ice flow patterns and glacial transport in the eastern hudson bay region implications for the late quaternary dynamics of the laurentide ice sheet recent field surveys in the eastern hudson bay region have led to the discovery of regional ice flow sequences that require a significant reassessment of the late quaternary dynamics of the laurentide ice sheet two regional ice flow phases can be recognized from till compositional data and from crosscutting relationships observed on striated bedrock surfaces the oldest is directed toward the northwest and north northwest while the youngest is directed toward the west and includes a late glacial deflection toward the southwest the wide regional distribution of striae formed during the early northwestward glacial movement together with the recognition of palimpsest glacial dispersal trains associated with this phase suggest that it was a long lived time transgressive regional event the ensuing glacial movement is a regionally dominant westward ice flow phase during which several large glacial dispersal trains were formed downglacier from distinctive bedrock sources the largest of these trains extends

impact of bedrock surface topography on spatial distribution of quaternary sediments and on the flow pattern of late weichselian glaciers on the east european craton russian plain this paper presents the first high quality maps of the bedrock surface topography and the quaternary sediment distribution for the late weichselian glaciation area on the east european craton eec and the reasoning behind the estimated sediment distribution geographic information system gis based three dimensional model of the bedrock surface topography the current digital terrain model dtm and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations in the current continental area of the western eec which was covered by the last scandinavian ice sheet sis the bedrock lies overwhelmingly 85 above the contemporary sea level with an average altitude of 54 m above sea level the topography of the bedrock surface is very well reflected in the current topography and an 80 m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow and thus all late weichselian ice streams were accommodated to the depressions in the subglacial surface such depressions are characterised by a thin 10 m layer of sediment with one till bed just 50 m upslope from the bottom of the depressions the thickness of the sediment increases two to threefold and comprises five or more till layers indicating relief enhancement through subsequent glaciations the average calculated thickness of the quaternary deposits in the study area was 49 m and the total volume of sediments was 20 280 km3

origin of a washboard moraine of the des moines lobe inferred from sediment properties abstract geometric characteristics of the washboard moraines of the des moines lobe dml of the laurentide ice sheet agree with their proposed origin as crevasse squeeze ridges but study of their sediments is required to help further test this hypothesis a 70 m long 3 5 m high section through a moraine ridge in central iowa revealed till with irregular isolated lenses of silt sand and gravel that dip to varying extents upglacier the texture and density of the till are like those of the basal till of the dml studied elsewhere in iowa and preconsolidation pressures determined from tests on till and silt of the ridge indicate that it developed subglacially rather than at the glacier margin preconsolidation pressures additionally demonstrate that pore water pressures in the bed supported most of the glacier s weight which would have contributed to till mobility fabrics based on the anisotropy of magnetic susceptibility of 3125 intact till specimens collected at 125 locations in the section indicate two end member states of strain that varied with location in the ridge and caused sediment mounding simple shear that was directed downglacier along shear planes inclined upglacier together with pure shear where an overlying crevasse allowed the sediment bed to extend upward and laterally meltwater that likely flowed along the crevasse deposited sorted sediments that were incorporated in till deformed and rotated this positive test of the crevasse squeeze hypothesis indicates that the dml was in longitudinal extension near its margin reinforcing previous arguments that the lobe surged the predominance of fabrics caused by simple shear demonstrates that crevasse filling was underway before the surge had fully halted this study should prompt caution in using similar transverse ridges such as those geophysically imaged in some submarine glacier forefields as indicators of retreat rates

the tekapo glacier new zealand during the last glacial maximum an active temperate glacier influenced by intermittent surge activity abstract quaternary glaciations have created impressive landform assemblages that can be used to understand palaeo glacier extent character and behaviour and hence past global and local glacier forcings however in the southern hemisphere and especially in new zealand the quaternary glacial landform record is relatively poorly investigated with regard to glaciological properties in this study a 1 m digital elevation model dem was generated from airborne lidar data and supplemented with aerial imagery and field observations to analyse the exceptionally well preserved glacial geomorphology surrounding lake tekapo new zealand we describe a rich suite of last glacial maximum lgm and recessional ice marginal subglacial supraglacial glaciofluvial and glaciolacustrine landform assemblages these represent two landsystems comprising i fluted till surfaces with low relief push moraine ridges and ii crevasse squeeze ridges zig zag eskers and attenuated lineations the former landsystem records the behaviour of an active temperate glacier and the latter landsystem which is superimposed upon and inset within the former strongly suggests intermittent surge phases the two landsystem signatures indicate a sequential change in ice marginal dynamics during recession that was likely to have been partially non climatically driven overall we present the first evidence of surge type glacier behaviour in new zealand

concentrated pulsed axial glacier flow structural glaciological evidence from kviarjokull in se iceland a detailed structural glaciological study carried out on kviarjokull in se iceland reveals that recent flow within this maritime glacier is concentrated within a narrow corridor located along its central axis this active corridor is responsible for feeding ice from the accumulation zone on the south eastern side of oraefajokull to the lower reaches of the glacier and resulted in a c 200 m advance during the winter of 2013 2014 and the formation of a push moraine the corridor comprises a series of lobes linked by a laterally continuous zone of highly fractured ice characterised by prominent flow parallel crevasses separated by shear zones the lobes form highly crevassed topographic highs on the glacier surface and occur immediately down ice of marked constrictions caused by prominent bedrock outcrops located on the northern side of the glacier close to the frontal margin of kviarjokull the southern side of the glacier is relatively smooth and pock marked by a number of large moulins the boundary between this slow moving ice and the active corridor is marked by a number of ice flow parallel strike slip faults and a prominent dextral shear zone which resulted in the clockwise rotation and dissection of an ice cored esker exposed on the glacier surface it is suggested that this concentrated style of glacier flow identified within kviarjokull has affinities with the individual flow units which operate within pulsing or surging glaciers

striated clast pavements products of deforming subglacial sediment studies of modern glaciers have recently drawn attention to the importance of subglacial sediment deformation to glacier dynamics and processes inferences regarding the probable shear strength of this sediment imply that large clasts may in some cases sink to underlying rigid sediment where abrasion by overlying deforming sediment could occur this scenario provides a formative mechanism for striated clast pavements commonly described from the base of fine grained massive diamictons associated with the late pleistocene laurentide ice sheet such a mechanism indicates that at the time of formation of clast pavements overlying diamictons associated with pavements had a low yield strength 0 5 kpa and were deforming mechanically like a debris flow clast pavements may therefore be an important criterion for recognition of sediments deposited by subglacial deformation transport

post stagnation behavior in the upstream regions of ice stream c west antarctica the region where two active tributaries feed into the now stagnant ice stream c isc west antarctica is thickening in this region we observe a correlation between faster ice flow the tributaries and elevated topography we conclude that stagnation of isc resulted in compression and thickening along the tributaries eventually forming a bulge on the ice sheet surface modern hydraulic potential gradients would divert basal meltwater from isc to ice stream b isb these gradients are primarily controlled by the bulge topography and so likely formed subsequent to trunk stagnation as such we argue against water piracy as being the cause for isc s stagnation kinematic wave theory suggests that thickness perturbations propagate downstream over time but that kinematic wave speed decreases near the stagnant trunk this and modest diffusion rates combine to trap most of the tributary fed ice in the bulge region using interferometric synthetic aperture radar velocity measurements we observe that half of the ice within isc s southern tributary flows into isb that flow pattern and other observations of non steady flow in the region likely result from stagnation induced thickening along upper isc combined with a longer period of thinning on upper isb if current trends in thickness change continue more ice from upper isc will be diverted to isb

positive mass balance of the ross ice streams west antarctica we have used ice flow velocity measurements from synthetic aperture radar to reassess the mass balance of the ross ice streams west antarctica we find strong evidence for ice sheet growth 26 8 gigatons per year in contrast to earlier estimates indicating a mass deficit 20 9 gigatons per year average thickening is equal to 25 of the accumulation rate with most of this growth occurring on ice stream c whillans ice stream which was thought to have a significantly negative mass balance is close to balance reflecting its continuing slowdown the overall positive mass balance may signal an end to the holocene retreat of these ice streams

steady state ditch drainage of two layered soil regions overlying an inverted v shaped impermeable bed with examples of the drainage of ballast beneath railway tracks water table heights due to steady surface accretion in drained two layered soil regions overlying an inverted v shaped impermeable bed are obtained using both the dupuit forchheimer approximate analysis with flow assumed parallel to the bed and also from numerical solutions of laplace s equation for the head distribution for illustration water table profiles obtained by the two procedures are compared for surface accretion draining to ditches in a typical two layered ballast foundation for a railway track where a very permeable ballast material overlies a less permeable sub grade on top of an inverted v shaped impermeable bed that slopes away both sides from a central line to drainage ditches these results are found to be in good agreement except very near the drainage ditches where the laplace numerical solution takes into consideration a surface of seepage that is ignored in the dupuit forchheimer analysis the dupuit forchheimer analysis is also in good agreement with results of a laboratory model experiment it is concluded that the approximate dupuit forchheimer analysis can be used with confidence in these situations it is used to investigate the effect on the water table elevation caused by the reduction of hydraulic conductivity of the porous materials due to clogging

