subglacial deformation associated with fast ice flow from the columbia glacier alaska abstract the columbia glacier is a large well studied tidewater glacier which has one of the highest non surging modern glacier flow rates previous studies from ice drilling have tentatively suggested the presence of a deforming bed this study examined the recently deglaciated since 1974 foreland and concluded there were features indicative of deforming bed conditions i e streamlined subglacial bedforms squeeze type push moraines and crevasse diapirs it was also argued that the elongation ratio and height of streamlined subglacial bedforms can be used as a proxy for shear strain and thickness of the deforming layer respectively and when these two parameters are combined together they produce a complex but predictable eigenvalue pattern in the tills comprising the landforms

the minch palaeo ice stream nw sector of the british irish ice sheet geophysical data from the uk continental shelf off nw scotland reveal strongly parallel subglacial bedforms on the sea bed field mapping on the adjacent landmass has identified closely spaced bedrock megagrooves and highly elongate drumlinoid forms all these mega scale glacial lineations probably formed beneath the same fast flowing zone of the last british irish ice sheet the minch palaeo ice stream this ice stream drained the nw sector of the british irish ice sheet during the late devensian glaciation marine isotope stage 2 and terminated near the edge of the continental shelf the size of the adjacent trough mouth sula sgeir fan and the presence of buried mega scale glacial lineations within the quaternary stratigraphic record imply that fast ice sheet flow in the minch has been a feature of several mid to late pleistocene glaciations

identifying palaeo ice stream tributaries on hard beds mapping glacial bedforms and erosion zones in nw scotland ice streams are fed by tributaries that can extend deep into the heart of ice sheets these tributaries are born at onset zones the abrupt transitions from slow sheet flow to fast streaming flow that often occur at significant topographic steps on hard beds bedrock dominated beds for this reason tributary onset zones leave only a subtle erosional geomorphic signature in the landscape record that is rarely studied this paper examines in detail the geomorphic signature of ice sheet flow on a hard bed at the head of a palaeo ice stream we use field survey techniques to map glacial bedforms within an 200 km2 area of hard crystalline bedrock in a landscape of areal scour around loch laxford in nw scotland the bedrock bedforms range from plastically moulded p forms and wholly abraded forms to stoss lee forms and plucked surfaces all on an outcrop scale 1 100 m we devise a five zone classification system to map in a gis the presence absence and abundance of glacial erosional forms within 619 500 m square grid cells we go on to use these erosional bedform zones along with known glaciological relationships to interpret the spatial and altitudinal pattern of palaeo ice sheet processes and glacier dynamics in this part of nw scotland our interpretation highlights the strong vertical thermal zonation on mountains and the spatial variations in ice rheology softness ice temperature and by inference ice velocity in troughs intimately associated with the onset of ice streaming in tributaries consequently we define the laxfjord palaeo ice stream tributary a feeder to the minch palaeo ice stream in nw scotland finally we suggest that this new mapping approach could be performed in other deglaciated hard bed terrain to examine more widely the subtle erosional signatures preserved in areas traditionally thought to represent ice sheet areal scour

the seafloor geomorphology of the windmill islands wilkes land east antarctica evidence of law dome ice margin dynamics abstract a high resolution multibeam sonar dataset covering an area of ca 33 km 2 was collected in the vicinity of the windmill islands 67 s 110 e wilkes land east antarctica the new data permit visualisation of the nearshore seafloor morphology in unprecedented detail providing invaluable insight into the ice sheet history of the region a range of geomorphic features are evident including prominent parallel northwest trending linear fault sets affecting mesoproterozoic metamorphic basement which appear to control the regional coastal physiography the fault systems probably formed during fragmentation of eastern gondwana during the mesozoic networks of sub glacial meltwater channels preserved on bedrock platforms and ridges indicate grounding of a thick ice sheet over the continental shelf during previous glaciations west trending subtle glacial lineations and streamlined landforms record evidence of the westward expansion of the grounded law dome ice sheet margin probably during the late pleistocene the direction of these features coincides with glacial striae on onshore crystalline bedrock outcrops perhaps the most striking glacial geomorphological features are sets of arcuate ridges confined mostly within glacially excavated u shaped troughs formed by erosion of the northwest trending bedrock fault sets these ridge sets are interpreted as push moraines or grounding zone features formed during episodic retreat of highly channelised topographically controlled ice streams following ice surging of the law dome margin this event was possibly triggered in response to local environmental forcing during the mid late holocene minor post glacial marine sedimentation is preserved in several small 1 km 2 isolated basins with shallow seaward sills

rock slope failure in the british mountains abstract in this first full review of extant quaternary rock slope failure rsf in the british mountains we provide a near complete inventory of 1082 sites 40 being rock slope deformations 40 arrested rockslides and 20 rock avalanches current rsf activity is negligible and this relict population is predominantly paraglacial with a parafluvial minority its spatial distribution is perplexing with rsf density varying greatly both regionally and locally in the scottish highlands eight main clusters account for 76 of rsf area in 15 of the montane area local concentrations occur in all the british ranges across high and low relief in core and peripheral locations and on varied geological and glaciological domains as conversely do extensive areas of sparsity even in similar lithologies generic interpretations are thus precluded geology is only a secondary control an association with concentrated erosion of bedrock ceb is proposed as a driver of intensified slope stresses ceb is most evident at those glacial breaches of main divides where the most vigorous recent incision is inferred and also in some trough heads a clear association between rsfs and these late developing breaches is demonstrated in the highlands in 42 localities with sparsity away from them it is also seen in seven lake district localities glaciological models identify ice sheet volatility capable of driving breach ramification high magnitude paleoseismic events are generally unlikely to have provoked rsf clusters a few candidates are considered rsf has been underrated as an agent of mountain landscape evolution in britain its spatio temporal incidence may assist in calibrating regional ice sheet models and in assessing climate change impacts we argue that the ceb rsf association has global relevance in identifying primary drivers of mass movement in bedrock

ploughing boulders along shorelines with particular reference to the st lawrence estuary abstract ploughing boulders are defined as large stones moved by drift ice with a ploughing action into the substrate leaving a linear depression on the back and building a frontal push ridge they are commonly found on unconsolidated tidal flats and on rock shore platforms along cold region shorelines in the marine lacustrine and fluvial environments it is an erosional process of great significance particularly for consolidated substrates a total of 176 ploughing boulders were measured in eighteen localities along the st lawrence estuary the largest boulder weighed 176 tons and was moved 3 m while a 50 t boulder was moved 40 m on shore platforms cut in shales one boulder 49 t has scoured a depression 19 m long and up to 40 cm deep and built a frontal ridge 40 cm high along the st lawrence estuary about 95 of ploughing boulders were moved seaward by ice floes entrained by ebb currents

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

ice caps existed throughout the lateglacial interstadial in northern scotland we constrain in detail fluctuations of two former ice caps in nw scotland with multibeam seabed surveys geomorphological mapping and cosmogenic 10 be isotope analyses we map a continuous sequence of 40 recessional moraines stretching from 10 km offshore to the wester ross mountains surface exposure ages from boulders on moraine ridges in assynt and the summer isles region show that substantial dynamic ice caps existed in nw scotland between 13 and 14 ka bp we interpret this as strong evidence that large active glaciers probably survived throughout the lateglacial interstadial and that during the older dryas period ca 14 ka bp ice caps in nw scotland were thicker and considerably more extensive than in the subsequent younger dryas stadial by inference we suggest that lateglacial ice cap oscillations in scotland reflect the complex interplay between changing temperature and precipitation regimes during this climatically unstable period ca 15 11 ka bp natural environment research council nerc copyright 2008 reproduced with the permission of nerc published by john wiley sons ltd

bedform signature of a west antarctic palaeo ice stream reveals a multi temporal record of flow and substrate control abstract the presence of a complex bedform arrangement on the sea floor of the continental shelf in the western amundsen sea embayment west antarctica indicates a multi temporal record of flow related to the activity of one or more ice streams in the past mapping and division of the bedforms into distinct landform assemblages reveals their time transgressive history which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo ice velocity as suggested previously a main control on the bedform imprint is the geology of the shelf which is divided broadly between rough bedrock on the inner shelf and smooth dipping sedimentary strata on the middle to outer shelf inner shelf bedform variability is well preserved revealing information about local complex basal ice conditions meltwater flow and ice dynamics over time these details which are not apparent at the scale of regional morphological studies indicate that past ice streams flowed across the entire shelf at times and often had onset zones that lay within the interior of the antarctic ice sheet today in contrast highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow and may be a truer representation of fast palaeo ice flow in these locations a revised model for ice streams on the shelf captures complicated multi temporal bedform patterns associated with an antarctic palaeo ice stream for the first time and confirms a strong substrate control on a major ice stream system that drained the west antarctic ice sheet during the late quaternary

