alps

breaches

troughs

diversion of drainage

Below - thumbnail looking towards the diffluent col of The Saddle and Strath Nethy

Above - thumbnail of the blocky debris pile of the RSF

Below - small scarps, like the one on the ridge above the figure, produced by fracturing in response to valley deepening

Above - Coire Dheirg and its major slope failures

Above - tensional gulls on the eastern spur above The Saddle

 

Rock Slope Failures by David Jarman

Definition: extensive loss of integrity in a mountain side resulting in slope deformation features, arrested landslipping, or cataclasmic landslides.

Strath Nethy

Rock slope failures (RSFs) are widespread in the Scottish Highlands, with over 600 significant cases ranging between 0.02 and 3.0 km2 (Ballantyne 1986). However, the main concentrations are in the southern and western highlands. Until recently, only one minor RSF had been recorded in the Cairngorms – the rockfall at the Shelter Stone, Loch Avon. Now an impressive RSF complex has been identified in Strath Nethy, which extends for 3 km and ranks among the largest in Scotland

The Strath Nethy RSF complex has four components:

  • most conspicuous is a blocky debris pile up to 10 m thick at the foot of the west valley wall. This was formerly described as a protalus rampart or protalus rock glacier (Ballantyne and Harris 1994) but has probably simply accumulated from a series of rockslides – there is a gap below the only break in the crags at Coire na Spreidhe. The debris is so coarse and free-draining that large parts of the block spread are still unvegetated after several thousand years, by contrast with the thick heather all around.
  • the NNE shoulder of Cairn Gorm narrows at 015077 where for 100 m it displays a ‘split ridge’ with a central depression. For about 40 m there is a distinct metre-high rock furrow/parapet backing this depression. Split ridges are common indicators of incipient RSF. Here, it is likely that a delayering/delaminating surface in the granite valley wall is daylighting behind the rim. If the failed mass were to collapse, it would form a northerly extension of the valley-floor debris. This may be how the main RSF evolved.
  • the east valley wall towards the head of Strath Nethy also has debris ramparts at a similar level to the pile opposite. Their provenance is unclear, as the river (aptly named Garbh Allt – rough stream) appears to have cut the ravine separating them. However at the head of the east valley side there are signs of incipient failure, with tension fissures or ‘gulls’ large enough to shelter in. A tor group near Ciste Mhearad is collapsing under the influence of slope failure processes.
  • further north, the open bay of Coire Dheirg shows slumping extensive enough for the OS map contours to depict, with signs of secondary slippage along the ‘corrie rim’ (this is unlikely to have been a functional corrie).

The Strath Nethy RSF complex is interesting for several reasons:

Granite geology: RSF is rare on granite because it lacks the extensive sliding surfaces found in metamorphic strata. This case shows that it can occur on granite where slope stresses are sufficient. The main west side deposits lie beneath 350m high cliffs, where scars suggest upward migration of failure episodes following slope-parallel stress-release joints to which granite is prone. By contrast, the slumps of Coire Dheirg are in a pocket of red granite which may have disintegrated to a depth of tens of metres by deep weathering long before failure was triggered. Curiously, the boundary of the Cairngorm granite pluton crosses Strath Nethy just north of the RSF zone, yet there is no RSF activity on the equally steep schist slopes, even though elsewhere schist is highly prone to failure.

Causes of RSF: the slope stresses needed to fail an intact rock mass are large. They require destabilisation by concentrated erosion at the slope foot, either deepening or widening the valley floor. This is exactly what vigorous glaciers do. Once the slope stresses are generated, failure can be by gradual creep, incremental slices, or large movements. It can be triggered by seismic shock, or by periglacial freeze-thaw action, or by high water pressures during ice melt or heavy rainfall. Strath Nethy displays several modes of failure, but stopping short of the cataclasmic landslide which in higher mountain ranges can block a valley and impound a temporary lake. There is no known evidence for strong earthquakes in the Cairngorms.

Age of RSF: most RSFs in Scotland occurred at or soon after final deglaciation. Because they are responding to glacial erosion, they are known as ‘paraglacial phenomena’ (Ballantyne 2002). The Strath Nethy RSFs have not yet been dated. If there was a deep and active glacier in Strath Nethy during the Loch Lomond Stadial, then the RSFs must postdate it and probably accumulated 8-11,000 years ago. If the Loch Lomond glacier was weak, the rockfalls could have accumulated on its surface, thus generating their ‘kettle-holed’ character when the ice melted. However the slope stresses were mainly generated during the much greater Late Devensian Stadial, and the high-level ridge splitting and slump scars date from its ending 13,000 years ago. Dating of the various RSF components will help to unravel the lateglacial history of the Cairngorms.

Glacial breaches: since this RSF complex is unique in the Cairngorms, it suggests that Strath Nethy underwent unusually intensive glacial erosion during the last stadials. There are many other valley sides as steep as Strath Nethy, but presumably they have evolved gradually, or were cut much earlier. One possible explanation is that only recently has ice begun overflowing into Strath Nethy from Loch Avon, thus suddenly trebling its catchment area and increasing the rate of glacial erosion in its floor. This process of glacial diffluence was first described here by Linton (1951). Before then the headwaters of the Nethy might have been a high-level zig-zag fluvial channel such as can still be seen on the flanks of Glen Feshie or east of Ben Avon. Strath Nethy is also notably lacking in the tributary corries which flank broad glacial troughs such as Glen Dee and Glen Derry.

Rates of erosion: the recent evolution of Strath Nethy fits the idea of selective linear erosion, with high rates of erosion concentrated along lines of channeled glacial outflow. Here, the col at the head of Strath Nethy (The Saddle) has probably been lowered by 100 m from its pre-glacial profile, and the valley floor would need to have been lowered by approaching that amount to set up slope stresses required for failure. The main blocky debris deposits are about 8.4 million m3 in volume. This would imply paraglacial valley wall retreat of about 40 m since ice retreat, a rate far more rapid than could be achieved by normal glacial erosion or slope processes.