Main image: Grey Bools storm deposits Wick aerial view (Credit: GmbH & Co. KG / Alamy Stock Photo / 2RTHK97)

Definition: wave-generated accumulations of large boulders on the tops of cliffs.

A striking feature at a few sites on rock coasts of the British Isles exposed to extreme waves is the presence of cliff-top storm deposits (CTSDs). These accumulations of angular boulders occur well above high water mark and relate to erosion and transport by the largest waves in major storms.

Cliff-top scouring and associated storm deposits are restricted in their development to the exposed coasts of Caithness along the Pentland Firth and the North Sea. CTSDs are best developed at the Grey Bools, Wick, and near Crosskirk (Woodman-Smith, 2004). At both these sites, a ramp-like cliff has facilitated wave run-up. Comparisons of edge rounding on storm blocks with that of flagstones in kirkyards suggests that the CTSDs have been stable since the 19th century. This is consistent with the mention of the Grey Bools by Hugh Miller (1841).

Two factors control the occurrence of boulder accumulations on cliff tops: bathymetry and cliff profile. Water depths of 10-20 m occur only a few tens of metres offshore. The presence of skerries precludes the formation of cliff-top deposits. The detailed configuration of the coast, particularly the size, orientation and form of geos and the height and form of cliffs, exerts a major control on the distribution, altitude, clast orientation and clast size of the cliff-top deposits.


Cliff-top scouring along the north coast.

A range of processes are involved in stripping the vegetation, soil and regolith including wave splash and dump, wind deflation and the effects of salt spray.

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