iceberg discharges into the north atlantic on millennial time scales during the last glaciation high resolution studies of north atlantic deep sea cores demonstrate that prominent increases in iceberg calving recurred at intervals of 2000 to 3000 years much more frequently than the 7000 to 10 000 year pacing of massive ice discharges associated with heinrich events the calving cycles correlate with warm cold oscillations called dansgaard oeschger events in greenland ice cores each cycle records synchronous discharges of ice from different sources and the cycles are decoupled from sea surface temperatures these findings point to a mechanism operating within the atmosphere that caused rapid oscillations in air temperatures above greenland and in calving from more than one ice sheet

late quaternary glacial history flow dynamics and sedimentation along the eastern margin of the antarctic peninsula ice sheet geological and geophysical data from the ne antarctic peninsula continental shelf are used to reconstruct the glacial history flow dynamics and sedimentation of the antarctic peninsula ice sheet apis along its eastern margin during the late quaternary ice advanced to the shelf edge during the last glacial maximum lgm and deposited a stiff till across the shelf the presence of highly attenuated bedforms indicates that fast flowing outlets drained the apis through cross shelf troughs to the outer shelf after this ice advance the bedforms are formed in deformation till in response to deforming bed processes deglaciation of robertson trough and the troughs of prince gustav channel larsen a and larsen inlet was continuous and possibly rapid on account of the absence of ice margin recessional features in contrast grounding zone wedges across the shelf of northern larsen a and south of prince gustav channel indicate that ice retreat was gradual and was punctuated by stillstands the transition from a grounded ice sheet to ice shelf conditions was completed before 11 12 14c ka bp on the shelf south of prince gustavchannel and is marked across the shelf by a change from subglacial till to a transitional heterogeneous unit dominated by coarse grained facies transitional sediments record mainly sub ice shelf rain out and restricted bottom current and sediment laden plume activity as well as localised debris flows meltwater derived facies are largely absent indicating that release of meltwater was not significant beneath polar ice shelves or during deglaciation of the apis in the broader context the colder eastern side of the apis was extensive and was drained mainly through fast flowing outlets palaeo ice streams therefore the eastern apis would have been an important contributor to sediment and iceberg flux to the weddell sea embayment during the lgm and subsequent deglaciation

tributaries of west antarctic ice streams revealed by radarsat interferometry interferometric radarsat data are used to map ice motion in the source areas of four west antarctic ice streams the data reveal that tributaries coincident with subglacial valleys provide a spatially extensive transition between slow inland flow and rapid ice stream flow and that adjacent ice streams draw from shared source regions two tributaries flow into the stagnant ice stream c creating an extensive region that is thickening at an average rate of 0 49 meters per year this is one of the largest rates of thickening ever reported in antarctica

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

basal processes beneath an arctic glacier and their geomorphic imprint after a surge elisebreen svalbard the foreground of elisebreen a retreating valley glacier in west svalbard exhibits a well preserved assemblage of subglacial landforms including ice flow parallel ridges flutings ice flow oblique ridges crevasse fill features and meandering ridges infill of basal meltwater conduits other landforms are thrust block moraine hummocky terrain and drumlinoid hills we argue in agreement with geomorphological models that this landform assemblage was generated by ice flow instability possibly a surge which took place in the past when the ice was thicker and the bed warmer the surge likely occurred due to elevated pore water pressure in a thin layer of thawed and water saturated till that separated glacier ice from a frozen substratum termination may have been caused by a combination of water drainage and loss of lubricating sediment sedimentological investigations indicate that key landforms may be formed by weak till oozing into basal cavities and crevasses opening in response to accelerated ice flow and into water conduits abandoned during rearrangement of the basal water system today elisebreen may no longer have surge potential due to its diminished size the ability to identify ice flow instability from geomorphological criteria is important in deglaciated terrain as well as in regions where ice dynamics are adapting to climate change

a groove ploughing theory for the production of mega scale glacial lineations and implications for ice stream mechanics mega scale glacial lineations msgls are longitudinally aligned corrugations ridge groove structures 6 100 km long in sediment produced subglacially they are indicators of fast flow and a common signature of ice stream beds we develop a qualitative theory that accounts for their formation and use numerical modelling and observations of ice stream beds to provide supporting evidence ice in contact with a rough scale of 10 10 3 m bedrock surface will mimic the form of the bed because of flow acceleration and convergence in ice stream onset zones the ice base roughness elements experience transverse strain transforming them from irregular bumps into longitudinally aligned keels of ice protruding downwards where such keels slide across a soft sedimentary bed they plough through the sediments carving elongate grooves and deforming material up into intervening ridges this explains msgls and has important implications for ice stream mechanics groove ploughing provides the means to acquire new lubricating sediment and to transport large volumes of it downstream keels may provide basal drag in the force budget of ice streams thereby playing a role in flow regulation and stability we speculate that groove ploughing permits significant ice stream widening thus facilitating high magnitude ice discharge

late glacial to holocene relative sea level change in assynt northwest scotland uk relative sea level change rsl from the late glacial through to the late holocene is reconstructed for the assynt region northwest scotland based on bio and lithostratigraphical analysis four new radiocarbon dated sea level index points help constrain rsl change for the late glacial to the late holocene these new data in addition to published material capture the rsl fall during the late glacial and the rise and fall associated with the mid holocene highstand two of these index points constrain the late glacial rsl history in assynt for the first time reconstructing rsl falling from 2 47 0 59 m od to 0 15 0 59 m od at c 14 000 15 000 cal yr bp these new data test model predictions of glacial isostatic adjustment gia particularly during the early deglacial period which is currently poorly constrained throughout the british isles whilst the empirical data from the mid to late holocene to present matches quite well with the recent gia model output there is a relatively poor fit between the timing of the late glacial rsl fall and early holocene rsl rise this mismatch also evident elsewhere in northwest scotland may result from uncertainties associated with both the global and local ice components of gia models

palaeo ice streams abstract the location and behaviour of ice streams is one of the most important controls on ice sheet configuration and stability in order to reconstruct former ice sheets we need to know ice stream location and timing once identified the beds of palaeo ice streams provide an unprecedented opportunity to glean information about their basal environment something that remains very difficult under contemporary ice streams this paper represents the first synthesis and discussion of palaeo ice stream research from a variety of former ice sheets and includes new insights relating their configuration and activity to the evidence they leave behind the ambiguous use of the term ice stream is addressed and four possible configurations for ice streams are presented diagnostic landforms and landform assemblages that we would expect an ice stream to produce are discussed and the temporal context of bedform generation is described as either a rubber stamped or smudged bedform imprint resulting from isochronous or time transgressive landform generation in particular we focus on the configuration of terrestrially terminating ice streams for which there are no modern analogues technological advances in marine geophysics are helping to generate a convergence of interest between antarctic glaciology and palaeo glaciology it is suggested that investigations around the fringes of contemporary ice sheets i e west antarctica can provide evidence that directly links the geomorphological record of palaeo ice streams with their contemporary counterparts in addition computational advances and modelling adaptations have permitted the incorporation of ice streams within ice sheet models we argue that data from palaeo ice stream beds are invaluable to ice sheet stream modelling experiments and will help us understand ice stream operation and the linkages between climate perturbations and both palaeo and contemporary ice sheets

quaternary glaciations of northern europe this paper presents the outcome of a workshop held in berlin in february 2009 concerned with current research on the glacial history of northern europe including the british isles the methodologies presently used to resolve this topic are outlined particular attention is given to new analytical methods deriving from high resolution remote imaging of glacial terrain both on land and on the sea bed key new stratigraphic sections higher resolution results from conventional geochronological methods like radiocarbon and more recently developed technologies such as luminescence and cosmogenic radionuclide dating the relationships between the results derived from these two methods are discussed in further detail along with possible explanations for these differences an outline of a most likely glacial history of the scandinavian and british and irish ice sheets is presented along with possible links to global climate change as represented by the marine isotope mis record tentative evidence for glaciation is identified in mis 22 16 12 10 8 6 4 and 2 and correlations of phases within the last glaciation are also explored for both the scandinavian and british and irish ice sheets the results show that the character and extent of glaciation in different parts of the region are not synchronous and much more geochronological work is required before regional correlations can be established with confidence

