The uniformity of the valley-side slope, in the dissected landscape of South Wales, might be interpreted in terms of rapid mass movement. Many samples of the debris mantle were taken from the test area to establish the engineering properties. The residual strength (φ′r = 35.3°) and saturated unit weight (γs = 17.4 kN/m3) were incorporated into a model of shallow sliding and gave threshold slopes of 35.3° (dry state) and 17.2° (saturated state). The slopes were randomly sampled to create a frequency distribution with a population mean for valley-side slope of 15.5° < μ < 17.8° equivalent to the limiting angle, so that rapid mass movement is important in interpreting valley side slope. The modes on the frequency distribution were also interpreted in terms of threshold values for mass movement processes and a comparison made with results from other environments. It is possible to interpret most of the British landscape as a relic feature of glacial, periglacial and mass movement processes superficially modified by current processes.