seismic reflection borehole and outcrop geometry of late wisconsin tills at a proposed landfill near toronto ontario the search for new landfill sites in the greater toronto area of southern ontario canada is producing a wealth of data regarding the subsurface stratigraphy and geometry of late wisconsin 25 ka

the stratigraphic and sedimentological significance of late devensian ice sheet surging in holderness yorkshire u k amino acid analyses of marine mollusc valves show that the basement till of holderness yorkshire is of late devensian age ca 20 000 bp and not pre ipswichian 125 000 bp as is traditionally supposed together with the overlying skipsea and withernsea tills the basement till is argued to be a deformation till resulting from the repeated onshore surging of late devensian ice over a muddy sea floor and the subglacial transport attenuation and mixing of marine sediment silts on the surface of the basement till yielding 14c dates of 18 500 and 18 240 bp have been regarded as minimum ages for late devensian glaciation in eastern england e g the dimlington stadial of rose 1985 1989 the amino acid data presented here indicates that the maximum of the late devensian glaciation in eastern england occurred earlier sedimentological interpretation of the late devensian tills of holderness as glacially reworked marine sediments supports previous glaciological models of the british ice sheet involving surging of ice lobes along the coast of eastern england the basement skipsea and withernsea tills are separated by shallow marine sediments and may be representative of deformable bed till facies deposited below pleistocene ice sheets moving over soft fine grained sediments

surface melt driven seasonal behaviour englacial and subglacial from a soft bedded temperate glacier recorded by in situ wireless probes we investigate the spatial and temporal englacial and subglacial processes associated with a temperate glacier resting on a deformable bed using the unique glacsweb wireless in situ probes embedded in the ice and the till combined with other techniques including ground penetrating radar gpr and borehole analysis during the melt season spring summer and autumn high surface melt leads to high water pressures in the englacial and subglacial environment winter is characterised by no surface melting on most days base apart from a series of positive degree days once winter begins a diurnal water pressure cycle is established in the ice and at the ice sediment interface with direct meltwater inputs from the positive degree days and a secondary slower englacial pathway with a five day lag this direct surface melt also drives water pressure changes in the till till deformation occurred throughout the year with the winter rate approximately 60 that of the melt season we were able to show the bed comprised patches of till with different strengths and were able to estimate their size relative percentage and temporal stability we show that the melt season is characterised by a high pressure distributed system and winter by a low pressure channelized system we contrast this with studies from greenland overlying rigid bedrock where the opposite was found we argue our results are typical of soft bedded glaciers with low englacial water content and suggest this type of glacier can rapidly respond to surface driven melt based on theoretical and field results we suggest that the subglacial hydrology comprises a melt season distributed system dominated by wide anastomosing broad flat channels and thin water sheets which may become more channelized in winter and more responsive to changes in meltwater inputs

the interrelation of glaciotectonic and glaciodepositional processes within the glacial environment abstract in recent years it has been recognised that ice sediment coupling occurred beneath the quaternary ice sheets that advanced over the soft sediments of lowland areas this paper looks in detail at the effects of this coupling on the sediments which results in glaciotectonic deformation and also discusses the interaction of deformation and deposition within the subglacial environment two types of glaciotectonic deformation are discussed that are produced by the active movement of ice 1 proglacial tectonics at the margin which include compressive fold styles such as listric thrusts and faults and open folding 2 subglacial tectonics formed beneath the glacier which include fold styles resulting from simple shear and represent a soft rock purely dynamic shear zone styles of deformation associated with stagnant ice are also investigated we argue that glaciotectonic deformation is a common phenomenon and an integral part of geological record from continental ice sheets it is suggested that zones of deformation within the sediment are related to similar zones of strain in the ice sheet and complex deformation sequences are produced by the superimposition of these differing styles upon one another as the ice sheet advances and retreats it is also argued that subglacial deposition and deformation are related and that on soft beds underformed till is rare whilst deformed till is very common

effects of basal debris on glacier flow glacier movement is resisted partially by debris either within glaciers or under glaciers in water saturated layers in experiments beneath a thick sliding glacier ice containing 2 to 11 debris exerted shear traction of 60 to 200 kilopascals on a smooth rock bed comparable to the total shear traction beneath glaciers and contrary to the usual assumption that debris bed friction is negligible imposed pore water pressure that was 60 to 100 of the normal stress in a subglacial debris layer reduced shear traction on the debris sufficiently to halt its deformation and cause slip of ice over the debris slip resistance was thus less than debris shearing resistance