glacial erosion and bedrock properties in nw scotland abrasion and plucking hardness and joint spacing subglacial erosion beneath glaciers occurs predominantly by abrasion and plucking producing distinct erosional forms the controls on the relative importance of abrasion vs plucking are poorly understood on the one hand glacial conditions that favour or suppress cavity formation ice velocity ice thickness and water pressure are thought to favour plucking or abrasion respectively conversely bedrock properties are also known to control landforms but this has rarely been analysed quantitatively in this study we compare landforms and bedrock properties of sandstone and quartzite at the bed of a palaeo ice stream near ullapool in nw scotland the boundary between the rock types is at right angles to the westward palaeo ice flow and palaeoglacial conditions on both rock types were similar we report quantitative parameters for bedrock properties schmidt hammer hardness and joint spacing and use morphometric parameters to analyse the landforms torridon sandstone is soft but thick bedded and with a wide joint spacing erosional bedforms include roche moutonnees with smoothed tops and concave stoss sides whalebacks and elongate p forms indicating a high proportion of abrasion over plucking cambrian quartzite is hard but thin bedded with narrow joint spacing erosional landforms are angular to subangular with abundant plucked lee faces suggesting a high proportion of plucking over abrasion hardness and joint spacing thus exert a strong control on subglacial erosional landforms and the mechanisms that formed them thus glacial conditions ice velocity ice thickness can only be inferred from glacial erosional landforms if the effects of bedrock properties of the substrate are considered

bedrock mega grooves in glaciated terrain a review abstract bedrock mega grooves are assemblages of straight and parallel troughs eroded in bedrock typically over 1000 m in length most sites occur within the limits of the last glacial maximum both on and off shore in this paper we review the current understanding of these important yet enigmatic landforms and propose a framework for their future research mega grooves are important to our understanding of ice sheet dynamics ice bedrock interactions and bedrock landscape evolution in glaciated areas the overall straightness of mega grooves across the landscape their parallel alignment to palaeo ice flow direction and occurrence below the general land surface level has led to their unanimous interpretation as landforms of subglacial erosion scenarios proposed for mega groove formation focus on either glacier ice or subglacial meltwater as the principal agent of erosion yet none offers a comprehensive explanation at locations where mega grooves occur along lines of structural geology their location formation and morphology were largely controlled by the bedrock characteristics where no underlying structural control is apparent mega grooves were likely initiated through glacial abrasion and subsequently modified through a range of erosional processes potentially involving multiple morphogenetic agencies and feedbacks operating between bedrock topography and basal ice flow in the absence of absolute dates morphostratigraphic analyses suggest mega groove survival through multiple glacial cycles no specific ice flow characteristics have been identified as a condition for bedrock grooving but it has been suggested that some bedrock mega grooves are related to ice streaming which deserves further study an initial analysis of bedrock grooves with seemingly similar morphology at a range of scales hints at a bedrock groove landform size continuum which could be a useful framework for exploring process landform relationships future research could usefully focus on quantitative analysis of mega groove morphology augmented with detailed field analysis of landform relationships to bedrock structure and lithology and thereby potentially provide further insight into the age and glaciological significance of these landforms

the use of nextmap britain for geological surveying in the vale of york the nextmap britain digital elevation model dem has opened many new opportunities that considerably help and enhance the way we undertake our geological mapping of bedrock structure superficial and artificial deposits the dataset has been successfully integrated into the digital and conventional mapping workflows of the vale of york mapping team a variety of visualisation and analysis techniques have been applied throughout the mapping process these techniques include an initial appraisal of nextmap with a comparison to existing geological mapping to define the field mapping strategy and site specific manipulation using tablet pcs nextmap interpretation has made an important contribution to the understanding of the extensive glacial and pro glacial deposits found in the vale of york such as sand bodies resting on lake deposits and identifying details within morainic and alluvial complexes for bedrock mapping nextmap has been used to identify landform features that relate to the underlying geology such as breaks in slope the extent of escarpments hillcrests and dip slopes provide an overview of the landscape and save time mapping out features in the field techniques have also been developed to automatically generate these landform features the dataset has also been used to identify areas where landsliding has occurred for accurate mapping of artificial ground and as a key surface for 3d geological models

bedrock megagrooves in assynt nw scotland geological mapping in assynt nw scotland has identified three areas of large scale linear bedrock grooving these closely spaced subparallel megagrooves occur predominantly on cambrian quartzite are not structurally controlled and trend generally in the direction of palaeo ice flow on the basis of geomorphological evidence most notably erosional relationships and s forms it is proposed that the megagrooves formed at the base of the last main late devensian ice sheet and are chiefly the result of high energy subglacial meltwater erosion the unusual concentration of bedrock cut nye channels in assynt indicates the presence of large volumes of meltwater beneath the last ice sheet in this region of nw scotland this high discharge of meltwater was probably accompanied by localized fast ice sheet flow akin to present day ice streams it is suggested that bedrock megagrooves may be the geomorphological signature of ice stream onset zones

exploring the ingredients required to successfully model the placement generation and evolution of ice streams in the british irish ice sheet abstract ice stream evolution is a major uncertainty in projections of the future of the greenland and antarctic ice sheets accurate simulation of ice stream evolution requires an understanding of a number of ingredients that control the location and behaviour of ice stream flow here we test the influence of geothermal heat flux grid resolution and bed hydrology on simulated ice streaming the palaeo record provides snapshots of ice stream evolution with a particularly well constrained ice sheet being the british irish ice sheet biis we implement a new basal sliding scheme coupled with thermo mechanics into the bisicles ice sheet model to simulate the evolution of the biis ice streams we find that the simulated location and spacing of ice streams matches well with the empirical reconstructions of ice stream flow in terms of position and direction when simple bed hydrology is included we show that the new basal sliding scheme allows the accurate simulation for the majority of biis ice streams the extensive empirical record of the biis has allowed the testing of model inputs and has helped demonstrate the skill of the ice sheet model in simulating the evolution of the location spacing and migration of ice streams through millennia simulated ice streams also prompt new empirical mapping of features indicative of streaming in the north channel region ice sheet model development has allowed accurate simulation of the palaeo record and allows for improved modelling of future ice stream behaviour

large scale ice moulding a discussion of genesis and glaciological significance abstract landsat images reveal a previously unsuspected large scale pattern of streamlining within drift which comprises a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain consideration of the genesis and glaciological significance of such landforms leads to a number of conclusions it is suggested that mega scale glacial lineations were formed under conditions of fast ice flow and their presence may thus record former locations of ice streams or surge events many lineations of varying scales display a pattern that reveals that they were not formed in sub marginal positions extensive sets of lineations must have been formed approximately synchronously thus indicating that lineation generation occurs over a wide range of glaciodynamic conditions from sub marginal positions to interior portions of ice sheets subglacial deformation of tills has been widely invoked to account for rapid glacier motion criteria for identifying such tills are restricted to structures viewed in cross section and it is thus hard to assess how widespread the processes of deformation may have been lineations provide a surface marker of structure that records subsequent deformation from other ice flow events it is argued that the palaeo flow record including cross cutting patterns provides the surface or plan expression of subglacial deformation the degree of modification to pre existing lineations should permit the identification of zones favoured by deformation and thus assist in an assessment of how pervasive deforming bed processes were in influencing ice sheet dynamics

evolution of subglacial bedforms along a paleo ice stream antarctic peninsula continental shelf geophysical data from the antarctic peninsula continental shelf reveal streamlined subglacial bedforms in a cross shelf trough bedforms exhibit progressive elongation with distance along the trough and record flow of a paleo ice stream from the antarctic peninsula ice sheet during the last glacial maximum downflow evolution of the bedforms indicates increasing flow velocities as the ice stream traversed the shelf this in turn is related to a transition from crystalline bedrock on the inner shelf to a soft sedimentary substrate on the outer shelf although streaming flow operated across both substrates the highest flow velocities occurred over the soft bed spatial variation in the inferred nature of fast flow from sliding to subglacial sediment deformation and or ploughing was also lithologically controlled these data highlight the control of subglacial geology on ice stream dynamics in the geological record and demonstrate a direct relationship between the formation of streamlined subglacial bedforms and paleo ice streams

glacial terrain zone analysis of a fragmented paleoglaciologic record southeast keewatin sector of the laurentide ice sheet abstract a highly fragmented subglacial landscape is recognized at the regional to sub regional scales in northeastern manitoba canada in the southeast keewatin sector a core region of the laurentide ice sheet new field based ice flow indicator measurements mapping of subglacial landforms remote sensing and aerial photograph and a re examination of previously published data from an 8100 km2 area in northeastern manitoba show that the preserved subglacial record reflects a complex and potentially long glacial history five streamlined landform flowsets are mapped a much higher degree of inheritance in the field based ice flow indicator data than previously reported allows for recognition of multiple ice flow phases analysis of the characteristics of the subglacial landscape combined with a relative age chronology established with field based indicators led to the recognition of disjoint zones with internally consistent glacial histories termed glacial terrain zones gtz these gtz were then classified as 1 relict glacial 2 palimpsest or 3 deglacial in nature our data suggest that while the southern keewatin sector was affected by regional ice divide translocation this alone cannot explain the fragmented high inheritance landscape we suggest that the subglacial landscape was continually evolving and subject to spatio temporal variations in intensity of erosion transportation and or deposition throughout multiple glacial events subglacial bed mosaic preservation of relict and palimpsest terrain likely occurred under large sticky low erosion regions these regions could have formed by at least two different mechanisms heterogeneous switch from warm based to cold based ice or within a warm based subglacial environment from wet to stiff dewatered till establishment of the regionally extensive 700 km wide by at least 500 km long dendritic esker channel system may have caused rapid spatially variable dewatering of the substrate far back under the ice sheet the gtz approach integrates all available data e g flowsets and other landform data striations to advance our interpretation of the spatio temporal evolution of subglacial dynamics in areas where the degree of landscape inheritance and overprinting is spatially highly variable this mosaic may be a characteristic net effect of landscape evolution beneath the core regions of ice sheets

