Modern Arctic glaciers as depositional models for former ice sheets

Sedimentary sequences currently forming at the margins of Spitsbergen glaciers are identical in thickness and detail to many Pleistocene and pre-Pleistocene glacigenic sequences. The transport of considerable volumes of englacial debris leads directly to the predominance of supraglacial till deposition, giving hummocky till surfaces and till plains. The association of supraglacial outwash with flow till produces tripartite till/outwash/till sequences, and multitill sequences, which are the result of a single glacier retreat phase. Complex tectonic structures, often with systematic regional trends are described, which are not the result of ice pushing but of downslope flow and collapse of supraglacial sequences above melting ice. New classifications are suggested for ice-contact stratified deposits and till, both of which depend upon position of deposition, supraglacial, englacial or subglacial It is suggested that existing models for the interpretation of ancient tills and the sequences in which they lie are often too simple and lead to erroneous stratigraphic and palaeogeographic conclusions. Till is too often interpreted solely as lodgement till, and it is suggested that many Pleistocene and earlier sequences, currently thought of as products of repeated glacier advance and readvance, may be perfectly normal products of a single retreat phase by a glacier with a thick englacial debris load. Ways of reconstructing the structural character of ancient ice margins are presented and it is also suggested that the thermal regimes of past ice sheets can be reconstructed from the nature of their deposits. The last Pleistocene ice sheet in Britain is thought to have been composed, at its maximum extent, of cold ice in the marginal zone and temperate ice in the internal zone.