and minor rotation are the predominant failure mechanisms; a glacially smoothed transition zone where the exact mode of failure are yet to clearly determined; and an outer zone along the shoreline of the peninsula where the failure is reactivated movement of Late Glacial rotated blocks in Jurassic mudstones that are now subject to erosion, toppling and extensive mudflows.