geomorphology under ice streams moving from form to process ice streams are integral components of an ice sheet s mass balance and directly impact on sea level their flow is governed by processes at the ice bed interface which create landforms that in turn modulate ice stream dynamics through their influence on bed topography and basal shear stresses thus ice stream geomorphology is critical to understanding and modelling ice streams and ice sheet dynamics this paper reviews developments in our understanding of ice stream geomorphology from an historical perspective with a focus on the extent to which studies of modern and palaeo ice streams have converged to take us from a position of near complete ignorance to a detailed understanding of their bed morphology during the 1970s and 1980s our knowledge was limited and largely gleaned from geophysical investigations of modern ice stream beds in antarctica very few palaeo ice streams had been identified with any confidence during the 1990s however glacial geomorphologists began to recognise their distinctive geomorphology which included distinct patterns of highly elongated mega scale glacial lineations ice stream shear margin moraines and major sedimentary depocentres however studying relict features could say little about the time scales over which this geomorphology evolved and under what glaciological conditions this began to be addressed in the early 2000s through continued efforts to scrutinise modern ice stream beds at higher resolution but our current understanding of how landforms relate to processes remains subject to large uncertainties particularly in relation to the mechanisms and time scales of sediment erosion transport and deposition and how these lead to the growth and decay of subglacial bedforms this represents the next key challenge and will require even closer cooperation between glaciology glacial geomorphology sedimentology and numerical modelling together with more sophisticated methods to quantify and analyse the anticipated growth of geomorphological data from beneath active ice streams

new boundary conditions for the west antarctic ice sheet subglacial topography beneath pine island glacier 1 predictions about future changes in the amundsen sea sector of the west antarctic ice sheet wais have been hampered by poorly known subglacial topography extensive airborne survey has allowed us to derive improved subglacial topography for the pine island glacier basin the trunk of this glacier lies in a narrow 250 km long 500 m deep sub glacial trough suggesting a long lived and constrained ice stream two tributaries lie in similar troughs others lie in less defined shallower troughs the lower basin of the glacier is surrounded by bedrock which after deglaciation and isostatic rebound could rise above sea level this feature would impede ice sheet collapse initiated near the grounding line of this glacier and prevent its progress into the deepest portions of wais the inland slope of the bed beneath the trunk of the glacier however confirms potential instability of the lower basin containing sufficient ice to raise global sea by 24 cm

subglacial hydraulic conditions and their possible impact upon subglacial bed formation abstract in reviewing hypotheses of subglacial bed formation it becomes apparent that a close relationship must exist between the development of these bedforms and subglacial hydraulic conditions a speculative classification of subglacial temperate beds is presented subglacial beds are subdivided into h q and m beds h beds are typically hard beds in which upper interface meltwater activity dominates m beds are those in whichd soft mobile subglacial debris conditions prevail and in which advective pervasive f flow of saturated debris occurs in q beds both of the other bed types occur but are of a spatially and temporally transitory nature thus this latter bed type may be the most common beneath ice masses it is under an m bed state that many drumlin subglacial bedforms and associated forms are thought to develop discussion on the processes involved in mobile debris entrainment and the effect of changing rheological parameters are developed it is argued that such bedform initiation occurs due to anisotropic rheological variations within the mobile debris setting up a feedback effect such conditions are thought to be found most likely at the lower interface of a fast moving ice mass possibly typical of an ice stream in using empirical analyses of mobile debris conditions a limited set of relationships between subglacial meltwater flux rates basal ice velocity debris porosity and bed width are established with reference to mobile debris thicknesses using this relationship a set of possible parameter values that would allow bed formation to be achieved are presented the impact of bed formation under these conditions upon possible sedimentological facies and deformation structures is postulated the wider effect of m bedd conditionsd on subglacial sedimentology is introduced and the need to investigate subglacial hydraulic and bed conditions under past glaciations from the evidence of their glacial sediments and landforms emphasised

geomorphological signature and flow dynamics of the minch palaeo ice stream northwest scotland large scale streamlined glacial landforms are identified in 11 areas of northwest scotland from the isle of skye in the south to the butt of lewis in the north these ice directional features occur in bedrock and superficial deposits generally below 350 m above sea level and where best developed have elongation ratios of 20 1 sidescan sonar and multibeam echo sounding data from the minch show elongate streamlined ridges and grooves on the seabed with elongation ratios of up to 70 1 these bedforms are interpreted as mega scale glacial lineations all the features identified formed beneath the minch palaeo ice stream which was ca 200 km long up to 50 km wide and drained ca 15 000 km2 of the northwest sector of the last british irish ice sheet late devensian glaciation nine ice stream tributaries and palaeo onset zones are also identified on the basis of geomorphological evidence the spatial distribution and pattern of streamlined bedforms around the minch has enabled the catchment flow paths and basal shear stresses of the palaeo ice stream and its tributaries to be tentatively reconstructed british geological survey natural environment research council copyright 2007 reproduced with the permission of bgs nerc published by john wiley sons ltd

ice streams in the laurentide ice sheet identification characteristics and comparison to modern ice sheets this paper presents a comprehensive review and synthesis of ice streams in the laurentide ice sheet lis based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed the inventory includes 117 ice streams which have been identified based on a variety of evidence including their bedform imprint large scale geomorphology topography till properties and ice rafted debris in ocean sediment records despite uncertainty in identifying ice streams in hard bedrock areas it is unlikely that any major ice streams have been missed during the last glacial maximum laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets large ice streams had extensive onset zones and were fed by multiple tributaries and where ice drained through regions of high relief the spacing of ice streams shows a degree of spatial self organisation which has hitherto not been recognised topography exerted a primary control on the location of ice streams but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock and where networks of ice streams switched direction repeatedly and probably over short time scales as the ice sheet retreated onto its low relief interior several ice streams show no correspondence with topography or underlying geology perhaps facilitated by localised build up of pressurised subglacial meltwater they differed from most other ice stream tracks in having much lower length to width ratios and have no modern analogues there have been very few attempts to date the initiation and cessation of ice streams but it is clear that ice streams switched on and off during deglaciation rather than maintaining the same trajectory as the ice margin retreated we provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30 of the margin was drained by ice streams at the lgm similar to that for present day antarctic ice sheets but this decreases to 15 and 12 at 12 cal ka bp and 10 cal ka bp respectively the extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate or internal ice dynamical feedbacks unrelated to climate or both is largely unknown and represents a key area for future work to address

reconstruction of loch lomond stadial younger dryas glaciers on ben more coigach north west scotland and implications for reconstructing palaeoclimate using small ice masses a significant inventory of evidence exists for loch lomond stadial lls younger dryas glaciation in scotland with recent work focused on satellite icefields however studies of more marginal settings are important for assessing the influence of topoclimatic factors on glacier functioning and crucially the impact of these on glacier derived palaeoclimatic reconstructions we present systematic assessments of snowblow and avalanching contributions informed by modern analogues and test these using the first detailed palaeoglaciological reconstructions of three corrie glaciers on ben more coigach north west scotland based on morphostratigraphic principles and lithostratigraphic evidence from the region these have been attributed to the lls with the reconstructions yielding an average equilibrium line altitude ela of 328 16 m a glacier derived sea level equivalent precipitation value of 1903 178 mm a 1 is inferred for the lls suggesting wetter conditions than currently and contradicting assertions of a more arid lls climate comparison with published palaeoprecipitation estimates indicates ben more coigach does not conform to expected regional precipitation gradients we argue that these discrepancies reflect topographically enhanced snow accumulation which lowered the ela from the true climatic ela this highlights the importance of assessing the influence of topoclimatic factors when applying small glaciers in palaeoclimatic reconstructions

