LARGE-SCALE ICE-MOLDING - A DISCUSSION OF GENESIS AND GLACIOLOGICAL SIGNIFICANCE

Landsat images reveal a previously unsuspected large-scale pattern of streamlining within drift which comprises a number of components. Drumlins and megaflutes form part of the pattern, but in addition there are two previously undocumented ice-moulded landform elements: streamlined lineations of much greater proportions, referred to as mega-scale glacial lineations, and a distinctive cross-cutting topology within the grain. Consideration of the genesis and glaciological significance of such landforms leads to a number of conclusions. It is suggested that mega-scale glacial lineations were formed under conditions of fast ice flow and their presence may thus record former locations of ice streams or surge events. Many lineations of varying scales display a pattern that reveals that they were not formed in sub-marginal positions. Extensive sets of lineations must have been formed approximately synchronously, thus indicating that lineation generation occurs over a wide range of glaciodynamic conditions, from sub-marginal positions to interior portions of ice sheets. Subglacial deformation of tills has been widely invoked to account for rapid glacier motion. Criteria for identifying such tills are restricted to structures viewed in cross-section, and it is thus hard to assess how widespread the processes of deformation may have been. Lineations provide a surface ''marker'' of structure that records subsequent deformation from other ice-flow events. It is argued that the palaeo-flow record including cross-cutting patterns provides the surface or plan expression of subglacial deformation. The degree of modification to pre-existing lineations should permit the identification of zones favoured by deformation, and thus assist in an assessment of how pervasive deforming bed processes were in influencing ice sheet dynamics.