Nebbifield, Foula. A 300m high cliff in Devonian sandstone
Lichen of the cliff face
Definition: steep to vertical or overhanging rock slope at the coast, free of soil and regolith
The outer coast of Shetland includes long sections of spectacular cliff coastline. The form of cliffs is intimately related to rock type and structure, together with the more muted influence of past processes, including glaciation and sea level history.
On the map and when walking around the coast, it is clear that the cliffs are highly indented by geos and voes. The larger inlets may show the effects of glaciation, with rear portions cut by meltwater or widened by the passage of ice. Many geos however are entirely the result of marine erosion of structural weaknesses, although the presence of till plugs in some geos indicate a history that spans more than one interglacial phase. From the sea, it is clear that the indentations seen from the cliff top are only part of the honeycombing of cliffs by marine erosion. Caves and tunnels may reach tens of metres inland. Joint widening by compressed air and water allows block loosening and further penetration of sea water. In extreme cases, the compressed water spurts from gloups or blowholes but the circulation of compressed air is neatly confirmed by Colin MacFadyen's observation on Orkney that cliff top pools fizz when high seas are pounding the cliffs.
Geikie's (1887) sketch of Noss Head, with overhanging cliffs developed on seaward-dipping sandstone
The detailed cross-sectional form of cliffs is a product of rock structure. Many cliffs drop into deep water and this precludes effective abrasion of the cliff base. The cliff face may be tens or even hundreds 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 20 m high can approach the shore, wave action reaches heights of 40 m on some cliffs, wash strips vegetation and loose debris and spray and air borne debris is carried many tens of metres inland. Rock fragments and granules thrown by the wind may create a depositional zone on cliff tops, a form of machair in which progressive soil thickening incorporates clasts from cobble size downwards.
Although the process zones vary in height, the cliff ecosystem is adapted to cope with all but the most extreme sea states. The cliff face is blackened by the lichen (Verrucaria maura). The uppermost part of the cliff, affected by frequent spray and occasional wash, has a cover of yellow (Caloplaca marina) and grey (Lecanora sp) lichens on exposed rock surfaces, sea pinks in crevices and patches of turf where finer material has been stabilised.
Comparisons with old photographs and visits to kirkyards can help to tie down the timescales for the development of different kinds of lichen cover. This is useful for indicating the minimum residence or exposure time of a rock surface of a cliff or a block on a cliff top. Recent rock falls from cliffs are indicated by fresh scars which either lack lichen or which have only limited weathering and discoloration of the exposed rock.