glacial to postglacial submarine landform assemblages in fiords of northeastern baffin island abstract fiord morphology plays a fundamental role in glacier flow dynamics and on ice margin stability as most of the present day margins of the greenland ice sheet lays in fiords there is a need for understanding short and long term glacial dynamics in fiord settings we investigate ice sheet retreat patterns in previously glaciated submarine terrain of northeastern baffin island fiords to provide analogues for modern and future ice sheet response to climate change and sea level rise geomorphological maps constructed from the interpretation of swath bathymetry imagery in fiords of northeastern baffin island reveal a wide range of glacial to postglacial landforms that allow the reconstruction of past ice sheet retreat dynamics ice flow landforms such as mega scale glacial lineations crag and tails and meltwater channels reveal the direction and behaviour of late foxe ice flow through the fiords the presence of undisturbed elongated landforms within the fiords suggests that ice streams have probably been active until the late stage of deglaciation landforms transverse to ice flow direction include grounding zone wedges frontal moraines grounding line fans recessional moraines and de geer moraines these landforms are interpreted as the result of former standstills of the ice margin during deglaciation the occurrence of grounding zones in deep 800 m part of the fiords contrasts with studies suggesting instability and rapid retreat of outlet glaciers over deep fiord basins sediment filled basins often characterised by the presence of turbidity channels gullies and mass movement scars occur in between the moraines sediment filled basins with a ponded architecture between sills illustrate that most of the sediment accumulation was ice proximal during deglaciation and characterised by gravity driven flows the proposed landform assemblage model for northeastern baffin fiords includes landforms typical of different fiord landsystems

flow dynamics and till genesis associated with a marine based antarctic palaeo ice stream geophysical and geological data indicate that during the last glacial cycle a palaeo ice stream drained the antarctic peninsula ice sheet apis through marguerite bay to the edge of the continental shelf via a bathymetric trough marguerite trough mega scale glacial lineations msgl in outer marguerite trough are formed in an acoustically transparent sediment unit consisting of a soft deformation till the association of msgl with deformation till indicates that bed deformation occurred beneath the ice stream and the thickness of the subglacial deforming layer was on the order of several metres msgl within outer marguerite trough are related to a pre existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove ploughing morphologically the msgl differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform an initial advance of the apis across the marguerite bay continental shelf at the lgm deposited a stiff subglacial till following this advance the ice margin within marguerite trough retreated 70 100 km before stabilising and depositing a grounding zone wedge an ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till development of the ice stream may have been a glacio dynamic response to regional deglaciation subsequent retreat of the marguerite trough ice stream was underway by 13 490 uncorrected 14c years before present based on the geophysical and core data ice stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo ice streams from other locations in antarctica this implies marked regional variations in antarctic ice stream dynamics during late quaternary deglaciation

a major ice drainage pathway of the last british irish ice sheet the tyne gap northern england the tyne gap is a wide pass situated between the scottish southern uplands and the english pennines that connects western and eastern england it was a major ice flow drainage pathway of the last british irish ice sheet this study presents new glacial geomorphological and sedimentological data from the tyne gap region that has allowed detailed reconstructions of palaeo ice flow dynamics during the late devensian marine isotope stage 2 mapped lineations reveal a complex palimpsest pattern which shows that ice flow was subject to multiple switches in direction these are summarised into three major ice flow phases stage i was characterised by convergent lake district and scottish ice that flowed east through the tyne gap as a topographically controlled ice stream this ice stream was identified from glacial geomorphological evidence in the form of convergent bedforms streamlined subglacial bedforms and evidence for deformable bed conditions stage ii involved northerly migration of the solway firth ice divide back into the southern uplands causing the easterly flow of ice to be weakened and resulting in southeasterly flow of ice down the north tyne valley and stage iii was characterised by strong drawdown of ice into the irish sea ice basin thus starving the tyne gap of ice and causing progressive ice sheet retreat westwards back across the watershed prior to ice stagnation copyright 2009 john wiley sons ltd

streamlined hard beds formed by palaeo ice streams a review fast flowing ice streams occur within modern ice sheets and also operated in pleistocene ice sheets the reconstruction of palaeo ice streams normally relies on the mapping of mega scale glacial lineations msgls and drumlins composed of soft sediment mainly till analysis of new satellite imagery and digital terrain models demonstrates the presence of large fields of kilometre scale glacial lineations comprising rock drumlins megagrooves and megaridges in this paper we describe and analyse a number of such hard bed landform systems from the former laurentide and british irish ice sheets occurring in a variety of palaeo ice stream settings these are attributed to erosion of crystalline and sedimentary rock below fast flowing ice streams bedrock properties such as hardness fracture spacing and bedding and their orientation with respect to ice flow have a profound effect on the occurrence and character of elongate rock bedforms elongate streamlined forms on hard crystalline rock as on the canadian shield only form under special circumstances in contrast sedimentary strata are highly susceptible to form streamlined hard beds specifically if bedrock strike is parallel to ice flow large scale elongate rock bedforms are erosional in origin formed by preferentially focused abrasion or by lateral plucking depending on bedrock type many palaeo ice stream footprints previously mapped in the laurentide ice sheet on the basis of soft bed bedforms are shown to be significantly larger extending up ice across sedimentary strata and onto precambrian crystalline rocks hard bed streamlined forms further show that ice streaming does not necessitate a deformable bed but can equally occur on smooth hard beds

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

erosion of bedrock by subglacial meltwater georgian bay ontario a regional view a spectacular series of sculpted erosional forms s forms is mapped and described from a 70 km wide area along the shore of georgian bay ontario which except for a scattered boulder lag has been swept clean of sediment a great variety of sculpted forms is described and illustrated and grouped into three classes transverse longitudinal and nondirectional forms transverse forms comprise transverse troughs muschelbruche sichelwannen and comma forms longitudinal forms comprise spindle flutes cavettos and furrows and nondirectional forms consist of undulating surfaces and potholes transverse forms are preferentially located on stoss slopes and longitudinal forms on lee slopes of rock rises undulating nondirectional forms are found on distal slopes and potholes at major breaks in slope this correlation of form and bed topography suggests that relief exerts considerable control on both form and location form geometry is also inferred to be related to coherent flow structures and their int

differentiation of subglacial conditions on soft and hard bed settings and implications for ice sheet dynamics a case study from north central poland we reconstruct patterns of subglacial processes on a hard bedrock and a soft bed under the southern sector of scandinavian ice sheet sis occurring in the basal till of the late saalian glaciation at the wapienno barcin and mlodocin sites north central poland based on detailed sedimentological studies two phases of sis transgression were recognised in the initial phase of the transgression the sis advanced onto a frozen substrate continuous permafrost the low permeability of the substratum led to a high subglacial water pressure swp and increased basal sliding the local increase of swp led to the development of different types of structures and sediments on a hard bedrock with low swp abrasion predominated and linear structures were developing while in the case of high swp the ice was decoupled from the hard substrate pressurised liquefied sediment flowed and structures of the p form and s form type developed on a soft bed the ice bed contact was of a mosaic type and the ice movement had an ice stream character the ice stream developed towards the east in the marginal zone of the sis and used a w e oriented valley filled by the wapienno formation fluvial complex during a later phase the ice movement was slower and did not have a stream character its direction changed to se the deposition of the main part of the diamicton occurred mainly as a result of the lodgement process

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

morphological characteristics formation and glaciological significance of rogen moraine in northern scotland rogen moraine are enigmatic landforms whose exact origin is still debated we use nextmap digital surface models and aerial photographs to map the distribution of previously unreported fields of rogen moraine in the vicinity of loch shin northern scotland existing models of formation are tested against detailed morphological rogen moraine characteristics obtained from the remote sensing data and field observations detailed morphometric analyses combined with their geographical setting lead us to postulate a likely mechanism of formation rogen moraine appear to have formed in areas where there were strong basal ice flow velocity gradients thrusting by compression or fracturing by extension of preexisting partially frozen sediment probably occurred in these areas resulting in rogen moraine formation a general down ice increase in ridge crest spacing suggests that the latter process may have been dominant and is consistent with the location of rogen moraine in the lee of topographic obstructions in areas that experienced overall extensional ice flow we also suggest that at least one field of rogen moraine formed where lateral basal ice flow velocity gradients were strongest possibly in a subglacial shear margin setting given their location the landforms may be consistent with formation during headward scavenging of the moray firth palaeo ice stream into a shrinking core of cold based ice

basal sliding of ice stream b west antarctica a tethered stake apparatus is used to measure basal sliding in a borehole on ice stream b west antarctica about 300 km upstream east from its grounding line near the head of the ross ice shelf a metal stake emplaced at the top of a layer of unfrozen till underlying the ice is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding the measured sliding motion includes any actual slip across the ice till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface this 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment based on features of the record of apparent sliding alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm in any case it is small compared to the seismically inferred till thickness of 9 m measured apparent sliding averages 69 of the total motion of 1 2 m d 1 over 26 days of observation if a 3 5 day period of slow apparent sliding 8 of the total motion is included in the average the occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c 80 and c 8 of the total motion on a time scale of a few days however it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole if the slow period is therefore omitted the indicated average basal sliding rate is 83 of the total motion in either case basal sliding predominates as the cause of the rapid ice stream motion in the last 2 days of observation the average apparent sliding rate reached 1 17 m d 1 essentially 100 of the motion of the ice stream if till deformation contributes significantly to the ice stream motion the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer these observations if applicable to the west antarctic ice sheet in general pose complications in modeling the rapid ice streaming motion