late pleistocene holocene retreat of the west antarctic ice sheet system in the ross sea part 1 geophysical results in the 1994 1995 and 1998 austral field seasons geophysical research efforts in the ross sea focused on acquiring a high resolution database designed to permit 1 reconstruction of the maximum extent and configuration of the ice sheet during the last glacial maximum i e oxygen isotope stage 2 2 reconstruction of conditions at the base of the ice sheet and 3 assessment of the relative retreat history of the ice sheet following the last glacial maximum five seismic facies are distinguished on the basis of external geometry relationships among features bounding surface amplitude intensity of internal acoustic signature and geometry of internal reflectors seismic facies 1 is a transparent draping unit correlated to diatomaceous mud interpreted as being deposited under open marine conditions seismic facies 2 and 3 are comprised of units displaying subdued massive internal signatures with smooth lower bounding surfaces and hummocky upper surfaces characterized by glacial lineations these units are interpreted to be grounding zone proximal deposits overridden by the expanded ice sheet seismic facies 2 may be a deforming till unit seismic facies 4a is characterized by an internally massive to chaotic signature an erosional often flat lower surface and a hummocky upper surface displaying glacial lineations this unit is interpreted to be till and is divided into deposits associated with the most recent glacial expansion 4a and with an earlier pre last glacial maximum expansion 4b seismic facies 5 is an acoustically laminated ponded and draping deposit interpreted to contain proglacial and sub ice shelf materials deposited continuously since the last interglacial period the associations of these units provide context for the interpretation of the ice edge maximum position conditions at the base of the ice sheet and the relative retreat history of the region there is compelling evidence for a much expanded ice sheet in the ross sea during the last glacial maximum in the western ross sea the maximum grounding position is marked by an isolated grounding zone wedge and placed in the vicinity of coulman island approximately 150 km from the continental shelf edge in the central ross sea the maximum grounding position is close to the continental shelf break based primarily on the presence of an extensive 60 m thick sheet like deposit with a fluted upper surface streaming ice may have occupied bathymetric lows on the continental shelf as suggested by 1 the configuration of bathymetry 2 the presence of glacially eroded troughs 3 the concentration of sediment interpreted to be deforming till within the middle to outer shelf reaches of the troughs and 4 the fluted nature of the upper surface absolute rates of streaming ice flow relative to inter ice stream areas are not implied the western ross sea continental shelf deposits record the retreat history of ice derived predominantly from the east antarctic ice sheet and glaciers of the transantarctic mountains ice flow on the continental shelf is interpreted to have remained fixed in position restricted by the walls of the troughs on the inner shelf of the western ross sea the ice flowed over and eroded lithified sedimentary strata slower moving ice occupied flat topped banks a single grounding zone wedge occurs on the western ross sea central shelf no substantial deposits are observed on the inner continental shelf this lack of grounding zone features reflects a restricted sediment supply and a relatively steady rapid retreat of the ice sheet during retreat ice flow acted independently in each major trough and on the bank tops ice retreated from victoria land basin drygalski trough before it retreated from joides basin ice remained on the bank tops shedding material into the abandoned troughs grounded ice in the central ross sea was derived predominantly from an expanded west antarctic ice sheet the ice sheet remained grounded on the continental shelf edge after retreat of ice from the western ross sea expanded ice eroded the inner shelf and deposited sheets of till on the central and outer shelf ice stream drainage shifted laterally as recorded by rounded laterally accreting ridges that separate bathymetric troughs the central ross sea is the repository for larger volumes of sediment derived from ice flow across basins of relatively thick unlithified sedimentary deposits two till sheets mark grounding zone positions in the central ross sea reflecting the higher sediment supply and a stepped deglaciation ice remained grounded on the pennell bank to the west a series of moraines marks the retreat of ice across the bank mega scale glacial lineations and other streamlined subglacial geomorphic features occur across the continental shelf primarily within the troughs the lineations substantiate the maximum reconstruction and support the interpretation of a deforming substrate beneath the outer reaches of the expanded ice sheet this deforming substrate may have contributed to the onset of deglaciation features associated with meltwater are rare or absent suggesting that basal meltwater played a minor role in retreat of the ice sheet

the signature of strain magnitude in tills associated with the vistula ice stream of the scandinavian ice sheet central poland abstract at oborki in central poland we examined a 3 m thick late weichselian basal till deposited by the vistula ice stream a multi proxy approach involving a study of grain size grain roundness crushing index petrographic composition till fabric and till micromorphology in a closely sampled vertical profile was applied to decipher till formation and ice movement mechanisms the till consists of a succession of four texturally and structurally different units generated by a combination of processes related to effective stresses exerted on the bed by active ice deformation intergranular advection clast ploughing and localized basal decoupling played important roles during till formation micromorphological analysis reveals that the most common s matrix microstructures are grain lineations grain stacks turbate structures till pellets and crushed grains microshears dominate whereas plasmic fabric is not well developed we use grain roundness crushing indices and microshear geometry to determine subglacial strain magnitude in the till matrix microfabric and macrofabric s 1 eigenvalues together with i l indexes indicate low shear strains in the order of 10 10 2 strain estimates vary depending on the particle sizes used suggesting a plastic mode of till deformation all proxies studied suggest complex and temporally variable subglacial conditions under the marginal part of the vistula ice stream probably caused by changes in till rheology related to fluctuating porewater pressures fast ice flow was facilitated by a combination of bed deformation and enhanced basal sliding on a thin water layer our results may also be representative for other terrestrially based pleistocene ice streams moving over soft beds

glacial characteristics of a pleistocene ice lobe in east central minnesota close scrutiny of the till deposited during the late wisconsin advance of the grantsburg sublobe across east central minnesota enables us to reconstruct the glaciological nature of a mid latitude pleistocene ice lobe the sublobe an offshoot of the des moines lobe advanced to the northeast about 16 000 yr b p overriding drift of the superior lobe an ice mass that had advanced from the lake superior basin approximately 20 000 yr b p because the tills are compositionally distinct variations in grantsburg tills can be discerned that resulted from incorporation of superior lobe drift patterns of grantsburg till dilution can in turn be interpreted to reconstruct some basic physical characteristics of the sublobe to quantify compositional variability analytical methods were selected to characterize the tills texture carbonate content clay mineralogy and magnetic susceptibility when considered jointly adequately distinguish the tills and constitute the means of detecting dilution of grantsburg tills results of analysis indicate that the composition of grantsburg till changes progressively downglacier as a consequence of prolonged contact with superior lobe drift terrane marked variations across the sublobe are also apparent with greater dilution at the margins and relatively lower levels of superior lobe drift incorporation at axial positions this pattern suggests that the grantsburg sublobe was a semipolar ice mass frozen to the substrate at its margin and at its pressure melting point along its axis

glacial landsystems and dynamics of the saginaw lobe of the laurentide ice sheet michigan usa the surficial terrain of the saginaw lobe in southern michigan is divided into 4 landsystems numbered sequentially from southwest to northeast containing related assemblages of sediments and landforms two of which appear to have a genetic relationship with the bedrock units over which they lie landsystem 1 consists of the sturgis moraine a terminal recessional moraine and an adjacent drumlin field the moraine is a high relief ridge with hummocky topography and ice walled lake plains at its crest and thick coarse outwash fans on its distal margin the drumlin field extends up glacier from the sturgis moraine across the subcrop of the mississippian coldwater shale to the subcrop of the overlying marshall sandstone the low permeability of the shale may have increased basal pore pressures into a range in which deformation of basal sediment into drumlins occurred landsystem 2 overlies the subcrop of the marshall sandstone stagnation of the lobe behind a discontinuous moraine in this area is indicated by high relief collapse topography composed of kames eskers and ice walled lake plains a system of tunnel valleys terminates and merges in this landsystem with large outwash fans and plains sloping away from the margin high basal drainage into the sandstone may have facilitated stagnation across a broad area to form this landsystem landsystem 3 may have also developed during stagnation of the lobe but it differs from landsystem 2 by its more subdued relief it contains a network of evenly spaced southwest trending open tunnel valleys that contain discontinuous esker segments landsystem 4 consists of till plains and low recessional moraines formed as the saginaw lobe retreated downslope into saginaw bay of the lake huron basin tunnel valleys are absent in this area