genesis of streamlined landforms and flow history of the green bay lobe wisconsin usa abstract the distribution of streamlined landforms in southeastern wisconsin suggests that drumlins and flutes formed during several phases of the green bay lobe between 18 and 14 000 bp the largest group of drumlins formed during a still stand of the ice margin during the johnstown phase presumably about 18 16 000 bp flutes and smaller drumlins are superimposed on larger forms and larger drumlins are remolded this indicates that drumlin modification continued during retreat three fields containing smaller drumlins formed after ice re advanced a short distance or stabilized during the green lake rush lake and st anna phases about 16 14 000 bp the drumlin forming process included erosion and deformation of pre existing ice marginal and proglacial sediments drumlins associated with the johnstown phase show an increase in length up glacier this is probably the result of variations in flow velocity length of time of drumlin formation and sediment availability flutes and small drumlins are associated with retreat moraines and formed near a thin retreating ice margin ice surface profile reconstructions suggest that margins were relatively steep during the johnstown milton green lake and early rush lake phases during the lake mills and late rush lake phases when flutes were forming the ice surface slopes were lower and margins were retreating during retreat minor sublobes developed some perhaps resulting from surges into small proglacial lake basins thus steep ice margins and driving stresses on the order of 15 25 kpa were typical during drumlin formation the formation of flutes and the remolding of larger drumlins into smaller forms took place beneath gently sloping ice of retreating margins associated with lower driving stresses

microscale evidence of liquefaction and its potential triggers during soft bed deformation within subglacial traction tills published conceptual models argue that much of the forward motion of modern and ancient glaciers is accommodated by deformation of soft sediments within the underlying bed at a microscale this deformation results in the development of a range of ductile and brittle structures in water saturated sediments as they accommodate the stresses being applied by the overriding glacier detailed micromorphological studies of subglacial traction tills reveal that these polydeformed sediments may also contain evidence of having undergone repeated phases of liquefaction followed by solid state shear deformation this spatially and temporally restricted liquefaction of subglacial traction tills lowers the shear strength of the sediment and promotes the formation of transient mobile zones within the bed which accommodate the shear imposed by the overriding ice this process of soft bed sliding alternating with bed deformation facilitates glacier movement by way of stick slip events the various controls on the slip events have previously been identified as i the introduction of pressurised meltwater into the bed a process limited by the porosity and permeability of the till and ii pressurisation of porewater as a result of subglacial deformation to which we include iii episodic liquefaction of water saturated subglacial traction tills in response to glacier seismic activity icequakes which are increasingly being recognized as significant processes in modern glaciers and ice sheets as liquefaction operates only in materials already at very low values of effective stress its process form signatures are likely indicative of glacier sub marginal tills

deforming beds viscous and plastic scales of deformation small scale field and laboratory observations of sub glacial sediment deformation suggest that failure is the predominant method of sediment deformation while large scale modelling studies using a viscous model of till deformation have been reasonably successful in predicting the geological consequences of ice sheet action this suggests that the cross over scale between viscous and plastic deformation occurs at a much larger scale than the grain grain interaction scale previously thought to be appropriate to deforming till sediment deformation and the drainage of sub glacial areas are considered at different scales and how these interact to produce failure and viscous behaviour is discussed a fluctuation length scale is proposed whose horizontal dimensions are of the order of the depth of the base of deformation 1 10 m on this length scale the glacier deforming bed system is highly variable in time as well as in space till deformation occurring at this length scale is plastic happening though failure events three other specific scales are proposed whose different properties depend on the relative spatial variability of static and seepage pressures and topography the key theoretical problem which has yet to be solved is how multiple small scale failure events combine into a viscous type flow this is discussed in the context of self organised criticality it is argued that the length scales appropriate for some drainage features are not appreciably larger than the scale for plastic failure events implying that modelling sub glacial drainage channels by balancing erosion rates against viscous closure of the channel is misconceived the outlines of an alternative model involving percolation theory are presented certain key questions which can only be settled through observations by glaciologists and glacial geologists are discussed it is suggested that the difference between lodgement tills and deformation tills reflects the rate of motion of till by deformation and that lodgement from the ice is unimportant

