shore platform             slowly changing shore platforms                      

inherited elements   new from NZ    ESPED 

Block quarry at the cliff

Abraded platform, inactive in the foreground, with storm beach and stripping of weathered sandstone

Block lift from the crest of small cuesta

Blackened and pitted surface on a relatively stable part of the platform

Shore platform processes

Significance: shore platform in many parts of the world are slow to change. The high wave energy environment of the exposed outer coast of Orkney provides high rates of erosion and allows the main processes which shape these platforms to be identified.

The shoreline between Stromness and Warebeth shows an actively eroding shore platform and low cliff. The platform is exposed to westerly storms and experiences a tidal range at springs of 2-3 m. The Devonian flagstone includes a range of massively bedded to flaggy sandstones. The sequence dips westward at 10
˚, with strike normal to the platform towards Stromness but turning parallel as the shoreline changes orientation at Warebeth.

Process zones

The platform displays a series of process zones from landward to seaward. The cliff is rarely more than 5 m high and postglacial weathering has often created a weathering profile of disaggregated and discoloured sandstone 1-3 m thick. Wave action is locally stripping this weathering profile and quarrying large blocks from dipping rock faces. Rapid coastal retreat has been partly stabilised by piecemeal coastal protection works.

At the base of the cliff is a storm beach, comprising cobbles and boulders of mainly rounded sandstone clasts but with occasionally large angular blocks.
At the foot of the beach is zone of abraded bedrock, with rounded joint edges and lacking significant lichen growth.

This passes seaward into rock surfaces which are neither frequently nor intensively abraded today. Although some rock surfaces may be smoothed they are blackened with Verrucaria maura and pitted by weathering. Small cuestas show no clear smoothing and a blackened and inactive dip slope and loss of blocks from the scarp face. Scars with little or no lichen cover shows sites of recent block removal. Block transport is from W-E, as show by the carry of blocks of distinctive size or lithology.

Towards the low water mark seaweed cover becomes extensive but evidence of recent block loss from scarp faces is provided by fallen and transported blocks in strike gulleys.

Processes of erosion

The platform is being extended to landward by cliff retreat but wave action is parallel to the shore, rather than normal, reflecting the orientation of dominant waves at this site. The platform is lowered by abrasion only at its inner edge, where mobilisation of coarse beach materials during storms allows scouring of rock surfaces. Elsewhere block quarrying is the main agent of lowering - these are wave-cut platforms. Blocks, slabs and flakes are lost to hydraulic quarrying from the scarp face of mini-cuestas, reflecting block lift during turbulent flow. Block loss from the scarp face leads to progressive elimination of the cuesta. Not all of the platform is actively eroding. Some elevated parts of the platform have blackened and pitted surfaces, indicating an absence of abrasion and block loss and sufficient time for significant chemical weathering to occur.

On the generally flat-lying Devonian sandstones and flagstones of Orkney and Caithness there is a strong association between platform width and the height of backing slopes. Platforms are wide and extensive wherever gentle slopes back the shore. This suggests that rock mass strength is the dominant control over platform extent. A corollary inference is that wave energy in even sheltered locations is or was sufficient to mobilise blocks from fissile lithologies, such as flagstone.