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

the morphology of ice streams a b and c west antarctica and their environs airborne radar soundings of the ice sheet surface made in 1984 1985 together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams a b and c and much of the region around them the flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking ice thicknesses were also measured along the flight lines but have not yet been mapped the surfaces of active ice streams a and b exhibit a longitudinal ridgetrough topography of uncertain origin prominent surface valleys are associated with most but not all of their marginal shear zones there is a deep subglacial trough beneath the grid northeastern side of ice stream a that connects to the subglacial trough beneath reedy glacier the boundary between ice streams bl and b2 the two tributaries of ice stream b persists as a suture zone for 250 km downstream from their merger ice stream bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream b until it disappears close to the grounding line between ice streams bl and b2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice which suggest to us that rafts of ice are being incorporated into the ice streams the previously reported negative mass balance for the drainage system of ice stream b may arise from the expansion of the ice stream into the interior of the west antarctic ice sheet crevasse bands at some places on ridge ab together with high glacial velocities reported elsewhere also suggest this expansion close to the ross ice shelf ice stream b widens into a nearly flat area where the mean surface slope is only 3 5 10 4 and the surface elevations are only a few tens of meters above hydrostatic equilibrium inactive ice stream c differs from active ice streams a and b in surficial and basal characteristics no elongated ridges and troughs are observed instead the ice stream surface exhibits several terraces including some maxima and minima in elevation radar sounding reveals areas where basal echoes are strong and steady indicating subglacial water alternating with areas of weaker echoes with short fading lengths calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick depending on the electrical conductivity of the water due to reversals in the surface slope the hydrologic potential gradient changes sign several times along ice stream c which implies that subglacial water should collect in some locations

geologic and topographic controls on fast flow in the laurentide and cordilleran ice sheets ice streams are fast flowing arteries which play a vital role in the dynamics and mass balance of present day ice sheets although not fully understood fast flow dynamics are intimately coupled with geologic topographic thermal and hydrologic conditions of the underlying bed these are difficult observables beneath contemporary ice sheets hindering elucidation of the processes which govern ice streaxn behavior for past ice sheets the problem is antithetic geologic evidence of former ice streams exists but spatim and temporal histories are uncertain however detailed knowledge of bed geology and topography is available in many places we take advantage of this information to compile terrain characteristics relevant to fast flow dynamics in the laurentide and cordilleran ice sheets using seed points where fast flowing wisconsinan ice has been geologically inferred discriminant analysis of a suite of north american geologic and topographic properties yields a concise measure of ice bed coupling strength our analysis suggests that the interior plains and continental shelf regions of north america have low basal coupling relative to areas of variable relief or exposed bedrock in the cordillera and on the canadian shield we conclude that the interior plains and continental shelves are both topographically and geologically predisposed to large scale basal flows i e ice streaxns or surge lobes this result holds independent of whether the mechanism of fast flow is sediment deformation or decoupled sliding over the bed

late glacial landforms of wollaston peninsula victoria island northwest territories product of ice marginal retreat surge and mass stagnation an analysis of glacial landforms on a regional scale leads to an interpretation of the dynamics of late wisconsinan glaciation on wollaston peninsula victoria island northwest territories the glacial record is dominated by four adjacent belts of landforms i ground moraine till plains and ice marginal drainage features ii hummocky moraine iii lateral and shear moraine and iv streamlined landforms the landform belts are considered as representing four distinct glacial ice conditions or regimes 1 ice margin retreat during extending flow of thin active ice 2 marginal ice stagnation following compressional flow 3 a surging ice margin producing massive shear moraines and 4 large scale flooding and mass ice stagnation following a surge these landform belts were arranged in zones by topographically controlled glacial dynamics the latter two defining a former ice stream glaciological inferences can be extended by examining the sediments and processes that produced each landform set

ice rafted debris associated with binge purge oscillations of the laurentide ice sheet the north atlantic sediment record suggests quasi periodic 7000 to 12 000 year period ice rafted debris ird depositions during at least the last glacial period the cause of these heinrich events as they are commonly known is not fully understood however they may point to surges of the ice stream that drained the hudson bay hudson strait region of the laurentide ice sheet we investigate a simple conceptual model of ice stream instability the binge purge model to suggest ways in which the ice stream could have entrained sufficient debris to account for the estimated mass of ird associated with a typical heinrich ird layer in the north atlantic 1 0 0 3 1015 kg we find that freezing of debris laden ice at the bed of the ice stream during the brief 750 years surge phase of the ice stream s hypothesized binge purge cycle can incorporate up to 5 1 1015 kg this amount is sufficient to meet the constraints of the north atlantic sediment record but by no means verifies the binge purge model as the cause of heinrich events

from morphostratigraphy to geochronology on the dating of ice marginal positions abstract this study focuses on the glacial landscape of northern germany as a part of the southwestern sector of the scandinavian ice sheet sis as it has a research history reaching back about 130 years since the glacial theory replaced the drift theory in the late 19th century it has experienced a number of key changes including the introduction of the concept of polyglaciation or the differentiation of ice advances within one glaciation which is closely linked to the morphostratigraphical definition of ice marginal positions with the development of numerical dating methods the rather static morphostratigraphically based chronologies which are deeply rooted in research history have been confronted with a growing number of geochronometrical data obtained from glacigenic deposits in order to date ice marginal positions the two most commonly used dating methods for that purpose are optically stimulated luminescence osl and surface exposure dating of erratic boulders this study highlights the dependency of the significance of results from different dating methods on the sampling position within the geomorphological framework results from osl and surface exposure dating for the same ice marginal position in theory cannot be the same as the methods date different processes in the development of the glacial landscape osl dating of fluvioglacial sandur sediments dating the process of active sedimentation at the ice front and surface exposure dating of boulders on end moraines dating the process of stabilisation of end moraines after the retreat of the ice front often qualify as the most useful combination of techniques in order to define a time interval for the formation of the landforms at a specific ice marginal position methodological drawbacks which might occur unfortunately tend to widen the process induced offset between osl and surface exposure ages incomplete bleaching of fluvioglacial sediments might cause an offset towards a maximum osl age for an ice marginal position whereas delayed stabilisation or post exposure relocation of boulders might cause a shift towards a minimum age of key importance are the influence of periglacial processes and the effects of melting of buried ice on landform stabilisation which remain to be quantified a possible solution to this problem combining osl and surface exposure dating is suggested in this study nevertheless the possibility exists to date different geomorphological processes by applying different dating methods to glacigenic deposits this allows for a differentiated reflection of morphostratigraphically based chronologies against the background of the modern concept of dynamic ice sheets in contrast to the traditional morphostratigraphical definition of ice marginal positions a time based definition is proposed because it offers the possibility of a more detailed reconstruction of ice sheet dynamics at a certain point in time

sedimentary imprints of former ice sheet margins insights from an end ordovician archive sw libya fromthe proterozoic to the quaternary the evolution of the earthwas characterised by recurrent periods of glaciation however the margins of many ancient ice sheets are poorly defined on palaeogeographic reconstructions the extent and outlines of ancient ice sheets can be better understood through careful documentation of sediments deposited at the ice sheet margin an outstanding example is provided herein based on an end ordovician archive in libya tihemboka area murzuq basin the four sets of structures include i subglacial glaciotectonic structures and soft sediment deformations from flowing glacier ice such as intraformational glacial striae intraformational deformation shear planes sheath folds normal microfaults and large scale glaciotectonic folds and thrusts ii structures resulting from overpressured subglacial meltwater flows such as clastic dykes and tunnel valleys iii proglacial depositional structures and facies related to highmagnitude meltwater floods such as sandstone intraclasts large scale bedforms resulting from supercritical flows climbing dune cross stratification and kettle holes and iv deformation structures resulting from free floating and nonglacier ice such as ice keel scours and ice crystal marks such a set of structures points to an ice marginal essentially continental depositional setting and provides an excellent suite of criteria to identify margins of ancient ice sheets in the stratigraphic record at a regional scale a reconstruction through time and space of the related depositional wedge is proposed this corresponded to a seismic scale n120 m in thickness 40 km in length ice marginal wedge in front of an essentially warm based ice sheet inducing concomitant large scale glaciotectonic deformation glacial basin and tunnel valley downcuttings the related ice front was associated with high energy meltwater flows feeding a network of deeply incised proglacial channels downstream and beyond them a fluvioglacial deltaic system shallow ice marginal permafrost most likely affected the depositional wedge at a larger scale the tihemboka ice marginal wedge is interpreted as related to a stillstand period over the gondwana platform developed over an estimated interval of a few thousands of years based on these data the conditions that arose in a particularly favourable context for the development the preservation and the identification of ice marginal wedges in the geological record are reviewed significant meltwater derived sediment deposition and aggradation in an accommodation space resulting either from preglacial inheritance glacial downcuttings and or glacio isostatic lithospheric flexure or active tectonic subsidence 1 myr are required for their formation and subsequent preservation

glacial curvilineations found along the southern sector of the laurentide ice sheet and a hypothesis of formation involving subglacial slope failure in tunnel valleys and subglacial lakes 2018 john wiley sons ltd first discovered in poland glacial curvilineations gcls are enigmatic landforms comprising parallel sets of sinuous ridges and troughs of metres amplitude and around 150m wavelength found within kilometres wide valleys interpreted as being produced by meltwater flowing subglacially their morphological and sedimentary characteristics and association with tunnel valleys has been described for some prominent polish examples from these observations the existing hypothesis is that they form as a consequence of erosion by longitudinal vortices that develop in subglacial floods here we report for the first time gcls found along the southern sector of the laurentide ice sheet in three northern states of the usa using mapping and topographic analysis from high resolution digital elevation models we report observations on their morphological properties and landform associations we find aspects of their context and morphology difficult to explain using the existing hypothesis we instead suggest that these glacial curvilineations are produced by subglacial bank and slope failures that locally widen tunnel valleys or that occur near subglacial lake shorelines further investigation is required to test this hypothesis and to ascertain the mechanisms of proposed mass movements which may have occurred by rotational or translational slope failure or by creep deformation our preferred mechanism is that such movements occurred where subglacial water was emplaced over previously perma frozen ground under such circumstances sediment blocks thawed by the water may then easily glide over a frozen decollement at low slope angles analogous to subaerial active layer glides in permafrost environments permafrost spring sapping may have provided lines of weakness for slope failure if the requirement for permafrost is found to hold then gcls may become an important indicator of the palaeo distribution of permafrost