ribbed moraine formation ribbed rogen moraines are conspicuous landforms found in interior parts of formerly glaciated areas two major theories for ribbed moraine formation have been suggested in recent years i the shear and stack theory which explains ribbed moraine formation by shearing and stacking of till slabs or englacially entrained material during compressive flow followed by basal melt out of transverse moraine ridges and ii the fracturing theory according to which ribbed moraines form by fracturing of frozen pre existing till sheets at the transition from cold to warm based conditions under deglaciating ice sheets in this paper we present new data on the distribution of ribbed moraines and their close association with areas of frozen bed conditions under ice sheets in addition we show examples of ribbed moraine ridges that fit together like a jig saw puzzle these observations indicate that fracturing and extension of a pre existing till sheet may be a predominant process in ribbed moraine formation in summary we conclude that all described characteristics of ribbed moraines are compatible with the fracturing theory while the shear and stack theory is hampered by an inability to explain many conspicuous features in the distribution pattern and detailed morphology of ribbed moraines one implication of the fracturing theory is that the distribution of ribbed moraines can be used to reconstruct the extent of areas that underwent a change from frozen bed to thawed bed conditions under former ice sheets

submarine geomorphology and glacial history of the sea of the hebrides uk the sea of the hebrides is an island studded region of complex bathymetry on the uk continental shelf west of the scottish mainland an extensive area 2200 km2 of recently collected multibeam bathymetry data combined with seismic reflection profiles reveal this part of the shelf to have been extensively modified by both glacial and modern processes our new geomorphological evidence strongly supports the contention that an ice stream drained ice from western scotland and the inner hebrides towards the barra fan at the continental shelf break at the height of the last glaciation marine isotope stage 2 3 convergent seabed glacial lineations and other subglacially streamlined features eroded in bedrock around the islands of canna and rum preserve the direction of ice sheet movement and strongly suggest the onset of ice streaming in a southwesterly direction on the continental shelf in the sea of the hebrides region this fast flow zone formed part of a larger convergent ice stream system draining much of western scotland and the north of ireland the southern part of which has been postulated already a number of rock basins linear troughs and deeps west and south of muck kept sediment free by the dominant modern tidal regime are interpreted as being at least partly subglacial in origin similarly a large complex of buried deeps between eigg and arisaig are interpreted as an overdeepened glacial drainage network it is suggested that intense bedrock erosion by focused subglacial abrasion and subglacial meltwater discharge over multiple glacial advance and decay cycles led to significant modifications of the pre quaternary bedrock surface other features such as moraines are only found in shallower water typically 50 m west of canna west of rum and south of skye their relative paucity indicates that the modern tidally dominated environment of the area may have removed much of the localised geomorphological evidence of offshore glacial limits within the sea of the hebrides or alternatively that the ice sheet margin was floating and retreated predominantly by calving leaving little geomorphological evidence of its retreat

laboratory simulations of glacial abrasion comparison with theory glacial abrasion was simulated in experiments in which a small artificial glacier bed was pushed beneath a fixed ice block under pressure the experiments provide a means of testing theoretical models of abrasion particularly those factors that govern the magnitude of stress concentrations beneath abrading rock fragments in preliminary experiments vertical ice flow around a sphere mounted on the bed was studied in subsequent experiments marble tablets were pushed beneath granitic rock fragments frozen into the base of the ice block unlike previous abrasion experiments the sliding velocity was realistic 25 mm d 1 and ice near the bed was at the pressure melting temperature resultant striations closely resemble those observed on glaciated bedrock as predicted by hallet 1979 the component of the ice velocity towards the bed strongly influenced stresses beneath fragments and classical regelation and creep theory provided an approximate estimate of the downward drag force on fragments half of the rock fragments rotated significantly accounting for 10 50 of their motion relative to the bed and influencing abrasion rates and the shear stress supported along the ice bed interface striation patterns indirectly suggest that fragment rotations were inhibited by increases in ice pressure which presumably increased the drag on roughness elements on fragment surfaces this may have resulted from a reduction in the thickness of the water film around fragments facilitated by leakage of water from the bed

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

flow pattern evolution of the last british ice sheet we present a 10 stage reconstruction of the evolution in ice flow patterns of the last british ice sheet from build up to demise derived from geomorphological evidence 100 flowsets identified in the subglacial bedform record drumlins mega scale glacial lineations and ribbed moraine are combined with ancillary evidence erratic transport paths absolute dates and a semi independently reconstructed retreat pattern to define flow patterns ice divides and ice sheet margins during build up maximum glaciation and retreat overprinting and cross cutting of landform assemblages are used to define the relative chronology of flow patterns and a tentative absolute chronology is presented based on a collation of available dates for ice advance and retreat the ice flow configuration of the last british ice sheet was not static some ice divides were remarkably stable persisting through multiple stages of the ice sheet evolution whereas others were transient features existing for a short time and or shifting in position 10s km the 10 reconstructed stages of ice sheet geometry capture two main modes of operation first as an integrated ice sheet with a broadly n s orientated ice divide and second as a multi domed ice sheet orientated parallel with the shelf edge a thick integrated ice sheet developed as ice expanded out of source areas in scotland to envelop southerly ice caps in northern england and wales and connect with the irish ice sheet to the west and the scandinavian ice sheet across the north sea following break up of ice over the north sea ice streaming probably drove mass loss and ice sheet thinning to create a more complex divide structure where ice flow patterns were largely controlled by the form of the underlying topography ice surface lowering occurred before separation of and retreat to multiple ice centres centred over high ground we consider this 10 stage reconstruction of the evolution in ice sheet configuration to be the simplest palaeo glaciological interpretation of the flowsets identified from the geomorphological record and their relative timing this empirically based reconstruction of flow pattern geometry provides a framework for more detailed local and regional studies and numerical modelling to provide robust explanations of the observed changes in ice sheet structure in terms of climate and glacial dynamics as a minimum numerical model outputs should be able to reproduce the identified flowset patterns in space and satisfy their chronological order

the dynamics of ice sheet outlets a comparison of data from aircraft altimetry landsat imagery and radia echo sounding has shown characteristic surface topographies associated with sheet and stream flow the transition between the two is abrupt and occurs at a step in the subglacial topography this marks the onset of basal sliding and high velocities caused by subglacial water it results in crevassed amphitheatre like basins round the head of outlet glaciers it is also the zone of maximum driving stress beyond which values decline rapidly as velocities increase this abrupt transition appears to be topographically controlled since basal temperatures are at the pressure melting point well inland of the change in regime the marie byrd land ice streams exhibit qualitative differences from other ice sheet outlets however the change to lower driving stresses is much more gradual and occurs several hundred kilometres inland such ice streams have particularly low surface slopes and appear in form and flow regime to resemble confined ice shelves rather than grounded ice the repeated association of the transition to rapid sliding with a distinct subglacial feature implies a stabilizing effect on discharge through outlet glaciers acceleration of the ice is pinned to a subglacial step and propagation of high velocities inland of this feature seems improbable rapid ice flow through subglacial trenches may also ensure a relatively permanent trough through accentuation of the feature by erosion this is concentrated towards the heads of outlet glaciers up stream of the region where significant basal decoupling occurs this may be a mechanism for the overdeepening of fjords at their inland ends and the development of very steep fjord headwalls

a highly mobile laurentide ice sheet revealed by satellite images of glacial lineations satellite images of the north american continent reveal previously unrecognized patterns of crossing drift lineations reflecting unexpected shifts of centres of mass of the laurentide ice sheet throughout the last glacial cycle such shifts must must have been associated with major global atmospheric circulation changes

new marine evidence for a late wisconsinan ice stream in amundsen gulf arctic canada abstract amundsen gulf and adjoining dolphin and union strait and coronation gulf form the southwestern end of the northwest passage adjacent to the beaufort sea extensive high resolution multibeam sonar imagery and sub bottom profiles of the seabed have been acquired primarily in amundsen gulf by arcticnet and the ocean mapping group at the university of new brunswick these data reveal a variety of seabed landforms including mega scale glacial ridge and groove lineations drumlins moraines iceberg scours bedrock outcrops and discontinuous sediment deposits of variable thickness the lineations are widespread especially in southeastern amundsen gulf they resemble modern and paleo bedforms reported from antarctica svalbard greenland and other canadian arctic channels where they have been ascribed to ice streams the glacial sole marks on the seabed in amundsen gulf and regional data from the adjacent mainland and islands outline the configuration of a glacial ice stream from the laurentide ice sheet that occupied amundsen gulf and adjoining waterways during the late wisconsinan part of the northwestward flowing ice stream was deflected around the colville mountains on victoria island and rejoined the main ice stream in amundsen gulf by way of prince albert sound the grounded amundsen gulf ice stream extended northwestward to the outer slope in the beaufort sea where it was buttressed by arctic shelf ice maximum ice stream extent is inferred to have been coincident with the late glacial maximum multi sequence ice contact sediments and stratigraphic relations with glaciomarine sediments indicate that several ice advances and retreats occurred in the northwestern part of the gulf final retreat from the maximum position began prior to 13 000 cal yr bp and terrestrial dates indicate that the retreating ice front had reached dolphin and union strait by about 12 5 cal ka bp

