Discussion on ‘Threshold Slopes in south Wales’ by W. C. Rouse & Y. I. Farhan

Abstract

The authors present an interesting account of the role of rapid mass movements in the slope morphology of South Wales. They attribute much of the slope form to mass movements during periglacial conditions, with more recent movements only superficially modifying that form. A similar conclusion regarding the role of periglacial conditions was made for colluvial masses in the Appalachian Plateau province of the United States (Gray et al . 1978, D'Appolonia et al . 1967, Philbrick 1961). However, large-scale movement of the type previously assigned to periglacial conditions recently occurred in a colluvial mass in that region, indicating that such movements may not be strictly related to past climatic conditions. Detailed study indicated that movements may have been triggered by a period of abnormally high precipitation that was preceded by at least 30 years of low precipitation (Gray & Gardner 1977). Large-scale movements are thought to occur infrequently and long intervals may occur where only smaller scale, superficial mass movements occur, such as those now observed in South Wales. Intervals between large-scale movements are marked by continued build-up of colluvium and decreased equilibrium in portions of the larger mass. Eventually, some triggering mechanism (e.g. high precipitation) initiates large-scale movements that continue until the mass reaches equilibrium. We suggest that a similar mechanism should be investigated for slope development in South Wales. This possibility seems to be supported by the authors' statement that ‘rapid mass movements may be on the increase’.