Summary The Dorset coast is an area with a long history of landslide activity and coastal erosion. Geologically the area is composed of the clays, mudstones and limestones of several divisions of the Lower Jurassic (Lower Lias) which are overlain unconformably by Cretaceous clays and sands (Gault and Upper Greensand.) Areas of serious instability appear to occur in close association with naturally occurring reservoirs of ground water, of which Black Ven at Char-mouth is a good example. Problems connected with a small coastal landslip near a private housing estate at Char-mouth were studied and shown to be related to a Pleistocene mudflow. The main purpose of the investigation was to suggest remedial works but the subsidiary aim, forming the theme of this paper, was to assess the usefulness of shallow depth geophysical and geotechnical methods not normally used in instability studies.

Summary A detailed geological and geotechnical survey was carried out in connection with the future development of the South Essex area for a possible third London airport on Maplin Sands. Information on the position of the old buried channels of the Crouch/Roach river system was obtained by carrying out a continuous seismic profiling survey along the present channels of the two rivers. The survey provided information on the depth to the London Clay surface, the thickness and depth of the alluvial deposits, the variation of lithology within these deposits, and the position of some of the buried channels.

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Summary In the west Dorset coastal area superficial materials include Head deposits which, by the nature of their origin, are variable in structure and composition. Two broad types are recognized depending on the provenance of the predominant constituent: Cretaceous Head and Lias Head. The Cretaceous Head was probably formed near to or at the end of the Devensian stage of the Late Pleistocene. Where the two types of Head deposit are superimposed, Cretaceous Head always lies above Lias Head. On many slopes Lias Head has topographic expression as lobate sheet forms which are frequently masked by a blanket of Cretaceous Head. The high permeability of Cretaceous Head together with the low strength of Lias Head produce slopes presenting potentially unstable conditions to depths of up to 4 m below ground surface. Three minor types of instability are generated in the Head deposits themselves: shallow translational movements, block slides, and shallow rotational slides. In addition, the high permeability of Cretaceous Head permits water to be introduced to undercliff areas, thus facilitating the development of instabilities in the solid formations below. This results in a mechanism of cliff-top retreat which is quite independent of cliff-toe erosion. The events at sites in Lyme Regis and Charmouth are summarized to illustrate the hazard presented by Head deposits in cliff-top areas.

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