ribbed moraines in sweden distribution pattern and palaeoglaciological implications abstract a detailed map of ribbed moraines in central and northern sweden is presented they are frequent features over much of the mapped area and the distribution has well defined limits the most abundant ribbed moraines are situated in close connection to areas that are inferred to have been cold based during the retreat of the late weichselian ice sheet while outer fully warm based areas lack ribbed moraine observations have also been made on a more local scale where cold based patches grade into ribbed moraines via a zone of a broken up till cover a model of origin is presented which includes contracting of a frozen core area of a retreating ice sheet at the transition from non sliding to sliding conditions there will be high stresses resulting in local extensional flow at the base of the ice sheet a rising phase change surface frozen thawed material would cause detachment and boudinage like breaking up of a pre existing till cover thawed at the bedrock contact but still frozen to the ice sole the orientation of ice flow during the formation of the ribbed moraines closely matches the ice flow directions of the deglaciating late weichselian ice sheet thus ribbed moraines are considered to be deglaciation landforms typical of shrinking cold based zones of a retreating ice sheet

ice keel seabed features in marine channels of the central canadian arctic archipelago evidence for former ice streams and iceberg scouring abstract the study area lies within the central part of the canadian arctic archipelago a region that was covered by the laurentide ice sheet during the late wisconsinan glaciation and earlier multibeam imagery from widely spaced transects indicates the presence of linear groove and ridge features on the seabed at several localities in peel sound franklin strait northern larsen sound and within m clintock channel these lineations resemble features in antarctica and in several formerly glaciated regions that have been interpreted to be sole marks emplaced beneath fast flowing ice streams based on these analogies a similar origin is inferred for the lineations on the channel floors within the study area the lineations are oriented parallel to the channel axes and margins they occur on all transects within the bathymetrically deeper area at the junction of franklin strait and peel sound northward in peel sound they occur extensively on the western and central transects and more locally on the eastern transect their north south orientation is normal to that of glacial flow features on somerset island and most of eastern prince of wales island which border peel sound to the east and west respectively the trend of the lineations is northeasterly parallel to the channel axis in franklin strait and mainly northerly in larsen sound and m clintock channel elsewhere the seabed imagery commonly displays scours of various sizes and orientations created by the keels of icebergs seabed sediments revealed by 3 5 khz sub bottom profiles are interpreted to consist primarily of ice contact sediments that in part are thinly mantled by draped water lain sediments the age of the lineations has not been established possibly their formation was coincident with an ice stream in the m clintock channel eastern victoria island region which formed an ice shelf in viscount melville sound that grounded on southern melville and byam martin islands at ca 10 4 9 6 14 c ka bp alternatively they could result from later glacial events

deep sea floor evidence of past ice streams off the antarctic peninsula the past existence of a giant ice stream off the northern end of the antarctic peninsula is suggested by a convex upward elongated sediment body now at a mean water depth of 1000 m because of its morphological characteristics i e a set of parallel to subparallel ridges and grooves to 100 km long with an overall width of 25 km we call this depositional body a bundle structure we hypothesize that bundle structures form by accumulation of basal deformation till under their parent ice streams from its location size and overall characteristics the bundle structure described here constitutes the best preserved largest deepest and relatively low latitude evidence of giant ice streams that flowed offshore antarctica during glacial maxima bundle structures reveal the very dynamic behavior of ice caps in the northern antarctic peninsula during the last glacial maximum with catchment areas draining rapidly under marine based critical subglacial conditions bundle structures represent a new megascale streamlined glacial landform

a ploughing model for the origin of weak tills beneath ice streams a qualitative treatment abstract glaciological studies of west antarctic ice streams have shown that weak sub ice stream tills provide the basal lubrication that makes fast ice streaming possible under low driving stresses given the significant current interest in time dependent ice stream behavior there is a clear need for a conceptual model of weak sub ice stream tills that treats in a simple but physically correct way the coupling between evolution of till properties and ice stream dynamics as a possible alternative to the previous viscous bed model we propose a ploughing model that is consistent with the experimentally determined coulomb plastic rheology of sub ice stream till in the ploughing model the till is a several meters thick layer of sedimentary material that is disturbed and transported by ploughing that occurs during sliding of a bumpy ice base the thickness of the till layer is determined in the ploughing model by the amplitude of the largest roughness elements ice keels there is no direct proof for the existence of ice bumps and ice keels beneath the modern west antarctic ice streams but bedforms e g megalineations and bundle structures left behind by pleistocene ice streams strongly support our assumption that an ice stream base is irregular generation of new till material occurs when ice keels protrude through the existing till layer and erode the top of the sub till preglacial sediments based on a single tethered stake measurement of engelhardt and kamb j glaciol 44 1998 223 made at the upb camp ice stream b west antarctica fig 1 we estimate that the till flux due to sliding with ploughing is there 3 yr 1 per meter width to balance the estimated till flux in the upb area substrata erosion by ice keels would have to take place at a high but not unreasonable non dimensional rate of 4 assuming 1 contact area in the case of the west antarctic ice streams erosion of sub till materials by ice keels may be particularly fast and unimpeded because these ice streams are overriding unlithified preglacial tertiary sediments the most significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocity independent plastic till rheology while allowing subglacial transport of till as in the viscous bed model models of ice streams with a plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds

megagrooves and streamlined bedrock in nw scotland the role of ice streams in landscape evolution abstract new multibeam bathymetry data onshore high resolution elevation data nextmap and fieldwork in the ullapool area of nw scotland reveal large scale megagrooves and streamlined bedrock forms in a well defined 20 km wide zone the landsystem is typical of a coherent flow corridor within a grounded ice sheet on bedrock dominated terrain we describe the morphology of the large scale features discuss their likely formation and consider the wider implications for ice sheet dynamics based on the strongly convergent bedform distribution the presence of megagrooves and highly elongate bedrock forms we interpret the erosional landscape to be the signature of a fast flowing tributary that once fed the the minch palaeo ice stream a major artery of the last british irish ice sheet the exact genesis of bedrock megagrooves remains uncertain although focused subglacial abrasion is likely to have carved most of the shallow strongly parallel features whilst glacial meltwater may have carved or modified others bedform morphometry is used to discriminate zones reflecting the degree of glacial streamlining elongation ratios 5 1 we interpret these zones to represent the transition from potentially cold based slow ice sheet flow to warm based fast flow based on these results and the presence of ribbed moraines we suggest a bedform continuum model for onset zones of palaeo ice streams on rigid beds rapid spatial bedform evolution is suggested to reflect an increase in subglacial erosive power that may be diagnostic of palaeo ice sheet thermal boundaries i e from cold to warm based and is also consistent with the expected downstream increase in ice velocity within an ice stream onset zone finally this study speculates on the role played by basal meltwater in ice stream initiation and the role of ice streams and their tributaries in landscape evolution

earthquake mechanisms in the hellenic trench near crete summary in the hellenic trench south of crete convergence between the southern aegean sea and africa occurs at a rate of at least 60 mm yr 1 previously published first motion fault plane solutions show a variety of different fault orientations and types but are not well constrained furthermore the lack of reliable focal depths for these earthquakes has obscured any simple pattern of deformation that might exist nonetheless the mechanisms of these earthquakes have strongly influenced views of the tectonics in the hellenic trench we have improved estimates of the fault parameters and focal depths for 14 of these earthquakes using long period p and sh waveforms the earthquake mechanisms fall into four groups a normal faults with a n s strike in the over riding material above the subduction zone b low angle thrusts with an e w strike at a depth of about 40 km c high angle reverse faults with the same strike but shallower focal depths than b d events within the suducting lithosphere with approximately e w p axes the thrusting in groups b and c is probably the mechanism by which the sediments of the mediterranean sea underplate and uplift crete these events have slip vectors in the direction 025 12 which represents the convergence direction between crete and africa along the sw facing boundary of the hellenic trench one of the events in group d occurred beneath the mediterranean ridge and involved high angle reverse faulting with a wnw ese p axis almost perpendicular to the direction of shortening deduced from folds at the surface the mediterranean sea floor in this region appears to be in a state of e w compression for reasons that are not clear

widespread complex flow in the interior of the antarctic ice sheet it has been suggested that as much as 90 of the discharge from the antarctic ice sheet is drained through a small number of fast moving ice streams and outlet glaciers fed by relatively stable and inactive catchment areas here evidence obtained from balance velocity estimates suggests that each major drainage basin is fed by complex systems of tributaries that penetrate up to 1000 kilometers from the grounding line into the interior of the ice sheet this finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing

