Subglacial Material Research Articles

Englacial Deltaic Sediments as Evidence for Basal Freezing and Marginal Shearing, Leirbreen, Southern Norway

AbstractA frozen englacial band of current-bedded silts, sands, and gravels were observed close to the snout of the temperate glacier Leirbreen. Little-disturbed sedimentary structures indicated a deltaic origin, the lack of disturbance showing that the sediments were frozen prior to their incorporation by the glacier and did not suffer shearing during glacial transport. It was concluded that they accumulated originally in a subglacial pool when the glacier was more extensive than today. Subsequent thinning of the glacier during recession allowed penetration of winter cooling to the base and freezing of subglacial sediments. The resulting layer of frozen sediment was protected from summer thawing by the overlying glacier and might therefore have been subject to further increase in thickness during successive winters. Incorporation of the frozen sediment band into the glacier resulted from compressive flow against the marginal zone of cold-based ice, leading to the development of thrust planes which penetrated through the glacier and into the frozen subglacial material. Shearing along such a thrust plane immediately below the frozen sediment layer, brought it through the glacier to the surface in a relatively undisturbed state.

Englacial Deltaic Sediments as Evidence for Basal Freezing and Marginal Shearing, Leirbreen, Southern Norway

AbstractA frozen englacial band of current-bedded silts, sands, and gravels were observed close to the snout of the temperate glacier Leirbreen. Little-disturbed sedimentary structures indicated a deltaic origin, the lack of disturbance showing that the sediments were frozen prior to their incorporation by the glacier and did not suffer shearing during glacial transport. It was concluded that they accumulated originally in a subglacial pool when the glacier was more extensive than today. Subsequent thinning of the glacier during recession allowed penetration of winter cooling to the base and freezing of subglacial sediments. The resulting layer of frozen sediment was protected from summer thawing by the overlying glacier and might therefore have been subject to further increase in thickness during successive winters. Incorporation of the frozen sediment band into the glacier resulted from compressive flow against the marginal zone of cold-based ice, leading to the development of thrust planes which penetrated through the glacier and into the frozen subglacial material. Shearing along such a thrust plane immediately below the frozen sediment layer, brought it through the glacier to the surface in a relatively undisturbed state.

Within‐valley asymmetry and related problems of Neoglacial lateral moraine development at certain Jotunheimen glaciers, southern Norway

A difference in the size of Neoglacial lateral moraines on either side of a valley axis (within‐valley asymmetry of lateral moraine development) is described. Analysis of clast roundness has revealed subangular material in latero‐terminal and terminal moraines; lateral moraines, however, exhibit a compositional gradient of increasing angularity with distance from the former glacier snout. Comparisons with clasts of known origin suggest that this ‘roundness gradient’ may be explained with reference to either or both of two hypotheses: (1) a variable proportion of supraglacial (or englacial) to subglacial transported material; and (2) the variable composition of regolith incorporated by a push mechanism from the valley sides. Within‐valley asymmetry is inferred to result where the supply of debris to lateral moraines from these sources is unequal either side of a valley axis. Both interpretations are also consistent with the relatively large size of latero‐terminal sections of end moraines. In order to account for the discrepancy between moraine size and apparent debris supply rates, it is suggested that the largest lateral moraines may have been formed over a longer time scale than the ‘Little Ice Age’, and that reworking of deposits may have occurred. The supply of debris to the north‐facing lateral moraine at Nordre Illåbreen has been so great that it has developed into a rock glacier; this suggests the possibility that subglacial material and valley‐side regolith, as well as supraglacial material, contributes to the formation of ice‐cored rock glaciers.

Freezing Fronts and their Possible Influence upon Processes of Subglacial Erosion and Deposition

Recent studies have shown the important influence of pore-water movement and the subglacial thermal regime on processes of erosion and deposition at the subglacial interface. the influence of migrating freezing fronts within subglacial material has been largely ignored. the phenomena of ice-water interface processes will be examined and their relevance to subglacial processes illustrated. four case studies are presented that deal with the various effects that freezing-front movement may have on pore water, consolidation, shear strength, and likely diagenetic characteristics of subglacial deposits. the influence upon erosional and depositional processes will be outlined, and a mechanism related to potential surge-like conditions within the subglacial zone postulated.

˝Stagnant-Ice˝ Topography and its Relation to Drumlin Genesis, with Reference to South-Central Ulster (Abstract Only)