phased occupation and retreat of the last british irish ice sheet in the southern north sea geomorphic and seismostratigraphic evidence of a dynamic ice lobe along the terrestrial margin of the southern north sea previous studies of the mis 2 glaciation impacting eastern britain have played a significant role in the development of principles relating to ice sheet dynamics e g deformable beds and the practice of reconstructing the style timing and spatial configuration of palaeo ice sheets these detailed terrestrially based findings have however relied on observations made from only the outer edges of the former ice mass as the north sea lobe nsl of the british irish ice sheet biis occupied an area that is now almost entirely submarine c 21 15 ka compounded by the fact that marine acquired data have been primarily of insufficient quality and density the configuration and behaviour of the last biis in the southern north sea remains surprisingly poorly constrained this paper presents analysis of a new integrated set of extensive seabed geomorphological and seismo stratigraphic observations that both advances the principles developed previously onshore e g multiple advance and retreat cycles and provides a more detailed and accurate reconstruction of the biis at its southern most extent in the north sea a new bathymetry compilation of the region reveals a series of broad sedimentary wedges and associated moraines that represent several terminal positions of the nsl these former still stand ice margins 1 4 are also found to relate to newly identified architectural patterns shallow stacked sedimentary wedges in the region s seismic stratigraphy previously mapped singularly as the bolders bank formation with ground truthing constraint provided by sediment cores these wedges are interpreted as sub marginal till wedges formed by complex subglacial accretionary processes that resulted in till thickening towards the former ice sheet margins the newly sub divided shallow seismic stratigraphy at least five units also provides an indication of the relative event chronology of the nsl while there is a general record of south to north retreat seismic data also indicate episodes of ice sheet re advance suggestive of an oscillating margin e g mis 2 maximum not related to first incursion of ice into region demonstrating further landform interdependence geographically grouped sets of tunnel valleys are shown to be genetically related to these individual ice margins providing clear insight into how meltwater drainage was organised at the evolving termini of this dynamic ice lobe the newly reconstructed offshore ice margins are found to be well correlated with previously observed terrestrial limits in lincolnshire and e yorkshire holderness e g mis 2 maximum and withernsea till this reconstruction will hopefully provide a useful framework for studies targeting the climatic mass balance and external glaciological factors i e fennoscandian ice sheet that influenced late stage advance and deglaciation important for accurately characterising both modern and palaeo ice sheets

what controls dead ice melting under different climate conditions a discussion abstract in the geological record hummocky dead ice moraines represent the final product of the melt out of dead ice processes and rates of dead ice melting in ice cored moraines and at debris covered glaciers are commonly believed to be governed by climate and debris cover properties here backwasting rates from 14 dead ice areas are assessed in relation to mean annual air temperature mean summer air temperature mean annual precipitation mean summer precipitation and annual sum of positive degree days the highest correlation was found between backwasting rate and mean annual air temperature however the correlation between melt rates and climate parameters is low stressing that processes and topography play a major role in governing the rates of backwasting the rates of backwasting from modern glacial environments should serve as input to de icing models for ancient dead ice areas in order to assess the mode and duration of deposition a challenge for future explorations of dead ice environments is to obtain long term records of field based monitoring of melt progression furthermore many modern satellite borne sensors have high potentials for recordings of multi temporal digital elevation models dems for detection and quantification of changes in dead ice environments in recent years high accuracy dems from airborne laser scanning altimetry lidar are emerging as an additional data source however time series of high resolution aerial photographs remain essential for both visual inspection and high resolution stereographic dem production

till stratigraphy and sedimentology at the margins of terrestrially terminating ice streams case study of the western canadian prairies and high plains complex glacigenic sediment sequences arranged in large arcuate assemblages and demarcating the lobate termini of fast ice flow corridors in western canada represent the terrestrial equivalents of the trough mouth fans of the marine palaeo ice stream record intensive sedimentological and stratigraphic investigations at a regional scale are employed in the interpretation of thick units or sequences of diamicton and two depositional scenarios are proposed first glacier marginal till thickening and the stacking of till wedges takes place in areas where an ice stream margin was stationary for short periods of time stacks of subglacial traction tills potentially record annual sub marginal incremental thickening at ice stream margins especially in areas where densely spaced recessional push moraines appear to record seasonal climatic forcing second proglacial lake and valley infilling with glacilacustrine rhythmites and mass flow diamictons is associated with the advection of sub marginal till into preglacial bedrock depressions and records ice stream marginal oscillations and debris influx into discontinuous proglacial lakes which act as subaqueous sediment sinks for materials that would otherwise have been used to construct sub marginal till wedges and moraines glacial overriding modifies the deposits produced in both scenarios through deformation and the dislocation and entrainment of bedrock and sediment rafts at the margins of bedrock valley walls resulting in the widespread development of glacitectonites and megablocks the occurrence of the thickest deposits as preglacial valley fills may be significant in groundwater siphoning away from subglacial deforming layers and the promotion of sticky spots

geomorphology and sedimentology of surging glaciers a land systems approach the identification of surging glaciers and ice streams in glaciated landscapes is of major importance to the understanding of ice sheet dynamics and for reconstructing ice sheets and climate no single landform or diagnostic criterion has yet been found with which to identify surging glaciers a surging glacier land system model is constructed using observations and measurements from contemporary surging glacier snouts in iceland svalbard u s a and canada for differentiating ancient surging margins from other non surging palaeoglaciers this integrates the suite of landforms sediments and stratigraphy produced at surging glacier margins landforms produced during surging include thrust moraines concertina eskers and subglacial crevasse squeeze ridges sedimentary sequences are usually characterized by multiple stacked diamictons and stratified interbeds which display severe glaciotectonic contortion and faulting hummocky moraine comprising interbedded stratified sediments and mass flow diamictons has also been associated with surge margins where large quantities of supraglacial and englacial debris entrained during the surge event have melted out in situ an example of the application of the land system model is presented for east central alberta canada a surging palaeo ice stream is identified within this part of the southwestern laurentide ice sheet where thrust block moraines crevasse squeeze ridges flutings hummocky moraine and glaciotectonized sediments are juxtaposed

geospatial analysis of controls on subglacial bedform morphometry in the new york drumlin field implications for laurentide ice sheet dynamics the outline and trend of 6566 subglacial bedforms in the new york drumlin field have been digitized from digital elevation data a spatial predictive model has been used to extend values of bedform elongation over an area measuring 200 km 110 km the resulting surface is used in conjunction with depth to bedrock data and an assumed duration of ice residence to test three proposed controls on bedform elongation upon comparison the resulting display of morphometry is best explained by differences in ice velocity across the field of study the existence of multiple zones of fast moving ice located along the southern margin of the laurentide ice sheet is implied by the observed patterns of bedform elongation and orientation we present two interpretations that are consistent with the observations first enhanced basal sliding caused by decreasing effective pressure near a calving margin is suggested as a possible mechanism by which localized fast ice flow is initiated and maintained second topographically controlled ice streams likely occupied the fjord like troughs of the appalachian upland northern rim contrary to previous understanding of the laurentide southern margin in new york state the resulting palaeoglaciological reconstruction illustrates a dynamic mosaic of ice stream and or outlet glacier activity copyright 2009 john wiley sons ltd

nature and origin of corrugated ground moraine of the des moines lobe story county iowa abstract corrugated ground moraine in story county iowa consists of low relief 1 2 m ridges which most commonly consist of late wisconsinan till the average spacing between ridges is about 105 m and they generally parallel the configuration of the bemis moraine the terminal moraine of the des moines glacial lobe till fabric pebble orientations and anisotropy of magnetic susceptibility facies variations and landform fabric analyzed at three sites in typical corrugated ground moraine landscape near ames in story county iowa indicate that at sites 1 and 2 the ridges consist of till probably deposited by lodgement at site 3 two ridges consist of till and massive to cross bedded sand till at site 3 is also probably lodgement till the nature of the till and related glaciofluvial sediments at each site suggests that the corrugation ridges formed in basal cracks or crevasses in the des moines lobe the cracks are postulated to have formed during extending flow of the des moines lobe as it advanced toward its terminal position retreat of the des moines lobe was rapid enough to preclude significant accumulations of supraglacial sediment upon corrugated ground moraine

