Modification of bedrock surfaces by glacial abrasion and quarrying: Evidence from North Wales
Introduction
Throughout the multiple ice age cycles that characterize the last ~2.6 Ma, subglacial erosion has had a profound influence on temperate and polar landscapes of both hemispheres (Sugden and John, 1976; Cook et al., 2020). Two of the single most important processes of glacial erosion beneath warm-based glaciers and ice sheets resting on hard beds are abrasion and quarrying. Abrasion describes the wear of a rock surface by rock debris in transport in basal ice (Hallet, 1979, Hallet, 1996; Cohen et al., 2005) and produces elongated, streamlined landforms with smoothed and ice-abraded surfaces (Evans, 1996; Roberts and Long, 2005). Quarrying describes the processes involved in the detachment and removal of blocks of bedrock (Glasser and Bennett, 2004). It has been suggested that abrasion may dominate over other erosional processes beneath fast-flowing glaciers (Herman et al., 2015; Yanites and Ehlers, 2016), however, quarrying is arguably the more important of the two processes because it provides the basal material required for glacial abrasion (Alley et al., 2019).
Quarrying theory defines the basal conditions that favour the growth of cracks in otherwise coherent bedrock (Iverson, 1991; Hallet, 1996). In addition to the role of basal sliding, which supplies the frictional energy, the important relationships are those of effective pressure of ice on the bed (ice overburden load minus subglacial water pressure) and the primary role of ice-bed cavities, usually in the lee of a rock step. The presence of cavities has the effect of concentrating stresses on rock protuberances. Under certain conditions, high effective ice pressures can hydraulically fracture and open existing or new micro-cracks normal to ice flow, and allow the removal of blocks of rock. Rapidly fluctuating water pressures in such cracks enhances fracture propagation and quarrying, which has been demonstrated by several empirical and theoretical studies (Iverson, 1991; Hallet, 1996; Cohen et al., 2005; Bartholomew et al., 2011; Anderson, 2014; Ugelvig et al., 2018). Diurnal and seasonal melt variations, and thus subglacial water pressure, can have an even more pronounced effect on the rates of quarrying since of the temporal amplification of differential bedrock stress around cavities and promotion of crack growth (Ugelvig et al., 2018). The imbalance between cavity size and water pressure drives episodes of elevated stress on the upglacier edges of steps in the bed, the so-called “hammer effect” by Anderson (2014), which is potentially of importance also for the quarrying rates integrated over longer periods (Ugelvig et al., 2018). According to Anderson (2014) thicker ice will generate a greater frequency of hammer events and thus is more effective at quarrying than thinner ice due to increased short-term oscillations in the sliding system. Stick-slip motion may also play a role in subglacial erosion (Zoet et al., 2013).
There are well-established links between lithology, bedrock structure and the geometry of glacial erosional landforms (Glasser et al., 1998). The importance of pre-existing fractures for quarrying processes is confirmed by field observations (Jahns, 1943; Gordon, 1981; Krabbendam and Glasser, 2011; Lane et al., 2015; Krabbendam et al., 2017), and the relationship between pre-existing fractures and quarrying has also been modelled theoretically (Hooyer et al., 2012; Iverson, 2012; Anderson, 2014). Geological data (e.g., measurements of concentrations of cosmogenic 10Be in glacial polish and bedrock fracture spacing) suggest that the distance between fractures in the rock are particularly important (Dühnforth et al., 2010). Quarried surfaces and major joint sets are often coincident (Hooyer et al., 2012). This is because the relative importance of abrasion and quarrying as geomorphic agents, and therefore bed roughness, is controlled by fracture spacing (Iverson, 2012; Anderson, 2014). Hardness and joint spacing also exert a strong control on subglacial erosional landforms and the mechanisms that form them (Krabbendam and Glasser, 2011).
Sugden et al. (1992) suggested that quarrying is also enhanced at the edge of a receding or thinning ice sheet, partly because of the changing relationship here between ice velocity and effective pressure. They argued that ice is thinner close to the margin and the normal load is lower, so cavities are likely to be more abundant and there is higher meltwater production. Assuming the ice is actively flowing, it becomes easier for overriding ice to dislodge and evacuate blocks close to an ice margin. These blocks often travel only short distances (metres) and their provenance can be traced to proximate source locations, further supporting the notion that an intense episode of quarrying accompanies deglaciation (Sugden et al., 2019). In numerical experiments, Ugelvig et al. (2018) also considered that meltwater variability enhances quarrying the most where there is a flat bed and thick ice, while abrasion is more sensitive to the variations in their steeper bed, thinner ice experiment. They observed that quarrying is most sensitive to variations in effective pressure, which are greatest in the thick-ice experiments, while larger variations in sliding speed boosts abrasion the most in the experiment with the steeper bed. Alley et al. (2019) noted that rapid, sustained bedrock erosion requires till removal at the ice/bed interface by sediment transport in subglacial streams.
