Scree

Definition: the loose debris or talus accumulated at the foot of a cliff comprising angular stones and boulders

The talus or scree slopes of the Lairig Ghru, Cairngorms, produced by a combination of rock fall, debris flow and avalanches

Scree is a product of rock fall. Frost weathering of cliffs, together with other weathering processes, leads to the detachment of blocks. It is most obvious when temperatures rise rapidly after a period of sub-zero temperatures and the stones rattle down the corrie headwalls. The blocks fall, bounce and slide until finally coming to rest at the base of the slope. The scree is highly unstable and comprises loosely fit angular blocks. The joy of scree running requires an even cover of relatively small fragments - but many famous scree runs are now downright difficult because the many runners over the years have shifted the fine debris to the base of the slope, leaving only the larger ankle-snapping blocks.

Scree often accumulates in cones at the base of gulleys. The scree may be reworked by debris flows, especially where the foot of the scree slope is eroded by a stream or river.

Crude size sorting of debris can be measured on scree slopes. The largest blocks are found at and beyond the base of the talus slope because these blocks have the greatest momentum as they bounce, roll and slide down the slope. Smaller blocks tend to stop earlier and become trapped between boulders.

Talus or scree slopes also have a model form, with three segments - a free face overlooking a transport slope down which the rocks roll and a lower accumulation slope. The accumulation slope is at the angle of rest, around 32°, in its upper part and the material is highly unstable. Settling and weathering give lower angles at the slope foot, provided there is no stream undercutting.

Rates of scree accumulation in the Cairngorms are uncertain. In his classic study in the Karkevagge in northern Sweden, Rapp (1960) measured rates of rock fall by gathering and measuring daily the clasts on the surfaces of snow patches that had fallen from cliffs the day before. No similar work has been attempted in the Cairngorms. There is a clear distinction, however, between the volume of scree in vegetated talus slopes inside and outside the limits of the Loch Lomond Stadial glaciers in the Cairngorms. Ballantyne (1996) draws attention to this clear contrast in the northern Cairngorms:

• inner parts of Coire an' t Sneachda and Coire an Lochan have active, largely un-vegetated talus slopes of modest volume. These parts of the corries were occupied by small glacial in the Loch Lomond Stadial
• the sweeping, thick talus slopes of the Lairig Ghru are largely vegetated but gulleyed by recent debris flows. This valley was last occupied by ice during deglaciation.

The contrast reflects the intensity of periglacial processes between 11 and 10 thousand years ago and the instability of newly deglaciated slopes.