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.
- © The Geological Society, London 1979