Lateral plucking as a mechanism for elongate erosional glacial bedforms: explaining megagrooves in Britain and Canada

Abstract

Megagrooves are kilometre-scale linear topographic lows carved in bedrock, separated by ridges, typically in areas of largely devoid of till. They have been reported from several areas covered by Pleistocene glaciations, such as Canadian NW Territories, Michigan and NW Scotland. Here we report two previously undocumented megagroove fields from Ungava, Canada, and northern England, and present new analyses of the megagrooves from NW Scotland. This paper seeks to determine the nature of the lithological and structural controls on the occurrence and formation of megagrooves. Analysis of both geomorphological and bedrock properties shows that megagrooves are generally: a) confined to well stratified or layered bedrock, such as (meta)sedimentary rocks with closely spaced joints, and tend not to occur on massive rocks such as gneiss or granite, or thick-bedded sedimentary rocks; b) subparallel to palaeo-ice flow and the strike of the strata; and tend not to occur where palaeo-ice flow is at high angles to the strike of strata; c) produced by significant glacial erosion by sustained unidirectional ice flow.Detailed analysis of megagrooves in NW Scotland shows that neither glacio-fluvial erosion, nor differential abrasion was the dominant mechanism of formation. A mechanism, here termed 'lateral plucking', is suggested that involves block plucking on rock steps parallel to ice flow. Removal of joint-bounded blocks from such rock steps involves a component of rotation along a vertical axis. Block removal may be enhanced by a direct component of shear stress onto the vertical stoss sides. The lateral plucking mechanism results in horizontal erosion at right angles to the ice flow, and enhances the groove/ridge topography. Megagrooves are potentially useful as palaeo-ice flow indicators in areas devoid of till, and can thus complement the palaeo-ice stream datasets which are presently largely based on soft-sediment landform studies.

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

“…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).…”

“…These linear valleys are probably formed by a combination of subglacial abrasion and lateral plucking (see Krabbendam and Bradwell, 2011 for an explanation of lateral plucking) and a repeated sequence of slope collapse and rock falls during interglacials and clearing out of debris during glacial erosion, similar to glacial valley widening (e.g. McColl, 2012) but on a smaller scale.…”

“…Cuesta-type landscapes can be modified by glacial erosion, but the resultant 'hard-bed landform assemblage' has a very different geomorphology, commonly characterized by bedrock megagrooves (e.g. Zumberge, 1954;Krabbendam and Bradwell, 2011;Eyles, 2012 (Mathes, 1930;Dühnforth et al, 2010).…”

Controls on bedrock bedform development beneath the Uummannaq Ice Stream onset zone, West Greenland

“…Previous 110 publications provide a full discussion of the processes that form roches moutonnées and whalebacks 111 (Glasser and Bennett, 2004;Roberts and Long, 2005). 112 113 6 Bedrock structure is well known to exert a direct control on subglacial bedform development, type, 114 and morphology (Gordon, 1981;Rea and Whalley, 1994;Roberts and Long, 2005; Dühnforth et al, 115 2010;Roberts et al, 2010;Krabbendam and Bradwell, 2011;Krabbendam and Glasser, 2011; Hooyer 116 et al, 2012). Variability in bedding plane strike, dip, and thickness can have a strong impact on 117 bedform evolution (Roberts et al, 2010).…”

“…If the chronology is incorrect, the events would 434 be reversed, making little difference to the landform development model below. During NE-SW 435 directed ice flow SSW-and S-facing faces were plucked, with block removal facilitated by E-W and N-436 has been outlined by Krabbendam and Bradwell (2011), termed lateral plucking. In their study, 470 erosion by this process led to the development of a series of negative relief megagroove features, 471 not the positive relief bedforms observed in the present study (Krabbendam and Bradwell, 2011).…”