Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland

Abstract

Glacial ripping is a newly recognized process sequence in which subglacial erosion is triggered by groundwater overpressure. Investigations in gneiss terrain in lowland Sweden indicate that ripping involves three stages of (i) hydraulic jacking, (ii) rock disruption under subglacial traction, and (iii) glacial transport of rock blocks. Evidence for each stage includes, respectively, dilated fractures with sediment fills, disintegrated roches moutonnées, and boulder spreads. Here, we ask: can glacial ripping also occur in sedimentary rocks, and, if so, what are its effects? The case study area is in hard, thinly bedded, gently dipping Cambrian quartz-arenites at Loch Eriboll, NW Scotland. Field surveys reveal dilated, sediment filled, bedding-parallel fractures, open joints, and brecciated zones, interpreted as markers for pervasive, shallow penetration of the quartz-arenite by water at overpressure. Other features, including disintegrated rock surfaces, boulder spreads, and monomict rubble tills, indicate glacial disruption and short distance subglacial transport. The field results together with cosmogenic isotope ages indicate that glacial ripping operated with high impact close to the former ice margin at Loch Eriboll at 17.6–16.5 ka. Glacial ripping thus can operate effectively in bedded, hard sedimentary rocks, and the accompanying brecciation is significant—if not dominant—in till formation. Candidate markers for glacial ripping are identified in other sedimentary terrains in former glaciated areas of the Northern Hemisphere.

Late erosion pulse triggered by rapid melt in the cold‐based interior of the last Fennoscandian Ice Sheet, an example from Rogen

“…Similar patterns of damage have been identified on low, abraded bedrock hills in Cambrian quartz arenites in northwest Scotland (Hall et al, 2021).…”

“…Where ribbed moraines occur in association with hydraulic jacking, disrupted bedrock, and with angular locally derived and glacially transported boulder fields (also termed boulder spreads, see Hall et al (2021)), the role of groundwater at overpressure must also be considered.…”

Drag forces at the ice-sheet bed and resistance of hard-rock obstacles: the physics of glacial ripping

“…To focus our modelling, this range of parameters needs to be restricted: some parameters are not explicitly modelled, but are discussed. Field evidence suggests that damage from glacial ripping was particularly effective below the ablation zone, close to the retreating margin of the Pleistocene ice sheets, and thus constituted an intensive phase of subglacial erosion just prior to deglaciation (Hall and others, 2020, 2021; Bukhari and others, 2021; Krabbendam and others, 2021, 2022). We thus focus on glaciological conditions beneath the last FIS during its late stage of final deglaciation.…”

“…Glasser and Bennett, 2004; Alley and others, 2019). Recently, a further effective subglacial erosion mechanism, glacial ripping , has been recognised to have operated in eastern Sweden (Hall and others, 2020; Krabbendam and others, 2022), NW Scotland (Hall and others, 2021) and possibly eastern Canada (Bukhari and others, 2021), below Pleistocene ice sheets that covered these areas. Glacial ripping can remove large parts or all of a rock hill such as a roche moutonnée (Hall and others, 2020; Krabbendam and others, 2022).…”

“…Glacial ripping is a ‘mass’ event, involving hundreds to thousands of blocks at more or less the same time, and affecting a large part or the entirety of a rock hill, distinct from classic lee side plucking, which only removes rock one block at a time (Krabbendam and others, 2022). While there is good field evidence (summarised below) to support this three-stage mass process (Hall and others, 2020, 2021; Krabbendam and others, 2021, 2022), the theoretical understanding of the process is still limited.

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