cliffs and structure                

A deep cleft on the west Rousay coast, the site of future cliff face failure

Cliffs: mass movement

Definition: subaerial processes of fall, slide, flow and creep that transfer debris to the foot of a slope

On the variably fractured, hard rocks of the outer coast of Orkney, the dominant processes of mass movement are fall and slides. Minor rock falls occur frequently, as shown by the scars left by the fallen blocks that contrast with the black staining of more stable surfaces. Larger slides are rarer but it is surprisingly common to find signs of incipient major rock slope failures on cliff tops. The pulling apart of the rock slopes as it tilts or slips seawards is marked by gulls - deep and narrow gullies above the cliff head.

The till cliffs found on more sheltered shores are susceptible to slumping. Where the base of the till passes below sea level then slope failure can occur along curved slide planes. More generally, till rests of rock terraces and ramps and here the rock acts to support the till face. The rate of cliff retreat is determined by the removal of debris from the base of the till.