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

sedimentological record of subglacial conditions and ice sheet dynamics of the vistula ice stream north central poland during the last glaciation abstract deposits of the vistula ice stream draining the scandinavian ice sheet during the last glaciation were investigated at four field sites in north central poland using micro and macroscale features the study reveals several till units with specific structural textural and lithological characteristics the individual till units are either macroscopically massive or bedded and the contacts between the units are either sharp or transitional the nature of the contacts with the underlying sediments ductile deformation structures largely undeformed clayey clasts tectonic lamination thin horizontal stringers of sorted sediments ploughing marks boulder pavements and striated upper surfaces of pebbles in the till indicate both bed deformation and enhanced basal sliding under high subglacial water pressure conditions it is suggested that the till is a hybrid deposit generated by some combination of lodgement deformation and ploughing punctuated by periods of basal decoupling the depth of deformation at any point in time was thinner up to several decimetres than the maximum till thickness c 2 5 m the ice sliding velocity estimations indicate velocities of less than 100 to over 2000 m yr 1 which suggests an unstable and highly dynamic ice lobe consistent with spatial variability of till characteristics sand wedges in the deposits beneath the till and the nature of the till bed interface indicate permafrost under the advancing ice sheet we suggest that under the increasing ice thickness a layer of thawed water saturated sediment formed on top of the still frozen ground due to inefficient drainage and contributed to ice streaming by promoting pervasive deformation and basal sliding

development of the topography controlled upper odra ice lobe scandinavian ice sheet in the fore mountain area of southern poland during the saalian glaciation abstract during the saalian drenthe glaciation the upper odra ice lobe constituted the most southern part of the scandinavian ice sheet in central europe the lobe was situated in an area with varied topography in the foreland of a mountain range the ice lobe had a complex form because it was subdivided into several smaller sublobes the primary factors that controlled the lobe were investigated it was found that the ice lobe development depended largely on the topography but the lithology of the ice sheet substratum was also highly important the topography controlled the main movement direction of the ice and caused a concentration of the ice flow in the axial part of the silesian lowland between subglacial morphological barriers which thereby formed a corridor of relatively fast flowing ice which was probably streaming and which supplied the lobe the topography also determined the ice flow within the lobe which strongly matched the valley system the presence of sediments with relatively low permeability at the base of the lobe the quaternary deposits are underlain by neogene clays caused high pore water pressure in the subglacial zone and triggered the basal movement of the ice sheet which promoted a rapid flow of the ice and favoured the development of the lobe the basal tills deposited from the active and passive ice and extant till patches without a fluvioglacial cover atop some parts of the studied area indicate spatially varying ice flow dynamics especially in the later stage of glaciation certain parts of the lobe underwent stagnation while in other parts the ice was probably still active the varied behaviour of the ice within the lobe was determined primarily by the topography and lithology of the substratum which produced spatially varied marginal compression and basal friction a model of the development of the lobe is proposed

uncovering the hidden part of a large ice stream of the laurentide ice sheet northern ontario canada abstract this investigation was prompted by an enigmatic ice flow anomaly area a on the glacial map of canada which covers about 10 000 km 2 in the hearst kapuskasing area of northeastern ontario it consists of streamlined landforms and striations indicative of a major ice flow toward 130 oriented at right angle to another toward 220 both are late glacial flows but long lasting disagreement exists regarding their relative age the analysis of aerial photographs and satellite images in conjunction with a detailed survey of bedrock cross striated surfaces over an area of about 30 000 km 2 within and around area a clearly indicate that the 130 flow preceded the 220 flow the earlier conflicting interpretations within area a are attributed mainly to the sporadic occurrence of relict striated surfaces formed by older southwestward 220 240 wisconsinan ice flows that have locally escaped destruction by late glacial flows with the result that the southwestward flows are older wisconsinan at some sites and younger late glacial 220 at others relative to the 130 flow when considered with other factors such as the maximum elevation reached by the youngest late glacial flow these ice flow relationships indicate that area a is the outcropping southern part of a much larger ese ice flow system which is probably related to a large fluted belt located to the north and that was identified as the winisk ice stream the distal part of the ice stream except for area a escaped detection by remote sensing mapping methods because depositional and erosional features associated with it are masked by deposits laid down by the younger 220 cochrane ice flow and or by postglacial marine and organic deposits or were destroyed by the younger ice flow the only reliable indicators of the passage of the ice stream in this buried section are ese relict striations crossed by sw striations the advancing ice stream toward the ese not only preceded the late cochrane 220 flow but probably outlasted it as suggested by the mapping of several thousand iceberg furrows in quebec and ontario also directed toward the ese and overprinted on flutes formed by the last glacial flow if this interpretation is correct this makes the winisk ice stream the largest terrestrial ice stream in the hudson bay basin an alternative interpretation associates this fluted belt to the exposed western fringe of streamlined bedforms from a late hudson lobe buried by younger sediments

large scale reorganization and sedimentation of terrestrial ice streams during late wisconsinan laurentide ice sheet deglaciation glacial geomorphological mapping of shuttle radar topography mission srtm data from the western canadian prairies demonstrates that during the last late wisconsinan deglaciation of the laurentide ice sheet terrestrial ice streams underwent a major reorganization of their flow configuration this reorganization involved a 90 shift in flow direction and was accompanied by a corresponding increase in the influence of topography on streaming flow ice streams included both topographically confined and pure ice streams that flowed independent of topography streaming flow is recorded by suites of highly elongate 60 km long subglacial bedforms bounded sharply at their lateral margins by prominent moraines initial streaming flow was unconfined by topography but was replaced progressively and crosscut by younger topographically confined flows flow reorganization is inferred to have been caused by temporal and spatial variations in the interaction between frozen and thawed bed conditions with thinning and shutdown of one ice stream ultimately triggering initiation of others this highlights the role of internal glaciodynamically driven reorganization in triggering streaming flow within large ice sheets and shows that large scale flow reorganization can occur over the time scale of a single deglaciation in terrestrial ice streams

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

sedimentary record of a scandinavian ice sheet drainage system and till deposition over subglacial obstacles promoting basal sliding an example from southern poland abstract subglacial obstacles occurring in the path of advancing ice sheets generally generate higher longitudinal compression and higher frictional drag than a flat substrate however in the case of a soft sediment substratum they can have a very different effect on ice sheet behaviour this study concerns a substrate composed of very fine grained sediments with low permeability the relationship between subglacial obstacles and the overriding scandinavian ice sheet was studied in an area of southern poland where a small intervalley neogene clay ridge 40 m high was present based on sedimentological and structural analysis of subglacial till and gravelly sandy sediments the basal depositional processes and subglacial conditions and their influence on ice sheet behaviour were analysed the till and related deposits within the ridge reflecting high water pressure conditions and lack of glacitectonic deformations indicate that the clay ridge did not generate much resistance against the advancing ice sheet but instead favoured basal slip the impermeable substratum weakened the ice bed coupling and promoted ice detachment from the substratum gravelly sandy inclusions at the till clay contact indicate that during the first stage of ice sheet overriding a canal drainage system developed at the ice substrate interface varied geometry size and location of inclusions of sorted sediments suggest periodic instability of the canal system which could lead to its transformation from initially uniform to being composed of conduits of different sizes during later stages of ice sheet overriding a traction till was deposited and occasional drainage through a water film was sufficient to evacuate basal meltwater the resulting change in the character of subglacial drainage was probably related to variations in water pressure gradient during progressive ice sheet advance

reconciling records of ice streaming and ice margin retreat to produce a palaeogeographic reconstruction of the deglaciation of the laurentide ice sheet abstract this paper reconstructs the deglaciation of the laurentide ice sheet lis including the innuitian ice sheet from the last glacial maximum lgm with a particular focus on the spatial and temporal variations in ice streaming and the associated changes in flow patterns and ice divides we build on a recent inventory of laurentide ice streams and use an existing ice margin chronology to produce the first detailed transient reconstruction of the ice stream drainage network in the lis which we depict in a series of palaeogeographic maps results show that the drainage network at the lgm was similar to modern day antarctica the majority of the ice streams were marine terminating and topographically controlled and many of these continued to function late into the deglaciation until the ice sheet lost its marine margin ice streams with a terrestrial ice margin in the west and south were more transient and ice flow directions changed with the build up peak phase and collapse of the cordilleran laurentide ice saddle the south eastern marine margin in atlantic canada started to retreat relatively early and some of the ice streams in this region switched off at or shortly after the lgm in contrast the ice streams draining towards the north western and north eastern marine margins in the beaufort sea and in baffin bay appear to have remained stable throughout most of the late glacial and some of them continued to function until after the younger dryas yd the yd influenced the dynamics of the deglaciation but there remains uncertainty about the response of the ice sheet in several sectors we tentatively ascribe the switching on of some major ice streams during this period e g m clintock channel ice stream at the north west margin but for other large ice streams whose timing partially overlaps with the yd the drivers are less clear and ice dynamical processes rather than effects of climate and surface mass balance are viewed as more likely drivers retreat rates markedly increased after the yd and the ice sheet became limited to the canadian shield this hard bed substrate brought a change in the character of ice streaming which became less frequent but generated much broader terrestrial ice streams the final collapse of the ice sheet saw a series of small ephemeral ice streams that resulted from the rapidly changing ice sheet geometry in and around hudson bay our reconstruction indicates that the lis underwent a transition from a topographically controlled ice drainage network at the lgm to an ice drainage network characterised by less frequent broad ice streams during the later stages of deglaciation these deglacial ice streams are mostly interpreted as a reaction to localised ice dynamical forcing flotation and calving of the ice front in glacial lakes and transgressing sea basal de coupling due to large amount of meltwater reaching the bed debuttressing due to rapid changes in ice sheet geometry rather than as conveyors of excess mass from the accumulation area of the ice sheet at an ice sheet scale the ice stream drainage network became less widespread and less efficient with the decreasing size of the deglaciating ice sheet the final elimination of which was mostly driven by surface melt