Studies of subglacial erosive processes generally focus on temperate/wet basal conditions because these promote sliding. It is generally held that cold or polythermal ice masses characterised by frozen bed conditions preclude such processes and act to protect their substrates and indeed, may even shield inherited pre-glacial landscapes. In recent decades, this view has been challenged both from process-glaciological and glacial geomorphological perspectives (e.g., Cuffey et al., 2000; Atkins et al., 2002; Atkins, 2013). Direct observation of active subglacial sediment entrainment under thermal conditions of −17 °C recorded in the basal layers of Meserve Glacier in the Dry Valleys, Antarctica, demonstrates that the assumption that cold-based glaciers do not slide and abrade their beds is incorrect (Cuffey et al., 2000). Although rates of subglacial abrasion may be an order of magnitude lower than those observed at warm-based ice masses, these observations suggest that even under prolonged cold-based conditions, given sufficient basal traction, basal sliding and associated erosion will occur. These findings are supported by geomorphological observations from formerly glaciated landscapes in the same region of Antarctica; for example, Atkins (2013) mapped the glacial geomorphological features across the forefield of frozen-based glaciers in the Dry Valleys, Antarctica.
Here, we address these questions about the styles and relative efficiency of glacial erosion with reference to a palaeo-glaciated bedrock surface at Moel Ysgyfarnogod in the Rhinog Mountains, Wales. We combine detailed analysis of DTM and fine-resolution imagery acquired by Unmanned Aerial Vehicle (UAV) with regional ice flow modelling to investigate the distribution and relative efficacy of abrasion and quarrying processes along with their former subglacial thermal regime across a 60 × 70 m area of the bedrock pavement. Our mapping and analysis of the distribution of geological, geomorphological and glacial characteristics of the Moel Ysgyfarnogod bedrock pavement enables us to qualitatively assess the relative efficacy and spatio-temporal distribution of the subglacial erosional processes operating beneath the ice mass that occupied the Rhinogs during the late Pleistocene. Similar glacial geomorphological mapping studies of bedrock pavements elsewhere in Wales, such as Snowdon (e.g., Sharp et al., 1989), provide additional context for our findings here.
Section snippets
Study area
The Rhinog Mountains (Rhinogydd in Welsh) are located in North Wales (Fig. 1). Geologically they are part of the Harlech Dome, a large anticline composed of Cambrian sandstones and mudstones of the Rhinog Formation and sandstones of the Hafotty Formation (EDINA Geology Digimap, 2014). The latter rock type forms the summit areas of most of the northern Rhinog Mountains, including the highest local summit at Moel Ysgyfarnogod (623 m asl), close to our chosen study site. The Rhinog Mountains were
Methods
Digital photographs of the bedrock surface were collected using an Unmanned Aerial Vehicle (UAV; DJI Phantom 4) equipped with a DJI FC300C camera (12 MP resolution) and GPS from c. 4–6 m above the ground surface. ‘Structure-from-Motion’ processing was undertaken in AgiSoft Professional software using standard and documented procedures for applied photogrammetric analysis in glacier and geomorphic studies (e.g., Westoby et al., 2012; Ryan et al., 2015; Jones et al., 2018). In total, 872
Results
Based on our DTM analysis, we identify the following five primary components of the glacially-eroded landscape (Fig. 3).
Interpretation and discussion
Well-developed striations and grooves are present across the entire surface of the bedrock outcrop, but quarried surfaces are restricted to the down-ice (lee) side of the bedrock at Moel Ysgyfarnogod. The distribution of abraded and quarried surfaces therefore fits existing glacial erosion theory, which predicts that ice-moulded and abraded surfaces should dominate the up-ice (stoss) side of bedrock outcrops and quarrying should dominate the down-ice (lee) side of bedrock outcrops. The overall
Conclusions
- •
The distribution of abraded and quarried surfaces at Moel Ysgyfarnogod confirms existing glacial erosion theory with ice-moulded and abraded surfaces dominating the up-ice proximal side of the bedrock outcrop and quarrying dominating the down-ice distal side of the bedrock outcrop.
- •
Glacial quarrying shows a strong relationship with bedrock structure; large blocks are detached and partially detached along pre-existing fracture and joint sets.