controls on bedrock bedform development beneath the uummannaq ice stream onset zone west greenland this paper investigates the controls on the formation of subglacially eroded bedrock bedforms beneath the topographically confined region upstream of the uummannaq ice stream uis during the last glacial cycle palaeoglaciological conditions are believed to have been similar for all sites in the study characterised by thick fast flowing ice moving over a rigid bedrock bed classic bedrock bedforms indicative of glacially eroded terrain were mapped including p forms roches moutonnees and whalebacks bedform long axes and plucked face orientations display close correlation parallel and perpendicular to palaeo ice flow directions inferred from striae measurements across all sites elongation ratios length to width varied by an order of magnitude between 0 8 1 and 8 4 1 bedform properties length height width and long axis orientation from four subsample areas form morphometrically distinct populations despite their close proximity and hypothesised similarity in palaeoglaciological conditions variations in lithology and geological structures e g joint frequency joint dip joint orientation bedding plane thickness and bedding plane dip provide lines of geological weakness which focus the glacial erosion in turn controlling bedform geometries determining the relationship s between bedding plane dip relative to palaeo ice flow and bedform shape relative length amplitude and wavelength has important ramifications for understanding subglacial bed roughness cavity formation and likely erosion style quarrying and or abrasion at the ice bed interface this paper demonstrates a direct link between bedrock bedform geometries and geological structure and emphasises the need to understand bedrock bedform characteristics when reconstructing palaeoglaciological conditions

landform signature of the laurentide and cordilleran ice sheets across alberta during the last glaciation government geological survey maps and research publications have portrayed the distribution of glacial landforms associated with the advance and retreat of the laurentide and cordilleran ice sheets across alberta at a local regional and continental scale to date this information has not been systematically synthesized into a single compilation at a consistent scale although this original work provided valuable information to constrain reconstructions of former ice sheet extent configuration and flow geometry its derivation primarily from the interpretation of aerial photographs and the shuttle radar topography mission 90 m digital elevation model dem may result in methodological inconsistencies and spatial biases these biases together with challenges associated with geomorphic mapping in densely forested areas of western and northern alberta limit the usefulness of previous mapping when applied to inversion based ice sheet reconstructions which have specific input data demands recently ligh

surface morphology and internal layer stratigraphy in the downstream end of kamb ice stream west antarctica satellite images of kamb ice stream formerly ice stream c west antarctica reveal several long curved linear features lineations oriented sub parallel to the ice flow direction we use ground based radar to characterize the internal layer stratigraphy of these lineations and the terrains that they bound some lineations are relict ice stream shear margins identified by hyperbolic diffractors near the surface interpreted to be buried crevasses and highly disturbed internal layers at depth satellite images show another set of lineations outside the relict margins that wrap around the ends of the surrounding inter ice stream ridges internal layers beneath these lineations are downwarped strongly into a syncline shape the internal stratigraphy of the terrain between these lineations and the relict margins is characterized by deep hyperbolic line diffractors our preferred hypothesis for the origin of this terrain is that it was floating sometime in the past the deep hyperbolas are interpreted to be basal crevasses and the strongly downwarped internal layers mark the position of a relict grounding line our study shows that lineations and intervening terrains have different internal layer characteristics implying different origins differentiation between these features is not possible using satellite images alone

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

quantifying bed roughness beneath contemporary and palaeo ice streams bed roughness is an important control on ice stream location and dynamics the majority of previous bed roughness studies have been based on data derived from radio echo sounding res transects across antarctica and greenland however the wide spacing of res transects means that the links between roughness and flow are poorly constrained here we use digital terrain model dtm bathymetry data from a well preserved palaeo ice stream to investigate basal controls on the behaviour of contemporary ice streams artificial transects were set up across the minch palaeo ice stream nw scotland to mimic res flight lines over institute and moller ice streams antarctica we then explored how different data resolution transect orientation and spacing and different methods impact upon roughness measurements our results show that fast palaeo ice flow can occur over a rough hard bed not just a smooth soft bed which much previous work has suggested smooth areas of the bed occur over both bedrock and sediment covered regions similar trends in bed roughness values were found using fast fourier transform analysis and standard deviation methods smoothing of bed roughness results can hide important details we propose that the typical spacing of res transects is too wide to capture different landform assemblages and that transect orientation influences bed roughness measurements in both contemporary and palaeo ice stream setting

radar images of the bed of the greenland ice sheet 1 in this paper we apply radar tomography methods to very high frequency airborne synthetic aperture radar data to measure the ice thickness field and to construct three dimensional basal image maps of a 5 20 km study area located along the southern flank of the jakobshavn glacier greenland unlike ice radar measurements typically made at nadir our approach uses radar echo phase and amplitude measured across an antenna array to determine the propagation angle and signal strength of pixel elements distributed on each side of the aircraft flight path that information combined with knowledge of aircraft position and the assumed dielectric properties of the glacier can be used to measure ice thickness and radar reflectivity across a 3 km wide swath combining ice thickness and surface topography data we estimate basal topography and basal drag we conclude that the glacier is sliding over the bed we use the three dimensional image maps of the bed to inspect the modern subglacial geomorphology and find for the first time beneath the greenland ice sheet assemblages of long ridge groove landforms that are oriented in the direction of the ice flow spatial dimensions 10 to 30 m depths 150 to 500 m spacing and lengths of 10 km or more and correlation with the current ice flow direction suggest that these are glacial erosional features similar to mega grooves observed on deglaciated terrain

formation of mega scale glacial lineations on the dubawnt lake ice stream bed 1 size shape and spacing from a large remote sensing dataset abstract mega scale glacial lineations msgls are the largest flow parallel bedforms produced by ice sheets and are formed beneath rapidly flowing ice streams knowledge of their characteristics and genesis is likely to result in an improved understanding of the rate at which ice and sediment are discharged by ice sheets but there is little consensus as to how they are formed and there are few quantitative datasets of their characteristics with which to formulate or test hypotheses this paper presents the results of a remote sensing survey of 46 000 bedforms on the dubawnt lake palaeo ice stream bed focussing on a central transect of 17 038 that includes highly elongate bedforms previously described as msgls within this transect lineations exceed 10 km in length max 20 km and 23 have elongation ratios 10 1 max 149 1 highly elongate features are interspersed with much shorter drumlin like features but longer bedforms are typically narrower suggesting that their length develops more quickly than or at the expense of their width bedforms are broadly symmetrical in plan form and have a preferred lateral spacing of 50 250 m which implies a regular rather than random pattern of corrugations comparison with drumlins reveals that the more attenuated msgls simply extend the tail of the distribution of data rather than plotting as a separate population taken together this supports the idea of a subglacial bedform continuum primarily controlled by ice velocity but existing hypotheses of msgl formation are either not supported or are insufficiently developed to explain our observations rather we conclude that under conditions of rapid ice flow msgls attain their great length relatively quickly decades through a probable combination of subglacial deformation which attenuates ridges and erosional processes that removes material from between them

field studies of bottom crevasses in the ross ice shelf antarctica analyse des crevasses observees par sondages radar aeriens et terrestres localisation de leurs points de formation

abraded rock landforms whalebacks developed under ice streams in mountain areas like many mountain ranges the coast mountains of british columbia canada have undergone both local and ice sheet glaciation effects of ice sheets are concentrated along major valleys and on adjacent spurs and passes which carried strong flows of diffluent ice the major valleys are broad glacial troughs with frequent rock basins their slopes are broken into rounded steep sided bosses whalebacks abraded on all sides they are of the order of 100 m to 1 km long and 10 m high in the southern coast mountains the distribution of these whalebacks is consistent with a proposed pattern of former ice streams 1 0 2 1 km thick within the cordilleran ice sheet they are best developed where geological structures parallel the valley and thus the former ice flow direction but they are found on a range of lithologies and some are transverse to structure the whalebacks provide an impression of glacial streamlining and occasionally grade into rock drumlins roches moutonnees are rare in the major troughs it is hypothesised that these whalebacks and rock drumlins develop under ice streams of greenland or east antarctic type sliding rapidly over bedrock and exploiting rock weaknesses to produce streamlined features lee slopes are abraded when thick ice suppresses bed separation even with rapid flow basal ice of low viscosity would aid this suppression water pressures under the ice streams may have remained high so that lee side plucking was rare such plucking is most likely where pressure fluctuates dramatically and especially when lee cavities under active ice reach atmospheric pressure

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

glacial landforms of extreme size in the keewatin sector of the laurentide ice sheet assemblages of glacial landforms of a mega scale are here identified in the keewatin sector of the laurentide ice sheet large till belts or ridges apparent only on satellite imagery and lyi

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock