Scat Ness - 3 m block and block beach on the cliff top
Recent block movement at Villians of Hamnavoe
Wave scour up to 40 asl on South Head
CTSD surveys February 2012 by Ian Napier
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 (Hall et al., 2006). 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 Shetland. They occur at sites where cliffs are exposed to the west on the Atlantic coast and to the south and south-east on the North Sea coast. There are no significant CTSD’s on the few, low cliffs exposed to the north.
Vementry, west coast. Wave scour on a ramp-like cliff. WW2 gun emplacements at c. 50 m OD
Cliff-top scouring by waves is more extensive than boulder accumulations, which rarely extend more than a 1-2 km along the coastline. This indicates that the formation of CTSD’s requires highly specific site conditions. Where cliff tops lie above 25 m OD, boulder accumulations are rare, as wave action generally does not reach to these elevations. Exceptions occur at South Head, Villians of Hamnavoe, at Esha Ness and at Virda Field, Papa Stour, where scoured cliff tops and boulder accumulations occur at up to 40 m OD. Most cliff-top boulder accumulations occur between 10 and 25 m OD.
Two factors appear to control the occurrence of boulder accumulations on cliff tops on Shetland: bathymetry and cliff profile. At the majority of CTSD locations, 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.
At a number of sites, cliff-top storm deposits lie close to but at a higher elevation than boulder beach ridges. On Mousa, wave transported boulders up to 1 m a axis reach an elevation of 20 m OD on Green Head, whilst boulder beach ridges line the margins of the tidal West Pool at elevations of up to 5 m OD. On Scat Ness, on the southern tip of Mainland, a contemporary boulder beach, with well-rounded clasts is developed to a height of about 6 m OD at the rear of Whale Geo. Angular boulders derived from the adjacent cliff during storms litter the turf behind the beach to a height of about 12 m. Such sites confirm that CTSD’s can be viewed as an extreme form of storm beach deposit.
Jonathan Willis (2007) has recently identified the importance of big storms and extreme waves in influencing the coastal configuration and settlement along the east coast of Bressay.