geosciences Article Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland Adrian M. Hall 1,* , Hannah Mathers 2 and Maarten Krabbendam 3 1 Department of Physical Geography, Stockholm University, SE-106 91 Stockholm, Sweden 2 School of Geographical & Earth Sciences, University of Glasgow, Glasgow G12 8QQ, UK; [email protected] 3 British Geological Survey, Edinburgh EH14 4AP, UK; [email protected] * Correspondence: [email protected] 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 Citation: Hall, A.M.; Mathers, H.; field results together with cosmogenic isotope ages indicate that glacial ripping operated with high Krabbendam, M. Glacial Ripping in impact close to the former ice margin at Loch Eriboll at 17.6–16.5 ka. Glacial ripping thus can operate Sedimentary Rocks: Loch Eriboll, NW effectively in bedded, hard sedimentary rocks, and the accompanying brecciation is significant— Scotland. Geosciences 2021, 11, 232. if not dominant—in till formation. Candidate markers for glacial ripping are identified in other https://doi.org/10.3390/ sedimentary terrains in former glaciated areas of the Northern Hemisphere. geosciences11060232 Keywords: glacial ripping; groundwater overpressure; breccia; rubble till; Cambrian quartz-arenite Academic Editors: Angelos G. Maravelis and Jesus Martinez-Frias 1. Introduction Received: 20 April 2021 Accepted: 27 May 2021 Recent work in lowland Sweden recognized a new process of erosion, termed glacial Published: 29 May 2021 ripping, that operated beneath the last Fennoscandian Ice Sheet (FIS) [1]. Ripping involved a sequence of process steps that comprise (i) hydraulic jacking and opening of subhorizontal Publisher’s Note: MDPI stays neutral fractures, (ii) consequent disruption of the near-surface rock mass under subglacial traction, with regard to jurisdictional claims in and (iii) transport and deposition beneath the ice sheet. Glacial ripping operated with high published maps and institutional affil- effectiveness in Sweden, affecting large areas to median depths of 1–4 m. Ripping was iations. triggered by build-up of groundwater overpressure in the shallow subsurface in the ice marginal zone of the retreating FIS [1,2]. In this paper, 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 Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. beneath the last British Irish Ice Sheet (BIIS). The study area is in Cambrian quartzite at This article is an open access article Loch Eriboll, NW Scotland (Figure1). Our field surveys identified many candidate features distributed under the terms and for groundwater overpressure and glacial ripping, but important differences exist which conditions of the Creative Commons reflect the distinctive structure of the Eriboll quartz-arenite and the local glaciological Attribution (CC BY) license (https:// conditions. Comparisons with other glaciated sedimentary bedrock terrains suggest that creativecommons.org/licenses/by/ glacial ripping operated widely beneath ice sheets during the Pleistocene. 4.0/). Geosciences 2021, 11, 232. https://doi.org/10.3390/geosciences11060232 https://www.mdpi.com/journal/geosciences Geosciences 2021, 11, 232 2 of 27 glaciological conditions. Comparisons with other glaciated sedimentary bedrock terrains suggest that glacial ripping operated widely beneath ice sheets during the Pleistocene. 2. Geology and Glacial Geomorphology Loch Eriboll is a NNE-SSW trending sea loch that occupies a 15 km long glacially over-deepened valley in the north-west Highlands, Scotland (Figure 1a). The western flank of the loch rises as a gently inclined, sub-planar surface that emerges from below sea level and extends to the summit of Cranstackie (802 m above sea level (a.s.l.). The topo- Geosciences 2021, 11, 232 graphic surface is an expression of structural control imposed by the dip of the Cambrian2 of 26 quartz-arenite strata and of the near-planar unconformity on the underlying Precambrian gneisses of the Lewisian gneiss complex. Figure 1. Topography and landforms around Loch Eriboll, NW Highlands. Images © CNS/Airbus, Google, Maxar Tech- Figure 1. Topography and landforms around Loch Eriboll, NW Highlands. Images © CNS/Airbus, Google, Maxar nologies. (a) Location and topography. Blue arrows indicate directions of ice flow. (b) Structural landforms at Hill 314 Technologies.