Structure, sedimentology and evolution of rockfall talus, Mynydd Du, south Wales

Gullies incised into relict rockfall talus slopes at Mynydd Du (south Wales) reveal stacked debris flow and slopewash deposits occasionally intercalated with buried peaty soils, reflecting episodic erosion and reworking of talus on the upper rectilinear slope. Radiocarbon-dating of the palaeosols indicates that reworking has been intermittently active since at least c. 4.8–44 cal ka bp. Particle-size analyses of bulk talus samples demonstrate that fine-grained (<2 mm) material represents on average c. 27% by weight of the Mynydd Du talus. Glacigenic and aeolian origins for this fine sediment are excluded, as is inwash of fines from the plateau above or in situ weathering of talus clasts. Flaking and granular weathering of the rockwall appear to be the most likely source of the fine sediment, the accumulation of which has probably proved instrumental in facilitating incision and reworking. Implications are considered for models of talus evolution, erosion histories of hillslope materials, and the role of small-scale weathering processes in rockwall retreat.