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.

Subglacial processes and drumlin formation in a confined bedrock valley, northwest Ireland

“…The western (down-ice) side of the whaleback is partly jagged and angular. The base of the whaleback and its bounding surface with overlying sediment is not seen clearly, but its overall morphology is similar to those recorded in adjacent valleys (Knight 2009). …”

“…Sediment-free areas located between the sediment-filled hollows are associated with meltwater scouring and other direct erosive processes at the ice bed (Knight 2009). Such scouring typifies the basal water-sorted gravels observed within drumlin B, where there are marked metre-scale spatial differences in sediment and meltwater availability, as evidenced by variations in sediment sorting, grain size and matrix content.…”

“…This evolutionary (temporal) change is not commonly considered in relation to sediment successions (Meriano & Eyles 2009), although it has important implications for subglacial processes, hydraulic pressure gradients, shear stress and ice dynamics (Marshall et al 1996;Piotrowski et al 2001;Knight 2002Knight , 2009.…”

Subglacial processes from drumlins in Clew Bay, western Ireland

“…This is seen in many areas of western Ireland (e.g. McCabe and Dardis, ; Knight, , , ). In addition, variations in bedrock type and sediment thickness overlying bedrock have significant impacts on drumlin size (Greenwood and Clark, ) and location (Raukas and Tavast, ).…”

“…Many studies have described drumlin sediments from specific sites within these and other west coast bays (e.g. McCabe et al, 1986;Hanvey, 1989Hanvey, , 1992Dardis, 1989b, 1994;Dardis and Hanvey, 1994;Knight and McCabe, 1997a;Knight, 1999Knight, , 2006Knight, , 2009aKnight, , 2011Knight, , 2014Coxon, 2001;Hart, 2005;Hiemstra et al, 2011), but overall patterns of subglacial processes and environments and their down-ice changes are still unclear. For example, studies from within Clew Bay highlight the role of changing subglacial processes as a result of basal hydrological regime which influenced the degree of ice-bed coupling (Knight, 2006;Hiemstra et al, 2011).…”