geomorphology and till architecture of terrestrial palaeo ice streams of the southwest laurentide ice sheet a borehole stratigraphic approach abstract a multidimensional study utilising geomorphological mapping and the analysis of regional borehole stratigraphy is employed to elucidate the regional till architecture of terrestrial palaeo ice streams relating to the late wisconsinan southwest laurentide ice sheet detailed mapping over a 57 400 km2 area of southwestern saskatchewan confirms previous reconstructions of a former southerly flowing ice stream demarcated by a 800 km long corridor of megaflutes and mega scale glacial lineations ice stream 1 and cross cut by three formerly southeast flowing ice streams ice streams 2a b and c analysis of the lithologic and geophysical characteristics of 197 borehole samples within these corridors reveals 17 stratigraphic units comprising multiple tills and associated stratified sediments overlying preglacial deposits the till thicknesses varying with both topography and distance down corridor reconciling this regional till architecture with the surficial geomorphology reveals that surficial units are spatially consistent with a dynamic switch in flow direction recorded by the cross cutting corridors of ice streams 1 2a b and c the general thickening of tills towards lobate ice stream margins is consistent with subglacial deformation theory and variations in this pattern on a more localised scale are attributed to influences of subglacial topography including thickening at buried valley margins thinning over uplands and thickening in overridden ice marginal landforms

evidence for a recently abandoned shear margin adjacent to ice stream b2 antarctica from ice penetrating radar measurements satellite imagery of the area between ice streams b1 and b2 antarctica shows a lineation on the surface of the ice sheet of uncertain origin ice motion in the area 2 m yr 1 is 2 orders of magnitude slower than that of the surrounding streams and shows no significant variation that could explain the feature a low power high resolution radar system was used to image the upper 80 m of the ice sheet between the two ice streams the survey shows that the lineation is associated with what is likely an abandoned shear margin the radar data show that a set of chaotic diffractors lies buried beneath two thirds of the area while the remaining one third is undisturbed to 80 m depth the chaotic ice ends abruptly along a boundary that is parallel to but offset 2 5 km from the surface lineation also isolated linear diffractors are commonly observed in the otherwise undisturbed ice immediately adjacent to the boundary the depth location orientation and curved form of the diffractors strongly suggest they are the tops of crevasses that were active at the time the chaotic ice was being strained and that they were formed by left lateral shear this is the same sense of shear that is presently active in the b2 margin 6 km away the depth to the chaotic diffractors suggests that the shear margin abandoned its prior position 190 years b p the burial depth decreases toward the b2 margin and suggests a migration rate of 100 m yr 1 in addition a separate high power radar system was used to image the entire thickness of the ice sheet 1 km in the same area these data show numerous linear diffractors near the base of the ice sheet it is very likely that at least some of these diffractors are entrained morainal debris others may be bottom crevasses or zones of wet reflective ice that developed in the high strain environment of the now abandoned shear margin

ice dynamics at the mouth of ice stream b antarctica field data collected at the mouth of ice stream b show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream surface slopes in this region are as low as ice shelf surface slopes yet with the exception of patches of ice which may be floating the ice is grounded basal shear stress is negligible the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress the surface is generally crevasse free features similar to ice rises are observed upstream of the grounding line their origin is uncertain but they move at velocities comparable to the surrounding ice the flow is laterally extensive and longitudinally compressive but there are large local variations of the strain rate from the regional trends the boundary between the two major tributaries to ice stream b followed with the radar is characterized by a band of strain rates much smaller than average detailed measurements at the downstream b network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates the strain rates indicate that the undulating topography is locally generated the lower elevation ice is thicker and moves faster a velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear a calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate the discharge of ice stream b does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin

landform and sediment imprints of fast glacier flow in the southwest laurentide ice sheet evidence for former fast glacier flow ice streaming in the southwest laurentide ice sheet is identified on the basis of regional glacial geomorphology and sedimentology highlighting the depositional processes associated with the margin of a terrestrial terminating ice stream preliminary mapping from a digital elevation model of alberta identifies corridors of smoothed topography and corridor parallel streamlined landforms megaflutes to mega lineations that display high levels of spatial coherency ridges that lie transverse to the dominant streamlining patterns are interpreted as a series of minor recessional push moraines b thrust block moraines or composite ridges hill hole pairs constructed during readvances surges and c overridden moraines cupola hills apparently of thrust origin together these landforms demarcate the beds and margins of former fast ice flow trunks or ice streams that terminated as lobate forms localised cross cutting and or misalignment of flow sets indicates temporal separation and the overprinting of ice streams lobes the fast flow tracks are separated by areas of interlobate or inter stream terrain in which moraines have been constructed at the margins of neighbouring competing ice streams outlet glaciers this inter stream terrain was covered by more sluggish non streaming ice during full glacial conditions thin tills at the centres of the fast flow corridors in many places unconformably overlying stratified sediments suggest that widespread till deformation may have been subordinate to basal sliding in driving fast ice flow but the general thickening of tills towards the lobate terminal margins of ice streams outlet glaciers is consistent with subglacial deformation theory in this area of relatively low relief we speculate that fast glacier flow or streaming was highly dynamic and transitory sometimes with fast flowing trunks topographically fixed in their onset zones and with the terminus migrating laterally the occurrence of minor push moraines and flutings and associated landforms because of their similarity to modern active temperate glacial landsystems are interpreted as indicative of ice lobe marginal oscillations possibly in response to seasonal climatic forcing in locations where meltwater was more effectively drained from the glacier bed further north the occurrence of surging glacier landsystems suggests that persistent fast glacier flow gave way to more transitory surging possibly in response to the decreasing size of ice reservoir areas in dispersal centres and also locally facilitated by ice bed decoupling and drawdown initiated by the development of ice dammed lakes copyright 2008 john wiley sons ltd

crevasse patterns and the strain rate tensor a high resolution comparison values of the strain rate tensor represented at a 20 m length scale are found to explain the pattern and orientation of crevasses in a 0 13 km 2 reach of worthington glacier alaska u s a the flow field of the reach is constructed from surveyed displacements of 110 markers spaced 20 30 m apart a velocity gradient method is then used to calculate values of the principal strain rate axes at the nodes of a 20 m 20 m orthogonal grid crevasses in the study reach are of two types splaying and transverse and are everywhere normal to the trajectories of greatest most tensile principal strain rate splaying crevasses exist where the longitudinal strain rate x is 0 and transverse crevasses are present under longitudinally extending flow i e x 0 the orientation of crevasses changes in the down glacier direction but the calculated rotation by the flow field does not account for this change in orientation observations suggest that individual crevasses represent local values of the regional flow field and are transient on the time scale of 1 2 years they are not persistent features that are translated and rotated by flow crevasse patterns are thus found to be a useful tool for mapping the strain rate tensor in this reach of a temperate valley glacier

propagation of a glacier surge into stagnant ice the propagation of the surge of variegated glacier into its terminal lobe was observed by daily surveying of a longitudinal line of closely spaced markers extending up glacier from a stagnant zone of thin 40 m nearly motionless ice near the terminus across a front zone to a surge zone of thicker 100 m rapidly moving 20 m d 1 ice above within the front zone there were large angles of upward motion 30 compressive longitudinal strain rates 0 1 d 1 and upward vertical velocities 5 m d 1 within the surge zone the ice motion was nearly parallel to the ice surface and longitudinal gradients were small the deformation pattern produced microcracking exfoliation and buckling of surface ice longitudinal cracking longitudinal chasms and an increase in glacier volume the front zone propagated at an average speed of about 40 m d 1 or roughly twice the surge speed of the ice above the front this factor is determined by the thickness jump across the front finite element modeling constrained by measured geometry and surface velocity was used to deduce the velocity and stress at depth the velocity in the surging zone occurs almost entirely by sliding the sliding rate drops abruptly beneath the center of the front zone to zero in the stagnant zone a comparison of basal velocity and shear stress distributions indicates a highly lubricated area beneath the upper part of the front zone and a locked area beneath the lower part of the front zone normal stress at the bed is not significantly different from overburden and there is no high compressive normal stress on the bed to act as a pressure dam to obstruct water flow through the front zone

glacial geology of western hudson strait canada with reference to laurentide ice sheet dynamics hudson strait is a large marine trough that connected the marine based interior of the laurentide ice sheet with the ocean field investigations of the glacial geology of western hudson strait have helped to define the role of the strait in laurentide ice sheet dynamics in general ice converged on western hudson strait from the north west and south upon reaching the strait ice flowing from foxe and ungava peninsulas turned eastward within less than 10 km of the coast to parallel eastwardly flowing ice in the strait glacial erosion of western hudson strait is identified as slight the texture carbonate content ph and the presence of several types of erratics distinguish till on nottingham salisbury and mill islands in western hudson strait from till on southern foxe peninsula and northern ungava peninsula till on the islands is characterized by higher ph and higher proportions of silt clay and carbonate than till on either adjacent peninsula in addition the presence of red sandstones porphyritic intrusives stromatolites omarolluks and iron formation are restricted to the islands with few exceptions that can be explained by ice rafting carbonate is present in the pebble and boulder fractions of till on foxe peninsula and in all fractions of till on the islands but is absent from till on ungava peninsula these data suggest that ice from distant and widely distributed source regions affected western hudson strait and along with evidence for a rapid change in ice flow direction along the margins of hudson strait suggest that the strait was occupied by a relatively fast moving ice stream that had a substantial impact on the interior of the ice sheet through downdraw

