THE ICE-CONTACT ENVIRONMENT

Water plays a key role in glacial sedimentation. Subglacial and englac ial hydraulic flow is through a unique form of karstic conduit network which is kept open trough a delicate balance between hydraulic and cr yostatic pressures. During periods of low water flow some conduits are kept open by the sediment which had previously been deposited during periods of maximum meltwater production. Esker sediments are deposited in increments sequentially younging towards the source as they are de posited in tunnels relatively close to the ice margin and only survive in situations where the ice margin is receding. Proglacial lakes seem to have triggered sedimentation by dampening flow in feeding conduits in some palaeogeological situations. Coarse grained alluvial braid pl ains are common in the ice marginal situation, but chaotic and ephemer al conditions are also characteristic and catastrophic flow events are common. Complex sequences of lacustrine, fluvial, debris flow and dir ect glacial sediments result. Deformation due to collapse of supportin g ice walls or buried ice blocks is common. Such is the volatility of the ice marginal environment that much of the sedimentary record is de stroyed almost as soon as it is deposited. (C) 1997 Elsevier Science L td.