A qualitative view of sub-ice-sheet landscape evolution

Abstract

Qualitative analysis of sub-ice-sheet landscapes is based on processes inferred from landform and sediment. Striation and plucking give information on the dynamic and thermody namic conditions at the ice bed. Lodgement and melt-out tills indicate that basal accretion and freezing conditions were followed by melting during till deposition, under sliding ice in the case of lodgement and stagnant ice in the case of melt-out. Surging is inferred from evidence of low ice- sheet profiles. Melt-out appears to have taken place over large areas in periods of stagnation following surges. Interbedded stratified sediment and melt-out till indicate decoupling of the ice from its bed during relatively minor subglacial drainage events. The bulk of glacial erosion, transport and deposition took place during ice-sheet advance. Over-ridden proglacial sediment, lodgement/deformation and melt-out tills, with subglacial sorted beds, were eroded by cataclysmic meltwater outburst floods that produced erosional and depositional drumlins, flutings, Rogen moraines and vast tracts of scoured bedrock. These floods marked a dramatic change in glacier regime over large areas of ice sheets. Low ice-sheet profiles caused widespread stagnation; glacial erosion and transport came to a halt. Extensive esker systems attest to a regional, integrated subglacial drainage network. Tunnel channels, glaciotectonic landforms and hummocky moraine are other elements of the subglacial landscape related directly or indirectly to meltwater storage and outburst floods. Models of landscape evolution are based on the conclusions derived from the analysis of sediment, form and process.