Subglacial erosion forms in northwest Ireland

Abstract

Subglacial erosional forms are commonly found on bedrock substrates inside the Late Weichselian ice margin in County Donegal, northwest Ireland, and can be used to provide detailed information on subglacial processes and environments. The erosional forms occur on spatial scales from whalebacks (tens of metres in scale), to asymmetric and channelized bedrock‐cut scours (tens of cm in scale) and striations (mm scale). Processes responsible for development of subglacial erosional forms occur along a continuum, from free meltwater existing as a laterally extensive sheet at the ice‐bed interface, to abrasion by basal ice. Channelized bedrock‐cut scours are particularly common in County Donegal, and show asymmetric and meandering thalwegs, U‐shaped cross‐profiles and steep lateral margins. Innermost parts of the scours are highly polished and have striations that follow thalweg direction. In places, bedrock surfaces are overlain by a delicate polish and thin calcite cement, and are buried beneath glacial till. Based on their morphology, the bedrock scours are interpreted as s‐forms caused by high‐pressure subglacial meltwater erosion. Striations within the scoured channels reflect periods of ice‐bed coupling and subglacial abrasion. The range of features observed here was used to consider relationships between subglacial topography, hydraulic processes and ice‐bed coupling. Precipitation of calcite cement took place in depressions on the bedrock surface by CO2 degassing. Infilling of depressions by glacial till formed a new type of ‘sticky spot’ related to spatial variations in subglacial water pressure. The temporal evolution of sticky spots reflects interactions within the subglacial environment between subglacial relief, hydraulic regime and ice‐bed coupling.