A recent landslide on the east Devon coast, UK

Abstract

The largest landslides on the south coast of England are those associated with failure surfaces in montmorillonite- rich mudstones close to the base of the Gault Formation (mid Cretaceous). Those at Folkestone, Sussex (Trenter & Warren 1996), Ventnor, Isle of Wight (Hutchinson 2001) and Black Ven, west Dorset (Brunsden 2002) are among the largest active landslide systems in the UK. Westwards from Dorset, the Gault facies of the Albian Stage is replaced on the east Devon coast by sandstones and sandy calcarenites of the Upper Greensand Formation. These are mostly moderately strong rocks that crop out in almost continuous, precipitous cliffs between Sidmouth and the county boundary at Lyme Regis. They are a major component of the landslide debris, but the formation itself has not previously been shown to have initiated a failure. For example, the extensive Bindon landslide [SY 277 895] near Axmouth, which incorporates large masses of Upper Greensand, was attributed to failure surfaces in the Gault (Conybeare et al . 1840). However, that formation has not been recorded in a coastal section west of Lyme Regis. Joint/bedding-bounded toppling failures have been observed from time to time in the highest, more massive part of the Upper Greensand along the east Devon coast. The most recent of these occurred after a prolonged period of wet weather in 2001 when a single block of about 3000 tonnes in the highest part of the Upper Greensand fell over 120 m from the western end of Higher Dunscombe Cliff, Salcombe Regis [SY 150 878] and broke up on the beach. A similar mechanism has until now seemed to be the most likely explanation to account for the accumulations of angular blocks of Upper Greensand up to several hundred tonnes in weight that are common in the intertidal area between Beer Head and Sidmouth …