Water plays a key role in glacial sedimentation. Subglacial and englacial hydraulic flow is through a unique form of karstic conduit network which is kept open trough a delicate balance between hydraulic and cryostatic 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 deposited 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 plains are common in the ice marginal situation, but chaotic and ephemeral conditions are also characteristic and catastrophic flow events are common. Complex sequences of lacustrine, fluvial, debris flow and direct glacial sediments result. Deformation due to collapse of supporting ice walls or buried ice blocks is common. Such is the volatility of the ice marginal environment that much of the sedimentary record is destroyed almost as soon as it is deposited.