The Nant Ffrancon ‘protalus rampart’: evidence for Late Pleistocene paraglacial landsliding in Snowdonia, Wales

Abstract

This paper assesses the validity of a protalus (pronival) rampart hypothesis and three alternative hypotheses (lateral moraine, protalus rock glacier and landslide) for the origin of a fossil talus-foot debris accumulation in North Wales. Whilst a rampart origin is supported by the landform's clear crestline, its morphological and topographic characteristics are unlike true protalus ramparts. In particular, its large volume implies Loch Lomond Stade (Younger Dryas) rockwall retreat roughly four times greater than that indicated by contemporaneous ramparts elsewhere in Britain. A glacial origin is dismissed on account of marked sedimentological and petrological contrasts between the landform debris and nearby till deposits. The absence of evidence for subsurface thrusting rules out a protalus rock glacier interpretation. The size and form of the landform and its constituent debris, the configuration of the rockwall, the presence of backward-tilted blocks and bedrock joints dipping out of the rockwall point to large-scale rocksliding as the most likely explanation for the landform, possibly associated with Late Devensian (Dimlington Stade) deglaciation. Three significant implications of these findings relate to (1) the identification of fossil, polygenetic talus-foot features, (2) Late Devensian climate and (3) paraglacial slope instability.