Cliffs are shaped by waves acting on complex rock structures. The nature of wave action varies according to the sea state and to the sea bed topography. Waves may arrive at cliffs breaking, plunging and, in deep water, unchanged and different processes of erosion occur under each condition. The up-rushing water from waves impacting on the cliff itself generates powerful hydraulic forces and the deluge of water on the cliff top washes and removes loose debris to create a scoured zone. During big storms, debris is removed from the edge of the cliff top and washed inland to form spreads or ridges of boulders. These cliff-top storm deposits give way inland to dumps of angular gravel and finally turf littered with pebbles and granules of air thrown debris.

Cliffs also can be regarded as rock walls subject to all the forces of rock mechanics. The cliff is affected by mass movement, especially by processes associated with lithologies with high rock mass strength. The cliff faces above the limit of common wave wash is also subjected to weathering. The influence of salt water spray may be particularly important here.

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.

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

  • The outer coast of Orkney is an outstanding location to study the erosion of hard rock coasts. Evidence of major erosion is not hard to find. The great bowl of Enegars corrie is losing its edge to the sea. The egg-shaped headland of Marwick Head is a dome half lost.

  • Amongst the many beauties of Orkney are the beaches which fringe the bays and ayres of the inner coast. These are dynamic forms, changing shape with the seasons and gradually retreating landward as sea level rises.

  • Orkney is a dissected landmass drowned by postglacial sea level rise. A drop in sea level of just 35 m would unite the archipelago into a single island. The individual islands may represent hills that formed the watershed areas of preglacial drainage basins but it is linear glacial erosion which has separated one island from another and severed Orkney from the rest of Scotland.

  • World coasts have seen sea-level variation of approximately 100 metres within the past 11,500 years through melting of the ice caps. When the ice caps melt the sea level rises globally (eustatically). The relative sea-level at any location is measured proportionate to the nearby land, which is itself subject to tectonic movement both up and down.