tsivat basin conduit system persists through two surges bering piedmont glacier alaska the 1993 1995 surge of bering glacier alaska occurred in two distinct phases phase 1 of the surge began on the eastern sector in july 1993 and ended in july 1994 after a powerful outburst of subglacial meltwater into tsivat lake basin on the north side of weeping peat island within days jokulhlaup discharge built a 1 5 km 2 delta of ice blocks 25 30 m buried in outwash by late october 1994 discharge temporarily shifted to a vent on weeping peat island where a second smaller outburst dissected the island and built two new sandar during phase 2 which began in spring 1995 and ended within five months continuous discharge issued from several vents along the ice front on weeping peat island before returning to the tsivat basin surge related changes include a five to six fold increase in meltwater turbidity the redirection of supercooled water in two ice contact lakes and an increase in the rate of glaciolacustrine sedimentation u s geological survey aerial photos by austin post show large ice blocks in braided channels indicating excessive subglacial discharge in a similar position adjacent to weeping peat island during the 1966 1967 surge during the subsequent three decades of retreat the location of ice marginal subglacial discharge vents remained aligned on a linear trend that describes the position of a persistent subglacial conduit system the presence of a major conduit system possibly stabilized by subglacial bedrock topography is suggested by 1 high level subglacial meltwater venting along the northern side of weeping peat island during the 1966 1967 surge 2 persistent low level discharge between surges and 3 the recurrence of localizing meltwater outbursts associated with both phases of the 1993 1995 surge

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

terrestrial ice streams a view from the lobe abstract the glacial landforms of minnesota are interpreted as the products of the lobate extensions of ice streams that issued from various ice sheds within the laurentide ice sheet low relief till plains trough shaped lowlands boulder pavements and streamlined forms make up the subglacial landsystem in minnesota that is interpreted as having been formed by streaming ice extremely uniform tills are created subglacially in a way that remains somewhat mysterious at the ice margins thrust moraines and hummocky stagnation topography are more common than single crested simple moraines if the ice lobes had repeated advances subglacial drainage features are obscure up ice but are present down ice in the form of tunnel valleys eskers spooner hills and associated ice marginal fans ice streaming may occur when basal shear stress is lowered as a result of high subglacial water pressure subglacial conditions that allow the retention of water will allow an ice lobe to extend far beyond the ice sheet as long as the ice shed also supports the advance by supplying adequate ice even with adequate ice flux however the advance of an ice lobe may be terminated at least temporarily if the subglacial water is drained through tunnel valleys or perhaps a permeable substrate thrust moraines and ice stagnation topography will result from sudden drainage although climate change is ultimately responsible for the accumulation of ice in the laurentide ice sheet the asynchronous advances and retreats of the ice lobes in the mid continent are strongly overprinted by the internal dynamics of individual ice streams as well as the interaction of ice sheds which obscure the climate signal

till beneath ice stream b 1 properties derived from seismic travel times seismic experiments conducted on ice stream b part of the marine ice sheet of west antarctica show a meters thick layer immediately beneath the 1000 m thick ice a seismic experiment consisting of wide angle reflection profiling along a line parallel to ice stream flow was conducted to determine the properties of this layer inversion of seismic travel times yields a compressional wave speed of less than 1700 m s 1 and a shear wave speed less than 160 m s 1 for the layer these very low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure based on wave speeds in other saturated unconsolidated sediments we believe that a porosity substantially greater than 0 32 probably around 0 4 and an excess of overburden pressure over pore pressure of only 50 kpa 0 5 bar characterize the layer at this location

the glacial origins of relict pingos wales uk ramparted depressions doughnut shaped debris cored ridges with peat and or sediment filled central basins are commonly perceived to represent the relict collapsed forms of permafrost ground ice mounds i e pingos or lithalsas in wales uk ramparted depressions of late pleistocene age have been widely attributed to permafrost related processes however a variety of alternative glacial origins for these enigmatic landforms are also consistent with the available geological and geomorphological evidence although previous studies have barely considered such alternative processes of formation from detailed geophysical sedimentological and remote sensing studies at two field sites we demonstrate that i the wastage of stagnating glacier ice is a viable alternative explanation for the formation of ramparted depressions in wales ii the glacial geomorphology and geology of these landforms is analogous to supraglacial and subglacial landforms from the last laurentide and fennoscandian ice sheets iii these landforms have significant potential for characterising the nature of deglaciation around the margins of the irish sea during the last glacial cycle and may record evidence for the overextension and stagnation of the south eastern margin of the irish sea ice stream and iv investigations of ramparted depressions within formerly glaciated terrains must consider both glacial and periglacial mechanisms of formation

solid phase arsenic speciation in aquifer sediments a micro x ray absorption spectroscopy approach for quantifying trace level speciation abstract arsenic as is a geogenic contaminant affecting groundwater in geologically diverse systems globally arsenic release from aquifer sediments to groundwater is favored when biogeochemical conditions especially oxidation reduction redox potential in aquifers fluctuate the specific objective of this research is to identify the solid phase sources and geochemical mechanisms of release of as in aquifers of the des moines lobe glacial advance the overarching concept is that conditions present at the aquifer aquitard interfaces promote a suite of geochemical reactions leading to mineral alteration and release of as to groundwater a microprobe x ray absorption spectroscopy xas approach is developed and applied to rotosonic drill core samples to identify the solid phase speciation of as in aquifer aquitard and aquifer aquitard interface sediments this approach addresses the low solid phase as concentrations as well as the fine scale physical and chemical heterogeneity of the sediments the spectroscopy data are analyzed using novel cosine distance and correlation distance hierarchical clustering for fe 1s and as 1s xas datasets the solid phase fe and as speciation is then interpreted using sediment and well water chemical data to propose solid phase as reservoirs and release mechanisms the results confirm that in two of the three locations studied the glacial sediment forming the aquitard is the source of as to the aquifer sediments the results are consistent with three different as release mechanisms 1 desorption from fe oxyhydr oxides 2 reductive dissolution of fe oxyhydr oxides and 3 oxidative dissolution of fe sulfides the findings confirm that glacial sediments at the interface between aquifer and aquitard are geochemically active zones for as the diversity of as release mechanisms is consistent with the geographic heterogeneity observed in the distribution of elevated as wells

multi phase evolution of gnammas weathering pits in a holocene deglacial granite landscape minnesota usa the morphometry of 85 gnammas weathering pits from big stone county in western minnesota allows the assessment of the relative ages of the gnamma population the ratio between maximum and minimum depths is independent of the initial size of the cavity and only depends on the weathering evolution therefore the distribution of depth ratios can be used to assess the gnamma population age and the history of weathering the asymmetrical distribution of depth ratios measured in big stone county forms three distinct populations when these sets are analyzed independently the correlation r2 between maximum and minimum depths is greater than 0 95 each single population has a normal distribution of depth ratios and the average depth ratios value for each population are 1 1 60 0 05 2 2 09 0 04 and 3 2 42 0 08 the initiation of gnamma formation followed the exhumation of the granite in the region this granite was till and saprolite covered upon retreat of the ice from the last glacial maximum nearby outcrops are striated but the study site remained buried until it was exhumed by paleofloods issuing from a proglacial lake these holocene aged gnammas in western minnesota were compared with gnammas of other ages from around the world our new results are in accordance with the hypothesis that values represent the evolution of gnammas with time under temperate to cold climate dynamics phases of the formation of new gnammas may result from changes in weathering processes related to climate changes copyright 2007 john wiley sons ltd

ice marginal thrusting of drift and bedrock thermal regime subglacial aquifers and glacial surges glacial thrust systems composed of blocks of drift and bedrock associated with hummocky stagnation moraine along the margin of the rainy lobe of the laurentide ice sheet in minnesota are used in

late pleistocene holocene retreat of the west antarctic ice sheet system in the ross sea part 2 sedimentologic and stratigraphic signature sedimentologic geotechnical geochemical and accelerator mass spectrometer ams radiocarbon data from two marine geologic cruises in the ross sea have allowed us to constrain facies relationships and temporal changes in the west antarctic ice sheet the selection of core sites was facilitated by the use of multibeam bottom imagery and a good quality chirp subbottom reflection system a complex but consistent succession of facies documents a number of environments from subglacial to open marine massive mud rich diamictons have low water contents contain in places a calcareous microfossil assemblage show minimal textural variation and contain low and uniform total organic carbon values this reflects a subglacial setting this unit passes upward into a stratified thin granulated facies consisting of pelletized sandy muddy gravel that is loosely compacted and contains a variable water content and concentrated horizons of pebble sized clasts this facies reflects the lift off zone or thin water film between the basal debris and sea floor overlying this unit are silty clays that contain a well sorted very fine grained sand component there are no coarse grains within this unit this facies is in part laminated and reflects deposition beneath an ice shelf near and away from the grounding line zone the ice shelf facies passes upward into a siliceous mud and ooze unit that represents deposition in an open marine setting a sandy volcaniclastic rich subfacies marks the transition from ice shelf to open marine environments found at the calving line the 86 ams radiocarbon dates on organic matter provide an accurate chronology for 19 cores ice shelf conditions were established in the outer drygalski trough by 11 0 25 ka and perhaps earlier this transition took place in the joides basin by 10 8 ka the calving front of the ross ice shelf passed over the drygalski trough at 74 s by 9 5 0 25 ka the timing of deglaciation in the ross sea calls into question current models for the contribution of antarctic glacial ice to holocene sea level rise and suggests that recession was relatively gradual and more closely aligned with northern hemisphere deglaciation and its associated eustatic pulse