(Figure 2c). 1. (a )Strike-parallel Location and valley, topography. deepened Blue by arrowsglacial erosio indicaten. 2. directionsFlank cliff offacing ice flow.up dip, (b with) Structural rounded landforms edge. 3. Back- at Hill 314slope, (Figure with2c). glacially 1. Strike-parallel abraded surface. valley, 4. Stepped deepened face by after glacial lee-side erosion. glacial 2. plucking. Flank cliff 5. Shallow facing uprock dip, basin with developed rounded after edge. 3. Backslope,excavation withof weathered glacially gneiss abraded on surface.the sub-Cambrian 4. Stepped unconf face afterormity. lee-side 6. Low glacial gneiss plucking. dome. Da 5.shed Shallow white rock line basin indicates developed the afterline excavation of unconformity of weathered between gneiss the Lewisian on the sub-Cambrian gneiss and the unconformity.overlying Basal 6. Quartzite. Low gneiss dome. Dashed white line indicates the line of unconformity between the Lewisian gneiss and the overlying Basal Quartzite. 2.1. Geology 2. GeologyThe Cambrian and Glacial Eriboll Geomorphology Formation in the NW Highlands was deposited unconforma- bly Lochupon the Eriboll Archaean-Palaeoproterozoic is a NNE-SSW trending Lewisian sea loch Gneiss. that occupies This unconformity a 15 km long is remark- glacially over-deepenedably flat and is valleya fragment in the of north-west the Great Unconformity Highlands, Scotland of Laurentia. (Figure The1a). Eriboll The western Formation flank ofis the overlain loch rises by the as avery gently thin inclined, (c. 30 m) sub-planar An-t-Sron Formation surface that (not emerges shown from on the below map), sea fol- level andlowed extends by the to thicker the summit units of of Ordovician Cranstackie limestones (802 m above and dolostones sea level (a.s.l.). of the Durness The topographic group surface(e.g., [3]). isan expression of structural control imposed by the dip of the Cambrian quartz- areniteThe strata Eriboll and Formation of the near-planar comprises unconformity two members: on the the Basal underlying Quartzite Precambrian Member and gneisses the ofPipe the Rock Lewisian Member gneiss [4]. complex. In this study, attention was focused on the lower, 75–125 m thick Basal Quartzite, comprising cross-bedded quartz-arenite (the unit name was historic 2.1.when Geology the term quartzite was also applied to sedimentary rocks with very high quartz The Cambrian Eriboll Formation in the NW Highlands was deposited unconformably upon the Archaean-Palaeoproterozoic Lewisian Gneiss. This unconformity is remarkably flat and is a fragment of the Great Unconformity of Laurentia. The Eriboll Formation is overlain by the very thin (c. 30 m) An-t-Sron Formation (not shown on the map), followed by the thicker units of Ordovician limestones and dolostones of the Durness group (e.g., [3]). The Eriboll Formation comprises two members: the Basal Quartzite Member and the Pipe Rock Member [4]. In this study, attention was focused on the lower, 75–125 m thick Basal Quartzite, comprising cross-bedded quartz-arenite (the unit name was historic when the term quartzite was also applied to sedimentary rocks with very high quartz content) (Figure2a). At Eriboll, the unconformity and the quartz-arenite beds dip 10–15 ◦ to the ESE and strike to ENE [5], more or less parallel to the long axis of the Loch. In the study area, the quartz-arenite comprises mainly quartz, with 5–10% feldspar grains. The grains are tightly packed with little matrix [6]. Porosity is mostly very low (<1%) [5]. The Basal Geosciences 2021, 11, 232 3 of 26 Quartzite in NW Scotland is very hard [7] with a median bed thickness of 0.16 m and a typical range in joint spacing of 0.13–0.47 m [8]. Individual quartz-arenite beds are rarely >1.5 m thick. Vertical fracture spacing and length follow power laws across scales [9], with wider spacing of vertical fractures in thicker beds (Figure2b). Long, straight vertical Geosciences 2021, 11, 232 4 of 27 fractures form dominantly