Glacial erosion and bedrock properties in NW Scotland: Abrasion and plucking, hardness and joint spacing

Abstract

Subglacial erosion beneath glaciers occurs predominantly by abrasion and plucking, producing distinct erosional forms. The controls on the relative importance of abrasion vs. plucking are poorly understood. On the one hand, glacial conditions that favour or suppress cavity formation (ice velocity, ice thickness, and water pressure) are thought to favour plucking or abrasion, respectively.Conversely, bedrock properties are also known to control landforms, but this has rarely been analysed quantitatively. In this study we compare landforms and bedrock properties of sandstone and quartzite at the bed of a palaeo-ice stream near Ullapool in NW Scotland. The boundary between the rock types is at right angles to the westward palaeo-ice flow, and palaeoglacial conditions on both rock types were similar. We report quantitative parameters for bedrock properties (Schmidt hammer hardness and joint spacing) and use morphometric parameters to analyse the landforms. Torridon sandstone is soft but thick-bedded and with a wide joint spacing. Erosional bedforms include roche moutonnées with smoothed tops and concave stoss sides, whalebacks, and elongate p-forms, indicating a high proportion of abrasion over plucking. Cambrian quartzite is hard but thin-bedded with narrow joint spacing. Erosional landforms are angular to subangular with abundant plucked lee faces, suggesting a high proportion of plucking over abrasion. Hardness and joint spacing thus exert a strong control on subglacial erosional landforms and the mechanisms that formed them. Thus glacial conditions (ice velocity, ice thickness) can only be inferred from glacial erosional landforms if the effects of bedrock properties of the substrate are considered.

Quaternary evolution of glaciated gneiss terrains: pre-glacial weathering vs. glacial erosion

“…Aydin andBasu, 2005, Krabbendam andGlasser, 2011). Fractures play an important role during weathering and erosion, and we thus include fracture analysis of a number of localities in our study area.…”

“…Eyles, 2012). It is increasingly recognised that 'hard-bed landform assemblages' are varied and influenced by bedrock type and structure, the relation between dominant bedrock structure and ice-flow direction, as well as glaciological controls such as ice thickness, ice velocity and thermal regime (Bradwell et al, 2008a;Bradwell, 2013;Eyles, 2012;Gordon, 1981;Johansson et al, 2001a, b;Krabbendam and Bradwell, 2011;Krabbendam and Glasser, 2011;Phillips et al, 2010;Zumberge, 1954).…”

“…Roche moutonnées show the classic distinct asymmetry -a smooth rounded stoss-side and a jagged plucked lee-side. Plucking generally exploits preexisting joints in the gneiss in the manner described by Rea (1994), Glasser (2011) andHooyer et al (2012). In most of the area, roche moutonnées are more common than whalebacks, but this is reversed in the Laxford area.…”

“…This 'background' jointing is represented in Fig 8A by the Loch na Facial and Ceannabeinne Beach localities, which joint spacing of 0.33 m and 0.74 m respectively (Table 1), although joint spacing and orientation varies greatly throughout the study area (Pless, 2011). Joints play a major role in glacial plucking (Rea, 1994;Krabbendam and Glasser, 2011;Hooyer et al, 2012) and many plucked faces coincide with joints (e.g. Fig 6A, D).…”

Morphotectonic implication of the Paleoproterozoic Mid-Lithuanian Suture Zone

“…The spac ing of frac tures also con trol the pre vail ing mecha nism (e.g. abra sion vs. pluck ing) of ero sion (Krabbendam and Glas ser, 2011). This can po ten tially ex plain the more in tense exca va tion of the MLSZ sur face by the ad vanc ing ice sheet, and the for ma tion of the Mid dle Lith u a nian trough.…”