PROGRESSIVE WEATHERING AND DEGRADATION OF MUDSTONE IN A COASTAL LANDSLIDE

The relationship between the processes of landsliding of a coastal cli ff and the progressive weathering of a Permo-Triassic mudstone is exam ined. The landslide, on the southern coast of Devon in southwest Engla nd, includes rockfalls, mudflows and a large mudslide. A stepped profi le is produced by occasional stronger beds within the mudstone, which is also capped by conglomerate. A combination of partially weathered c lasts and disaggregated matrix develops in debris fans and mudflows wh ich feed the main mudslide. Weathering, which increases the plasticity index of the degrading mudstone, is found to be crucial to the genera l profile being maintained. If the plasticity index becomes too high, mudflows are inhibited because too much water is required, while too l ow a plasticity index results in material which is reluctant to flow o n the slopes beneath the main cliff.