Multidisciplinary studies carried out in advance of site investigations of the areas at Lyme Regis, Dorset most threatened by a\ud combination of landslides and marine erosion included sidescan-sonar, bathymetric and seismic-reflection surveys in the adjacent\ud offshore area. These revealed a large area (over 1500 m long x 700 m wide) 500 to 700 m offshore from the present-day coastline\ud in which an irregular sea bed is strewn with rock debris. This area is underlain by a layer of heterogeneous material up to 30 m\ud thick with traces of disturbed bedding and shear planes. Comparison of the stratigraphy and structure of the underlying in situ\ud beds, as determined from seismic-reflection surveys, suggests that the disturbed material is the residue from a single large\ud (> 10 million tonnes) landslide that resulted from a shear failure in seaward-dipping mudstones in the lower part of the Jurassic\ud Charmouth Mudstone Formation. A reconstruction of the geology immediately prior to the landslide indicates that the failure\ud occurred at a time when the cliff line was c. 350 m south of its present position, possibly in the mid Holocene 5000 to 6000 year\ud ago when sea level was sufficiently high to re-erode a Pleistocene cliff line
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