Academic paper in MARGO

Recent block fracture and movement at The Nev

Inga Ness, Westray. Old lichen-covered CTSDs and platform at 25 m above sea level. The blocks were moved by storm waves approaching from the WNW more than 60 years ago.

Key sites

Quoy Geo  The Nev  Yesnaby

CTSDs on Shetland                                                 

Cliff-top storm deposits

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 Atlantic waves is the presence of cliff-top storm deposits (CTSDs). These boulder accumulations occur well above high water mark at elevations of up to 50 m OD (Williams and Hall, 2004). The boulder accumulations have been referred to as storm beaches (Mykura, 1976) and block beaches (Kinehan et al., 1871) and share some of the characteristics of storm boulder beaches close to modern sea level, notably the localised development of beach ridges with seaward-dipping, imbricate boulders. The angularity, lack of sorting and frequent presence of boulders over 1 m in length are not features commonly associated with storm beaches and point to affinities with tsunami deposits (Dawson, 1994).

Cliff-top scouring and associated storm deposits are restricted in their development to the exposed outer coast of Orkney. They occur at three sites where cliffs are exposed to the west on the Atlantic coast and one site on the North Sea coast.

Recently moved block at The Nev. Book is 24x15 cm.

Cliff-top scouring by waves is more extensive than boulder accumulations, which rarely extend more than a few hundred m along the coastline. This indicates that the formation of CTSD’s requires highly specific site conditions. Where cliff tops lie above 22 m OD, boulder accumulations are absent, as wave action generally does not reach to these elevations. Most cliff-top boulder accumulations occur at around 10 m OD.

Two factors appear to 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 generally precludes the formation of cliff-top deposits. In cross-section, the cliffs with boulder accumulations have a lower near-vertical section which extends down to at least low-water mark and an upper ramp, which may slope to seaward or landward. 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.