subglacial conditions and scandinavian ice sheet dynamics at the coarse grained substratum of the fore mountain area of southern poland abstract the fore mountain areas of southern poland are locally composed of the coarse grained sediments of alluvial fans which created unusual conditions under the advancing scandinavian ice sheet during the elsterian glaciation this highly permeable substratum potentially enabled rapid outflow of meltwater from the ice sheet base thereby reducing the water pressure and strongly influencing the ice sheet dynamics the subglacial conditions and the relationship between the ice sheet behaviour and its coarse grained substratum were studied at the foreland of the western carpathian mountains the sedimentological and structural analysis of the till and related sediments that were deposited above the alluvial gravel of the fore mountain fans are presented the study indicates that despite the high permeability of the coarse grained substratum it did not slow the ice sheet movement conversely the ice sheet moved mainly due to basal slip and locally shallow deformations this was a consequence of very high basal water pressure which resulted largely from the presence of permafrost that restricted subglacial groundwater outflow in addition the ice sheet substratum was inclined opposite to the direction of its movement increasing the pressure of the subglacial water numerous subhorizontal sandy laminae within the till indicate that the meltwater from the ice sheet base was drained by a water film along the ice bed interface the water escape structures within the till and subtill sediments indicate the occasional instability of the ice sheet hydrological system and suggest that the meltwater was periodically stored in the ice sheet base temporal changes occurring in the ice sheet hydrological system might indicate variations in the ice sheet behaviour i e phases of relatively fast ice flow and phases of ice stagnation the latter were probably correlated with the freezing of the ice margin to its base the study shows how the coarse grained substratum could potentially influence ice sheet behaviour and provides a broader look on the factors controlling the dynamics of the pleistocene ice sheets which soft sediment substrata in many areas were texturally varied

influence of subglacial geology on the position of a west antarctic ice stream from seismic observations ice streams drain much of the interior west antarctic ice sheet and buffer the main ice reservoir from oceanic influences1 2 the slow flowing interior feeds the floating ross ice shelf with ice via fast flowing ice streams3 that are believed to modulate sea level change through their control of inland ice storage understanding ice stream behaviour and predicting the response to climate change4 requires a better knowledge of the subglacial geology5 6 it is known that a thawed ice bed and high pressure basal water are necessary but not sufficient conditions to cause ice streaming7 8 moreover it has been hypothesized that a soft sedimentary bed is also required because of its intrinsic low frictional resistance to flow9 and owing to its high erodibility so as to generate till that can deform and lubricate ice motion10 11 or to bury rough features and smooth the bed for sliding here we use seismic observations to provide evidence that one margin of the upglacier part of an ice stream is directly above the boundary of a basin with such sedimentary fill the ice stream is within the basin and the ice outside the basin is slow flowing the basin fill presents an order of magnitude lower frictional resistance to ice flow than the subglacial material outside the basin we conclude that the ice stream position is dependent on subglacial geology

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

record of the vistula ice lobe advances in the late weichselian glacial sequence in north central poland during the last glacial maximum lgm the vistula ice lobe was one of the key morphological elements at the southern margin of the scandinavian ice sheet in north central poland the area of the vistula ice lobe experienced two ice sheet advances of varied extent the older one is correlated with the leszno brandenburg phase and the younger one is correlated with the poznan frankfurt phase sedimentological research and luminescence dating of the late weichselian glacial sequence in the vistula ice lobe area was conducted a new scenario of the late weichselian ice events in the area is presented the limit of the leszno phase within the vistula ice lobe was much smaller than had been accepted previously and its age is estimated to be about 20 300 bp significant ice sheet retreat was

changes in the configuration of ice stream flow from the west antarctic ice sheet surface based ice penetrating radar profiles on the northeast flank of siple dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream the scar defines the southwestern boundary of an ice stream flowing from ice stream c to ice stream d close to where it enters the ross ice shelf our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across underlain by a zone of disturbed internal stratigraphy revealed by the radar burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1 3 ka the configuration of the ice streams draining the west antarctic ice sheet into the ross ice shelf evidently changes with time and attempts to predict the evolution of the ice sheet must incorporate this observation

episodes of tidally forced swale erosion on the inner shelf interspersed with millennial fluviodeltaic progradational interludes insights from northern bohai bay china abstract erosional swales are common features on the seafloor with vigorous bottom flow s on continental shelves with energetical tidal currents and limited river discharge the initiation and evolution of tidally induced erosional depressions and bedforms have been extensively discussed but little is known about the evolution of such features when interspersed with active deltaic progradation over millennial timescales a recent boomer seismic investigation revealed a swale erosional complex within and proximal to the subaqueous delta in northern bohai bay nbhb china providing us with an opportunity to investigate such features here using the same seismic dataset we analyze the morphology reflective characteristics and evolution of the erosional and depositional structures the erosional swale complex consists of two episodes of tidally induced incised channels within a northern progradational system nps located in the northernmost bohai bay and prograde to the southeast followed by an episode of extensive trough and bump formation negative and positive relief within the middle and northern nbhb which are also erosional features these features are all highly lineated oriented wnw ese we attribute their formation to the similarly oriented tidal flows in this region the multi episode evolution of the swale erosional complex is predominately associated with progradation of the nps whose lobes cap the pre existing erosional features and provide substrate for new erosion the trough and bump complex was subject to long term erosion and may represent an evolution from the previous linear tidally forced marine incisions the complex was subsequently buried in part by the step wise progradation of a southern progradational system sps developed within the southern 2 3 bohai bay and prograde to the north northeast in addition we also observed clusters of longitudinal bedforms with convex internal reflectors superimposed on the erosional remnant bumps coeval with the erosion dominated environment they are observable on both sandy and muddy substrates

the last and penultimate glaciation in the north alpine foreland new stratigraphical and chronological data from the salzach glacier abstract the northern alpine foreland was repeatedly covered by massive piedmont glaciers during quaternary glacial maxima the salzach palaeoglacier lobe austria germany was the easternmost of a series of pleistocene piedmont glaciers entering the foreland through major alpine valleys it covered an area of more than 1000 km2 during at least four glacial maxima here we aim to bring more light into its history by analyzing multiple drill log data two major outcrops topographic data and absolute chronologies of sediments stratigraphic and lithofacies investigations are focused on proximal i e near the lobe axis and distal i e near terminal moraine deposits of the salzach glacier lobe the glacial carving into the miocene bedrock occurred during early glacial maxima and was rather uniform across the lobe with larger values only in the proximal parts of the glacier more than 100 m of accumulated sediments during later glacial maxima indicate a change in ice sheet dynamics and a characteristic sequence development which varies from proximal to distal lobe positions new luminescence ages suggest a depositional focus at the penultimate glacial period while the impact of the lgm was rather minor sediments of gravelly braided rivers dominate the proximal parts of the former lobe where meltwater discharge was generally high while sheetflood deposits dominate the distal near terminal lobe positions

sedimentary processes at the base of a west antarctic ice stream constraints from textural and compositional properties of subglacial debris samples of sediments from beneath ice stream b at camp upb west antarctica provide the first opportunity to study the relationship between sediment properties and physical conditions in a sub ice stream environment piston coring in holes bored by hot water drilling yielded five 1 3 m long undisturbed subglacial sediment cores we analyzed granulometry composition and particle morphology in these cores the upb cores are composed of a clay rich unsorted diamicton containing rare marine diatoms sedimentary particles in these cores bear no evidence of the recent crushing or abrasion that is common in other subglacial sedimentary environments the presence of reworked diatoms and their state of preservation as well as the relative spatial homogeneity of this diamicton suggest that the upb cores sampled a several meter thick till layer and not in situ glacimarine sediments the till does incorporate material recycled from the subjacent poorly indurated tertiary glacimarine sediments of the ross sea sedimentary basin which extends beneath this part of the west antarctic ice sheet we propose that the lack of significant comminution in the upb till is ultimately due to its setting over these easily erodible clay rich source sediments the resulting fine grained till matrix inhibits glacial comminution because it facilitates buildup of high pore water pressures and hinders interparticle stress concentrations our observations are consistent with the conjecture that subglacial deformation of weak fine grained tills does not produce significant comminution of till debris elson 1988 based on our findings we hypothesize that extensive layers of weak till may develop preferentially where ice overrides preexisting poorly indurated fine grained sediments since such weak till layers create a permissive condition for ice streaming sub glacial geology may have an indirect but strong control over the location extent and basal mechanics of ice streams

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