A talus rock glacier in Nantlle Valley, North Wales

ABSTRACT Landforms comprising mixtures of fine and coarse sediment and deposited during late glacial times are ubiquitous in the British mountains. In many cases, their interpretation has often proved contentious; as a result, the non‐generic term discrete debris accumulations (DDA) has been used to describe them and, over the last 50 years, some have been interpreted as fossil rock glaciers of late Quaternary age. However, detailed accounts of their sedimentology and structural assessments of surrounding rock slopes have been lacking, despite the insights into processes that such investigations would allow. Here, and for the first time in the British Isles, we use detailed lithological and structural modelling and analyses to investigate a well‐defined DDA in the Nantlle Valley of North Wales. We show that component clasts within the landform are from rockfall debris originally detached from upslope rockwalls above by relatively small‐scale failure events. The results of 2D rockfall analysis demonstrate that a significant proportion of component debris could not have been deposited directly from rockfall. Instead, we conclude that the DDA was formed by downslope remobilisation of thick talus deposits, and movement was associated with strong compressional flow in the frontal margin. We argue that the only plausible mechanism that could have been responsible was the internal creep of ground ice that had developed within the talus. As a result, we argue that the DDA is a fossil rock glacier. Its formation in western Britain with former high levels of precipitation calls into question some of the palaeoenvironmental implications often applied to such landforms.