Abstract
Fast-flowing ice streams occur within modern ice sheets and also operated in Pleistocene ice sheets. The reconstruction of palaeo-ice streams normally relies on the mapping of mega-scale glacial lineations (MSGLs) and drumlins composed of soft sediment, mainly till. Analysis of new satellite imagery and digital terrain models, demonstrates the presence of large fields of kilometre-scale glacial lineations comprising rock drumlins, megagrooves and megaridges. In this paper we describe and analyse a number of such ‘hard-bed’ landform systems from the former Laurentide and British–Irish ice sheets, occurring in a variety of palaeo-ice stream settings. These are attributed to erosion of crystalline and sedimentary rock below fast flowing ice streams. Bedrock properties such as hardness, fracture spacing and bedding and their orientation with respect to ice flow have a profound effect on the occurrence and character of elongate rock bedforms. Elongate streamlined forms on hard crystalline rock, as on the Canadian Shield, only form under special circumstances; in contrast, sedimentary strata are highly susceptible to form streamlined hard beds, specifically if bedrock strike is parallel to ice flow. Large-scale elongate rock bedforms are erosional in origin, formed by preferentially focused abrasion or by lateral plucking, depending on bedrock type. Many palaeo-ice stream footprints previously mapped in the Laurentide Ice Sheet on the basis of soft-bed bedforms are shown to be significantly larger, extending up-ice across sedimentary strata and onto Precambrian crystalline rocks. Hard-bed streamlined forms further show that ice streaming does not necessitate a deformable bed, but can equally occur on smooth hard beds.
Highlights
Introduction and scopeIce streams have flow velocities many times greater than surrounding areas of sluggish or intermediate-velocity ice; they form the arteries of ice sheets and are crucial for regulating the flow dynamics of ice masses (e.g., Bentley, 1987; Joughin et al, 2001; Bennett, 2003; Rignot et al, 2011; Stokes et al, 2015)
Ice streams can be mapped using direct satellite observations of surface ice velocities (e.g., Joughin et al, 2010; Rignot et al, 2011), whereas palaeo-ice streams in Pleistocene ice sheets can be reconstructed by mapping out their geomorphological footprints (Figs. 1, 2), consisting primarily of discrete flow sets of elongate bedforms such as mega-scale glacial lineations (MSGLs) (e.g., Clark, 1994; Patterson, 1997; Stokes and Clark, 2001, 2002, 2003; Everest et al, 2005; Golledge and Stoker, 2006; Hughes et al, 2014; Spagnolo et al, 2014)
We show that such bedrock megalineations are common, and occur in a variety of glaciological settings, ranging from strongly topographically constrained palaeo-ice streams (e.g., Nass River, British Columbia); to palaeo-ice streams that are moderately topographically constrained, commonly in broad breaches (e.g., Tyne Gap, Ullapool; UK); and including palaeo-ice streams in areas where topographic control is much more subtle (e.g., Ontario, Alberta, Saskatchewan)
Summary
Introduction and scopeIce streams have flow velocities many times greater than surrounding areas of sluggish or intermediate-velocity ice; they form the arteries of ice sheets and are crucial for regulating the flow dynamics of ice masses (e.g., Bentley, 1987; Joughin et al, 2001; Bennett, 2003; Rignot et al, 2011; Stokes et al, 2015). 1, 2), consisting primarily of discrete flow sets of elongate bedforms such as mega-scale glacial lineations (MSGLs) (e.g., Clark, 1994; Patterson, 1997; Stokes and Clark, 2001, 2002, 2003; Everest et al, 2005; Golledge and Stoker, 2006; Hughes et al, 2014; Spagnolo et al, 2014). Within the former Laurentide Ice Sheet, for example, at least 100 such palaeo-ice streams have been recognised (Margold et al, 2015). Analysis of hard-bed streamlined forms can aid in reconstructing palaeo-ice streams, assist in understanding the primary controls on ice streaming itself (see discussions in Winsborrow et al, 2010; Livingstone et al, 2012) and has potential implications for the origin of MSGLs in general (Eyles et al, 2016)
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