Landslides in rock and soil debris

This page looks at the range of types of landform created by the movement of weathered rock, till and soil due to gravity. Many landslides occur due to sudden rainstorms, prolonged very wet conditions, and snowmelt. However, unstable slopes can also be caused by the melting of frozen ground on Scotland's mountains. Landslide movement can be sudden (especially those caused by intense rainstorms) or it may be so slow that we hardly notice, except for a few tell tale signs like cracks in the ground, and subsidence on affected roads. 

All landslides are potential hazards, but most occur in remote mountain areas away from people. Debris flows in particular are a regular hazard that disrupts transport routes in the Highlands during severe weather conditions.

Creep

Creep is the slow downslope movement of material under gravity. It generally occurs over large areas. Three different types of creep are recognised: Seasonal movement or creep within the soil, due to seasonal changes in soil moisture and temperature (for example frost heave processes). Continuous creep where soil and rock debris is not strong enough to resist gravity. Progressive creep where the soil and rock debris suddenly reaches a critical state where it will fail due to other factors.

Solifluction

Solifluction sheets and lobes are found on steeper slopes, where the loosened boulders and soil have been moved downslope by 'solifluction' (a mixture of soil creep and flow) processes, forming distinctive sheets, terraces and lobes of debris and boulders. Larger boulder lobes in Scotland were last active during the Loch Lomond Readvance (12,900 - 11,500 years ago), but some smaller solifluction features are active most winters.

Translational slides

These types of slumps and slides occur during very wet weather, when the near surface soil and rock debris becomes saturated with water, and slides and flows off downslope. They can develop into more dangerous debris flows on steep ground, where the landslide may travel a long distance churning up the debris into a slurry.

Debris flows

Debris flows are Scotland's most common type of landslide hazard. They range in character from sloppy wet mud flows to a mixture containing more rock debris and similar in consistency with a slurry of wet concrete. They  occur on steep slopes, greater than 20°, and once started can continue to travel over much gentler ground where slopes may be only 10°. How far a debris flow travels depending on the mix of debris to water being churned about. Many debris flows start off as translational slides, but as the slide moves down slope the water and rubble gets mixed up and forming a slurry that can flow. Debris flows often leave a trail of abandoned rubble in their wake, forming distinctive ridges or levees.

Debris flows range in size from 1 to 10 metres across, carrying 1 to several cubic metres of debris. Debris flows may occur as one offs from open hillsides, but repeated debris flows are more common below gullies.

Debris cones form where repeated debris flows accumulate, usually beneath gullies. Many debris cones in the Highlands have periods of activity and quiet, depending not just on suitable weather conditions needed to trigger them, but also on the availability of mud and rock collected higher up in the gully floor.