- •
There is evidence that glacial quarrying can rapidly
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
MR's contribution was supported by the Masaryk University project MUNI/A/1356/2019 and The Ministry of Education, Youth and Sports of the Czech Republic projects LM2015078 and CZ.02.1.01/0.0/0.0/16_013/0001708. AH and HP acknowledge support by the Research Council of Norway through its Centre of Excellence funding scheme (grant 223259) and HP the Akademia Programme at Equinor.
References (53)
- et al.
Control of glacial quarrying by bedrock joints
Geomorphology
(2012) - et al.
Rapid thinning of the Welsh Ice Cap at 20 ka based on 10Be ages
Quat. Res.
(2016) - et al.
Glacial erosion and bedrock properties in NW Scotland: abrasion and plucking, hardness and joint spacing
Geomorphology
(2011) - et al.
Joint-bounded crescentic scars formed by subglacial clast-bed contact forces: implications for bedrock failure beneath glaciers
Geomorphology
(2017) - et al.
Controls on bedrock bedform development beneath the Uummannaq Ice Stream onset zone, West Greenland
Geomorphology
(2015) - et al.
The Glacial History of the British Isles during the Early and Middle Pleistocene: implications for the long-term development of the British Ice Sheet
- et al.
‘Structure-from-Motion’ photogrammetry: a low-cost, effective tool for geoscience applications
Geomorphology
(2012) - et al.
Intermittent glacial sliding velocities explain variations in long-timescale denudation
Earth Planet. Sci. Lett.
(2016) Classic Glacial Landforms of Snowdonia
(1997)- et al.
Glacial erosion: status and outlook
Ann. Glaciol.
(2019)
Evolution of lumpy glacial landscapes
Geology
Geomorphological evidence of cold-based glacier activity in South Victoria Land, Antarctica
Cold glaciers erode and deposit: evidence from Allan Hills, Antarctica
Geology
Large sorted stone-stripes in the Rhinog Mountains, North Wales
Geografiska Annaler: Series A: Physical Geography
Supraglacial forcing of subglacial drainage in the ablation zone of the Greenland ice sheet
Geophys. Res. Lett.
Debris-bed Friction of Hard-bedded Glaciers
The empirical basis for modelling glacial erosion rates
Nat. Commun.
Entrainment at cold glacier beds
Geology
Amplified melt and flow of the Greenland ice sheet driven by late-summer cyclonic rainfall
Nat. Geosci.
Bedrock fracture control of glacial erosion processes and rates
Geology
British Geological Survey (BGS) Data
Abraded rock landforms (whalebacks) developed under ice streams in mountainous areas
Ann. Glaciol.
On the geology of Arenig Fawr and Moel Llyfnant
Q. J. Geol. Soc.
The Glaciation of the Harlech Dome
Establishing the age and geomorphological significance of sorted stone-stripes in the Rhinog Mountains, North Wales
Geografiska Annaler, Series A, Physical Geography
Sarn Badrig, a submarine moraine in Cardigan Bay, north Wales
Z. Geomorphol.
Cited by (12)
<sup>10</sup>Be and <sup>26</sup>Al exposure history of the highest mountains in Wales: Evidence from Yr Wyddfa (Snowdon) and Y Glyderau for a nunatak landscape at the global Last Glacial Maximum
2022, Quaternary Science ReviewsCitation Excerpt :In addition to these large boulders, a number of smaller boulders are also present. The subangular and subrounded morphology of the boulders and the fact that they are upslope of major rock outcrops indicates that they are derived from quarrying (plucking) of bedrock blocks and moved by ice and their form partially modified by subglacial transport (cf. Hallet, 1996; Glasser et al., 2020). Visual examination combined with geochemical comparisons of the boulders with the underlying bedrock shows that some are technically ‘erratics’.
Geomorphology and surficial geology of the Femmilsjøen area, northern Spitsbergen
2021, GeomorphologyCitation Excerpt :However, while most of the case studies of thermal switch are from small valley glaciers, we assume that it would also occur within large ice caps (e.g., Humlum et al., 2005; Lovell et al., 2015). Cold-based glaciers were for a long time thought to protect the substrate rather than modify it (Glasser et al., 2020, and references therein). During the last two decades, this view has been challenged by studies indicating that cold-based glaciers modify landscapes, however, at a rate of an order of magnitude lower (Cuffey et al., 2000; Atkins et al., 2002; Atkins, 2013).
Subcritical crack propagation in glacial quarrying during subglacial water pressure variation
2023, Journal of GlaciologyCoevolving edge rounding and shape of glacial erratics: the case of Shap granite, UK
2024, Earth Surface DynamicsThe interplay of bedrock fractures and glacial erosion in defining the present-day land surface topography in mesoscopically isotropic crystalline rocks
2023, Earth Surface Processes and Landforms