Cliff processes

The strike-aligned cliffs at Sandside Head, developed in seaward-dipping flagstones. A zone of stripped vegetation is evident on the cliff top, a product of deflation, spray and wave-splash. Cliff retreat is slowly encroaching on the local watershed.

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 rocks of 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.

The cliff face may be tens of metres high but can be divided into a number of zones according to the influence of waves and spray. At the base is the submerged zone, hidden from view. Close to sea level lies a zone of inundation which is exposed to the air only between waves. Above this lies a wave impact zone, which is pounded by waves under higher sea states. The highest marine influence is in the spray and wash zone, where water from breaking waves shoots upwards as droplets or sheets. All four zones vary in height according to the sea state. Only during conditions of low tide and low swell can the inundation zone be observed easily and at these times wave and spray action will be confined to the lowest parts of the cliff. Under high sea states, where deep water waves up to 15 m high can approach the shore, wave action reaches heights of 25 m on some cliffs, wash strips vegetation and loose debris and spray and air borne debris is carried many tens of metres inland.