Previous work on drumlin genesis considers the problem at the ice-rock interface and ignores the environmental significance of inter-drumlin sediments and superimposed landform associations. Present work by the author examines two drumlin fields in south-central Ulster, Northern Ireland, and proposes a mechanism of drumlin formation which is time-dependent and results from changes of thermal regime through time.Drumlin exposures in south-central Ulster reveal a particular facies association in drumlin tills showing considerable lithological variation; striae, roches moutonnées, and basal lodgement till beneath drumlins indicate an event early in the last glaciation characterized by a temperate thermal regime. This association is overlain by a basal melt-out till, characterized by striated clasts and containing numerous pockets of stratified sediment. The melt-out till is related to changes in provenance of erratics in the till, associated with a shift in the centre of ice accumulation during the course of glaciation.The stratigraphy suggests that a considerable amount of drift was formed prior to drumlin formation and that a change of thermal state may have characterized the shift in the ice accumulation zone. This change is also suggested by evidence of high-level melt-water transport in the Sperrin Mountains, indicating a sub-polar or sub-temperate thermal regime existing in the early part of the late Weichselian deglaciation.A model based on thermal convection theory of polar ice sheets (Hughes 1976) is proposed in an attempt to relate the juxtaposition of drumlins and glaciofluvial assemblages in southcentral Ulster. In an ice accumulation zone, as characterized this area during the late Weichselian glacial maximum, advection is generally absent and convection is the dominant flow in the ice sheet. In this environment motive ice may develop in the basal zone, resulting from vertical buoyancy stresses acting downward on a column in the ice sheet. This flow environment may coexist within an ice sheet which is "stagnant" and perhaps subject to supraglacial insolation melting, depending on the overall climatic regime. If convection is dominant, it may create areas of basal compression and tension, acting to establish co-existing zones of "hot" and "cold" ice; hot-ice zones are subject to basal erosion, leading to migration of subglacial sediments to points of englacial storage above the density inversion layer. Cold-ice zones act to protect underlying sediments and result in isolation of drift nuclei, protection being afforded by the yield strength of frozen drift being greater than either debris-laden or clean ice.If the model is tenable, it seems likely that drift nucleation in drumlin fields may have been accomplished before the drumlin formation or streamlining event. Vertical migration of subglacial materials also explains the similarity of sediment sources of glacigenic and glaciofluvial assemblages which occur in juxtaposition, and, without reduction in drumlin density. The sequence of events in south-central Ulster may be summarized as follows: (1)Build-up of ice, associated with sub-drumlin erosion forms and basal lodgement till. Change of thermal regime from temperate to sub-temperate.(2)Shift in the ice-accumulation zone from highland to lowland, indicated by erratic dispersals of glaciogenic sediments. Change of thermal regime from sub-polar to polar at the late Weichselian glacial maximum.(3)High-level insolation melting with concomitant basal freezing. Initiation of polygonal array in basal zone. Change of thermal regime from polar to sub-polar or sub-temperate.(4)Down-wastage of ice and nunatak formation. Formation of high-level melt-water phenomena. Deposition of basal melt-out till. Further development of basal polygonal array. Till nuclei established and englacial sediment isolated. Change of thermal regime from sub-polar to sub-temperate.(5)Change of thermal state to temperate. Uncoupling of ice from bedrock. Streamlining of till nuclei and reworking of englacial sediment by melt water.(6)Final disintegration of ice.

Processes of Glacier Erosion on Different Substrata

AbstractMost theories of glacier movement and subglacial erosion have assumed that glaciers rest on rigid bedrock surfaces. Whilst this is probably correct for much of the bed area of most modern glaciers, deformable sediments do occur beneath them and formed a substantial area of the beds of large ice sheets during glacial periods. Observations and theories are presented and reviewed about the processes of glacier erosion of rock and unlithified sediment beds both when they are frozen and unfrozen.Erosional bedrock landforms, such asroches moutonnées, indicate two principal subglacial erosional processes, plucking and abrasion. Where supraglacially derived debris is unimportant, plucking provides the tools which abrade the bed, and must be a quantitatively more important process than abrasion, though more localized. Where plucking is suppressed, erosion rates must be slow. Subglacial measurements of abrasion rates beneath a temperate glacier are used to test an earlier abrasional theory (Boulton, [C1974]). The form of the predicted abrasion-rate curve for changing ice velocity and pressure is verified. This theory successfully simulates two-dimensional erosional bedforms. Subglacial observations demonstrate how flow basal ice around the flanks of bedrock obstacles causes streaming of debris to occur. It is suggested that this streaming process is primarily responsible for the longitudinally lineated form of large-scale surfaces typical of glacially eroded bedrock.Plucking and abrasion also occur beneath cold ice, though at slow rates, and are probably restricted to places where the ice thickness is small.Where the glacier bed is composed of unlithified sediment, subglacial measurements show that deformation can produce very large discharges of subglacial material, which makes this a potential agent of very rapid subglacial landform production. The heterogeneity of subglacial sediment leads to spatially variable rates of deformation, and it is suggested that relatively stronger parts of the sediment body may form the nuclei for drumlin and mega-flute formation.Whereas unlithified unfrozen sediment deforms beneath the glacier rather than being incorporated within it, ice-cemented subglacial sediments can behave like bedrock, because of their relative rigidity, and are readily plucked and incorporated englacially. They may also deform beneath the glacier.

