Rock-slope failure in parts of the Scottish Highlands

Abstract

The deposits and scars of 432 rock-slcpe failures (RSFs), affecting over 74 km² of the Scottish Highlands, have been mapped from aerial photographs. Some 80% of the RSFs have formed in schists. Most RSFs lie within or close to the limits of the Loch Lomond Advance glaciation. RSFs usually occur midway up a slope often to where the upper surfaces of Advance glaciers reached. A spatial correlation has been established between RSF positions and the upper limits of the Advance.

Despite this correlation the RSFs were not, generally, triggered by glacial oversteepening. Mechanical analyses of 27 slopes prove that most would not have displaced unless some force augmented gravity. High cleft-water pressures, progressive-failure and earthquakes were probably the most important catalysts of slope failure.

A classification of RSFs into plane-sliding, wedge-sliding and toppling modes of failure provides a useful framework for the analysis of the smaller case studies. Large-scale RSFs (up to 112 million m³), often displaying extensive obsequent-scarps, are an amalgam of various forms of failure and are more difficult to classify and analyse.

Assuming a constant frequency of rock-slope failure in the Scottish Highlands over the last 12,500 years (the generally accepted time of final Scottish ice sheet deglaciation), a rock-slope failure should have occurred on average every 29 years. However, field evidence suggests that a phase of high rock-slope failure incidence accompanied the deglaciation of the Loch Lomond Advance and perhaps other earlier readvances. The case study RSFs are thought to range in age from greater than 11,000 years to c. 250-300 years old.

A probabilistic model, incorporating factors that are intrinsic and extrinsic to the rock-slope system, is advanced to explain the delayed response of some rock-slopes to deglaciation. An attempt is made to quantify the denudation achieved by RSFs.