THEY are the spectacular natural events that changed the face of the Scottish landscape forever. In past millennia, hundreds of giant rockslides triggered by climate change have sheared off mountainsides.
But a new dating technique suggests that the threat is far from over. Experts estimate the country could be hit at any time by up to three massive landslips over the next 1,000 years.
The most likely areas are Torridon, Skye and the western Cairngorms down to the Angus Glens, where huge rock falls have been most common in the past.
Further, if a rock face slides off in a coastal region, it could create a giant wave that would swamp surrounding areas.
Their scale would be vastly greater than the boulder and mud slides that have become increasingly prevalent in Scotland.
The biggest dated so far occurred when nine million tonnes of Beinn Alligin, which dominates Glen Torridon, broke off 4,000 years ago with such power that it travelled a mile along a valley floor.
The prediction of future major rock falls has been made by Professor Colin Ballantyne, of the School of Geography and Geosciences at St Andrews University, who has been studying Scotland's post-glacial landslides for more than 15 years.
He said: "Periods of exceptionally heavy rainfall bring the threat of landslides that may block or destroy roads in the hillier parts of Scotland.
"These modern landslides, however, are tiny when compared with some that occurred in the past. Until recently, little was known about the timing and causes of huge landslides in the Highlands, but new ways of dating landslide deposits are showing that there is still a possibility of future catastrophic landslides in Scottish glens."
Ballantyne's work covers the period between the peak of the last Ice Age around 17,000 years ago, when most of Scotland was covered in an ice sheet up to 1,200 metres thick, to the present day. He has pioneered the use of a new technique, cosmogenic isotope dating, that can pinpoint the age of a rockslide.
When a slide occurs, previously buried rock surfaces are exposed to a bombardment by cosmic radiation. This prompts the slow build-up of tell-tale traces of certain isotopes – atoms – in minerals such as quartz. As the concentrations of isotopes are known to increase predictably over time, geologists can work out when the landslide occurred.
Working with the University of Washington in the US and at the Scottish Universities' Environmental Research Centre in East Kilbride, Ballantyne has dated 15 major landslides, with another 22 in the pipeline.
What surprised the team was that although some dated back 16,000 years, the most recent was a rock fall that dammed up the famous Lost Valley in Glencoe 1,900 years ago. "Some big rockslides took place pretty soon after deglaciation. For example, in the Cairngorms two occurred about 16,000 years ago, immediately after the ice had gone," said Ballantyne.
"Other results have proved more surprising. A major rockslide on The Storr, in northern Skye, did not occur until about 6,500 years ago, over 10,000 years after deglaciation.
"The most recent of the big rock avalanches that we've dated so far is one that dams up the Lost Valley near Glencoe. About half a million tonnes of rock came down about 1,900 years ago. On a geological timescale, it's very recent."
The slides have occurred as a consequence of the retreat of the ice-sheets. As the ice melted, the pressure eased and "rock relaxation" took place.
"When the weight is taken off, the rocks expand again like a sponge," said Ballantyne. "But they can't just spring back into place. As they expand, they tend to crack open near the surface and when they reach a certain critical point they fail and break apart. It is usually caused by slow effects of deglaciation, but earthquakes, even small ones, may trigger them as well."
Large rockslides directly into the sea or lochs can generate local 'tsunami' effects. Ballantyne said: "This happened in Norway in 1928, when an entire fishing village was wiped out. Any future big rockslide or rock avalanche in the Highlands would probably take place in a remote glen, so the hazard rating is pretty low. The most likely effect is that it could take out a few sheep."
The humans most likely to be affected would be mountaineers. Told of the findings, David Gibson, a spokesman for the Mountaineering Council of Scotland, said: "Interesting, yes. Are we scared? No. Mountaineers have to be prepared for any eventuality."
One possible landslip site is The Snub, the hill above Loch Brandy in Glen Clova in Highland Perthshire. Gibson said: "There is huge crack near the cliff edge above the loch. If that goes down, there is going to be an almighty splash."
The Scottish Executive ordered a report into landslide risks after main routes were blocked by mud and rock after heavy rainstorms in the summers of 2004 and 2005.
A family had a narrow escape from their car when a landslide blocked the A9 near Dunkeld. Then 50 people had to be airlifted to safety after huge mudslides trapped their vehicles on the A85 in Glen Ogle, near Lochearnhead. The slide caused 1m of damage to the road, paths and bridges, which took months to repair.
But Ballantyne said: "They were 1,000th of the size of some of the older ones."