Processes of Glacier Erosion on Different Substrata

AbstractMost theories of glacier movement and subglacial erosion have assumed that glaciers rest on rigid bedrock surfaces. Whilst this is probably correct for much of the bed area of most modern glaciers, deformable sediments do occur beneath them and formed a substantial area of the beds of large ice sheets during glacial periods. Observations and theories are presented and reviewed about the processes of glacier erosion of rock and unlithified sediment beds both when they are frozen and unfrozen.Erosional bedrock landforms, such as roches moutonnées, indicate two principal subglacial erosional processes, plucking and abrasion. Where supraglacially derived debris is unimportant, plucking provides the tools which abrade the bed, and must be a quantitatively more important process than abrasion, though more localized. Where plucking is suppressed, erosion rates must be slow. Subglacial measurements of abrasion rates beneath a temperate glacier are used to test an earlier abrasional theory (Boulton, [C1974]). The form of the predicted abrasion-rate curve for changing ice velocity and pressure is verified. This theory successfully simulates two-dimensional erosional bedforms. Subglacial observations demonstrate how flow basal ice around the flanks of bedrock obstacles causes streaming of debris to occur. It is suggested that this streaming process is primarily responsible for the longitudinally lineated form of large-scale surfaces typical of glacially eroded bedrock.Plucking and abrasion also occur beneath cold ice, though at slow rates, and are probably restricted to places where the ice thickness is small.Where the glacier bed is composed of unlithified sediment, subglacial measurements show that deformation can produce very large discharges of subglacial material, which makes this a potential agent of very rapid subglacial landform production. The heterogeneity of subglacial sediment leads to spatially variable rates of deformation, and it is suggested that relatively stronger parts of the sediment body may form the nuclei for drumlin and mega-flute formation.Whereas unlithified unfrozen sediment deforms beneath the glacier rather than being incorporated within it, ice-cemented subglacial sediments can behave like bedrock, because of their relative rigidity, and are readily plucked and incorporated englacially. They may also deform beneath the glacier.

The Origin of Glacially Fluted Surfaces-Observations and Theory

Abstract Studies of fluted surfaces beyond the margins of glaciers in Spitsbergen, Iceland, Norway and the Alps show that almost all emanate from rigid obstructions, commonly boulders in till. Field relations of flutes are described and it is shown that a relationship exists between flute height and the height of the initiating obstruction. Subglacial observations indicate that flutes form when till is intruded into tunnels which tend to open up in the lee of obstacles. The pattern of strain implied by this process is shown to be reflected by micro- and macrofabrics in the till. The commonly found occurrence of an average spacing between flutes does not arise because of some rhythmic or periodic mechanism in the glacier, but is produced by the random seeding of boulders which themselves generate flutes. It is suggested that the term flute be used as a genetic rather than a descriptive term, and be restricted to long parallel-sided ridges which reflect accurately the direction of ice movement and which form when deformable subglacial materials are intruded into tunnels which tend to open up on the lee sides of single rigid obstructions on the glacier bed.

The Origin of Glacially Fluted Surfaces-Observations and Theory

AbstractStudies of fluted surfaces beyond the margins of glaciers in Spitsbergen, Iceland, Norway and the Alps show that almost all emanate from rigid obstructions, commonly boulders in till. Field relations of flutes are described and it is shown that a relationship exists between flute height and the height of the initiating obstruction. Subglacial observations indicate that flutes form when till is intruded into tunnels which tend to open up in the lee of obstacles. The pattern of strain implied by this process is shown to be reflected by micro- and macrofabrics in the till. The commonly found occurrence of an average spacing between flutes does not arise because of some rhythmic or periodic mechanism in the glacier, but is produced by the random seeding of boulders which themselves generate flutes.It is suggested that the term flute be used as a genetic rather than a descriptive term, and be restricted to long parallel-sided ridges which reflect accurately the direction of ice movement and which form when deformable subglacial materials are intruded into tunnels which tend to open up on the lee sides of single rigid obstructions on the glacier bed.

A scanning electron microscope study of surface textures of quartz grains from glacial environments

ABSTRACTSurface textures of quartz sand grains from several glacial environments at the Feegletscher, Switzerland examined by means of scanning electron microscopy are described. The difference between supraglacial and subglacial material is very slight. At the moment the interpretation of these textures must be done with care until the full statistical relation to environments can be ascertained. This does not necessarily invalidate earlier investigations although it does mean that glacial and extraglacial environments in the vicinity of glaciers are apparently not distinguishable. Examination of surface precipitation features suggests a sequence of events which can be used to help discriminate between different ages of deposits in an area. A similarity between some of the surface debris from moraine samples and those seen in loess and quickclay deposits is also suggested.

Glacial Studies in Canada

DR. WILLIAM H. SHERZER has published in the Smithsonian Miscellaneous Collections (pp. 453–496) a handsomely illustrated preliminary report entitled “Glacial Studies in the Canadian Rockies and Selkirks.” The five glaciers selected are conveniently located in Alberta and British Columbia, and the line of the Canadian Pacific Railway passes near them. Observations have been made on the rate of motion of the Victoria Glacier, which is as low as about 52 feet a year, and on the lowering of its surface by ablation. The front of this glacier shows a shearing movement of one layer over another, as was tested by the pushing forward of iron spikes driven into an upper and a lower stratum. The right lateral moraine receives a certain amount of ground-moraine or sub-glacial material from a hanging glacier on Mount Lefroy, which breaks away in avalanches on to the main Victoria flow. This incident, which is well illustrated, serves to warn us from assuming that all sub-glacial material at a glacier's edge results from plucking action on the wall or floor in contact with the local ice.