Modification of bedrock surfaces by glacial abrasion and quarrying: Evidence from North Wales

Abrasion and quarrying are significant processes of subglacial erosion for ice masses in direct contact with hard substrates, yet their relative efficacy and spatio-temporal variability is unclear. Here, we investigate the glacial impact of these processes on a 70 m by 60 m bedrock surface at Moel Ysgyfarnogod in the Rhinog Mountains, Wales, using a combination of high-resolution digital photographs, analysis of a Digital Terrain Model derived from an Unmanned Aerial Vehicle survey, and regional ice sheet modelling. We map and analyze the distribution of grooved and striated surfaces, abraded surfaces, quarried blocks and open fractures in addition to the orientation of pre-existing bedrock fractures and joints. The grooves and striations are orientated in a single, consistent direction across the bedrock surfaces related to regional ice flow during the Late Pleistocene. Abraded and smoothed bedrock dominates the proximal edges of the bedrock outcrop and quarrying prevails on the distal edges of the bedrock outcrop, which are dominated by detached and partially detached blocks. We propose these blocks were removed during the final stages of the last glacial cycle when subglacial meltwater was plentiful in this otherwise predominantly frozen subglacial setting. A minimum estimate of 2000 m3 displaced material at this site implies that subglacial quarrying would have been an important erosional process during final stages of deglaciation.