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.
Highlights
Recent work in lowland Sweden recognized a new process of erosion, termed glacial ripping, that operated beneath the last Fennoscandian Ice Sheet (FIS) [1]
We apply a checklist of features identified in basement gneisses in Sweden as markers for glacial ripping to test if similar features developed in sedimentary rocks in Scotland beneath the last British Irish Ice Sheet (BIIS)
The simple homoclinal structure of the quartz-arenite exerted a strong control over landforms of glacial erosion at Loch Eriboll (Figure 1b)
Summary
Recent work in lowland Sweden recognized a new process of erosion, termed glacial ripping, that operated beneath the last Fennoscandian Ice Sheet (FIS) [1]. We consider if and how glacial ripping may operate in hard sedimentary rocks. We apply a checklist of features identified in basement gneisses in Sweden as markers for glacial ripping to test if similar features developed in sedimentary rocks in Scotland beneath the last British Irish Ice Sheet (BIIS). Our field surveys identified many candidate features for groundwater overpressure and glacial ripping, but important differences exist which reflect the distinctive structure of the Eriboll quartz-arenite and the local glaciological conditions.
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