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GGR317H5 (1)

Chapter 8 ggr317.docx

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David Passmore

Glaciers terminating on land can deposit sediment in two ways idirectly that is from the ice itself or iiindirectly for example through the action of meltwaterDirect deposition by a glacier is restricted to its immediate vicinity whereas meltwater may carry sediment far beyond the glacier margin Glacial sediments are traditionally divided into those that are nonsorted and formed by direct glacier sedimentation and those that are sorted or stratified and usually deposited by meltwater In practice this is a rather artificial distinction because meltwater deposits are not always stratified In this chapter we describe the products of direct glacier sedimentation before considering deposition from meltwater81 DIRECT GLACIAL SEDIMENTATIONDebris deposited directly by a glacier is known as till A till is defined as sediment deposited by glacier ice but one which has not been disaggregated although it may have suffered glacially induced flow either in the subglacial or supraglacial environment It normally consists of large pebbles cobbles or boulders referred to generally as clasts set within a finegrained matrix of silt and clayDiamictionThere are four primary processes by which debris in transport within a glaciermay be deposited i lodgementthis occurs when the frictional resistance between a clast in transport at the base of a glacier and the glacier bed exceeds the drag imposed by the overlying ice such that the clast ceases to move ii meltoutthe direct release of debris by melting iii sublimationvaporisation of ice causing the direct release of debris and iv subglacial deformationthis involves the assimilation of sediment into a deforming layer beneath a glacier The process of sublimation is currently only documented from Antarctica and occurs there due to the extreme cold and aridity of this environmentTraditionally these process distinctions are used to recognise a range of differenttill types on the basis of their macromorphology namely Table 81 i lodgementtill ii subglacial meltout till iii deformation till iv supraglacial meltout and flow tilland v sublimation tillTill may accumulate in a subglacial setting via a range of processes including idirect lodgement of debris in traction over the glacier bed ii basal melting anddebris release iii deposition in basal cavities and iv deformation and assimilationof overridden sediment812 Supraglacial TillMelting on the surface of a glacier driven by solar radiation releases debris that canproduce supraglacial meltout till Figure 87 This debris may be confined to debris
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