A Discussion on valley slopes and cliffs in southern England: morphology, mechanics and Quaternary history - The evolution of landslide slopes in Dorset

Abstract

The form and evolution of Fairy Dell, an active landslide complex on the Dorset coast, is described using a combination of cartographic, air photograph and field survey techniques. Erosion rates for the main landslide scars, undercliffs and sea cliffs are calculated and the spatial patterns of landslide evolution demonstrated by the use of sequences of maps and geological cross sections. Two dominant mechanisms are identified and described: (1) rotational landsliding and (2) block disruption, the breakup of originally large landslide units as they move downslope and over time. The role of small-scale erosion, in combination with the infilling of depressions by scree, wash debris and mudslides so as to produce an increasingly subdued topography as the landslides degrade, is emphasized and simple evolutionary models are proposed. The active landslide complex is then compared with the now stable, degraded landslide slopes inland. It is shown how the spatial patterns of landforms recognized in these areas on morphological maps and the complex subsurface forms revealed in sections can be better understood by reference to the suggested evolutionary sequence developed for the active complex. The sections in the stable but degraded slopes clearly support the idea of retrogressive rotational landsliding followed by block disruption, infilling and the downslope reduction of topographic expression of landslide units. Finally, it is suggested that this evolutionary interpretation might assist in the understanding of similar areas elsewhere and, if used in conjunction with geomorphological surveys, could result in the planning of more efficient site investigations.