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 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.

Academic paper in MARGO

  • The outer coast of Orkney is an outstanding location to study the erosion of hard rock coasts. Evidence of major erosion is not hard to find. The great bowl of Enegars corrie is losing its edge to the sea. The egg-shaped headland of Marwick Head is a dome half lost.

  • Amongst the many beauties of Orkney are the beaches which fringe the bays and ayres of the inner coast. These are dynamic forms, changing shape with the seasons and gradually retreating landward as sea level rises.

  • Orkney is a dissected landmass drowned by postglacial sea level rise. A drop in sea level of just 35 m would unite the archipelago into a single island. The individual islands may represent hills that formed the watershed areas of preglacial drainage basins but it is linear glacial erosion which has separated one island from another and severed Orkney from the rest of Scotland.

  • World coasts have seen sea-level variation of approximately 100 metres within the past 11,500 years through melting of the ice caps. When the ice caps melt the sea level rises globally (eustatically). The relative sea-level at any location is measured proportionate to the nearby land, which is itself subject to tectonic movement both up and down.