Abstract

The relationship between the processes of landsliding of a coastal cliffand the progressive weathering of a Permo-Triassic mudstone is examined. The landslide, on the southern coast of Devon in southwest England, includes rockfalls, mudflows and a large mudslide. A stepped profile is produced by occasional stronger beds within the mudstone, which is also capped by conglomerate. A combination of partially weathered clasts and disaggregated matrix develops in debris fans and mudflows which feed the main mudslide. Weathering, which increases the plasticity index of the degrading mudstone, is found to be crucial to the general profile being maintained. If the plasticity index becomes too high, mudflows are inhibited because too much water is required, while too low a plasticity index results in material which is reluctant to flow on the slopes beneath the main cliff.

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