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

migration of seismic activity along intraplate seismic belts in the japanese islands abstract migration of seismic activity after eight large earthquakes which occurred in and near japan is investigated it is found that migration velocities are in most cases in the range between several km yr and 100 km yr however in some cases seismic activity in the regions more than a few hundred kilometers distant from the focal regions of great earthquakes are induced just after the earthquake occurrence in these cases apparent migration velocities are more than 100 km day although large earthquakes above m 6 seldom or never occur in almost all cases seismic activity migrates along intraplate seismotectonic belts containing the focal region of the large earthquakes which have triggered the migrational activity it is interesting to note in relation to the prediction of large intraplate earthquakes that the activity of small earthquakes of about m 4 are sometimes observed in the whole of the seismic belts in a relatively short period preceding the commencement of migration of larger earthquakes in those belts when a gap remains behind the migration front it is likely to be filled later a seismic gap concept seems to be most effectively applicable to these gaps as far as intraplate seismic activity is concerned although the physical mechanism of migration has not been found as yet the phenomena are sometimes useful for forecasting future large earthquakes and help us to understand intraplate seismotectonics

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

application of seismic stratigraphy and sedimentology to regional hydrogeological investigations an example from oak ridges moraine southern ontario canada hydrogeological models need to be supported by a clear understanding of the subsurface geology to provide effective assessment flow modelling or management of groundwater regimes this paper illustrates how geophysical and sedimentological data can be used to significantly improve watershed scale hydrostratigraphic models by advancing our understanding of the subsurface through regional hydrogeological investigations in the greater toronto area the example of a 3 km shallow seismic reflection survey that traverses a buried channel within bowmanville creek watershed oak ridges moraine ontario illustrates a basis for linking geophysical and sedimentological properties to regional hydrostratigraphic parameters seismic reflection methods plus seismic stratigraphy and a well constrained three dimensional geological framework have helped to i identify regional hydrostratigraphic units ii define properties and trends of these units facies iii improve depositional models that assist hydrogeological

shear resistance and continuity of subglacial till hydrology rules the field observations of g s boulton stimulated widespread interest in deformable beds shear resistance of till in its critical state is insensitive to strain rate and increases linearly with effective pressure during unsteady deformation pseudo viscous shear resistance can be caused by dilation of consolidated tills and resultant pore pressure decline this effect is probably uncommon however because susceptible tills of low hydraulic diffusivity are also those least likely to consolidate significantly during effective pressure transients stick slip motion at whillans ice stream antarctica indicates that its basal till must weaken during rapid slip and strengthen during longer periods of slower slip recurrence intervals for rapid slip episodes there 6 18 hours indicate that till strength variations if driven by changes in pore pressure either related or unrelated to basal freezing are focused in the uppermost several centimeters of the bed ploughing of grains at the bed surface and associated excess pore pressures in adjacent till can account for rate weakening during rapid slip with pore pressure decay causing strengthening between slip episodes by promoting shallow sluggish subglacial water flow and low effective pressure soft beds may help sustain themselves by slowing their own transport soft bed shear resistance kinematics and continuity are problems rooted in subglacial hydrology

flow mechanism of the des moines lobe of the laurentide ice sheet rapid flow of the des moines lobe of the laurentide ice sheet may have been related to its unlithified substrate new reconstructions of the lobe based on moraine elevations sediment subsidence during moraine deposition and flow direction indicators indicate that the lobe may have been 3 times thicker than in previous reconstructions nevertheless implied basal shear stresses are 15 kpa so internal ice deformation was not significant instead the lobe likely moved by a combination of sliding plowing of particles through the bed surface and bed shear consolidation tests on basal till yield preconsolidation stresses of 125 300 kpa so effective normal stresses on the bed were small a model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed resulting in too small a shear traction on the bed to shear it at depth consistent with this prediction measurements of orientations of clasts in basal till yield a weak fabric implying pervasive bed shear strain less than 2 although some stronger fabrics have been reported by others we infer tentatively that movement was principally at the bed surface by plowing

magnetic fabrics of the douglas till of the superior lobe exploring bed deformation kinematics abstract the superior lobe of the laurentide ice sheet during its last advance into northwestern wisconsin may have moved and transported sediment primarily through deep widespread shear of its bed to high strains 100 to study the kinematics of this deformation we measured the anisotropy of magnetic susceptibility of 3750 intact samples of its basal till the douglas member of the miller creek formation collected from eight profiles at 0 2 m depth increments a benchmark for interpretations was provided by results of ring shear experiments conducted on the douglas till which provided fabric characteristics for the case of horizontal simple shear orientations of maximum susceptibility k1 were controlled by preferred orientations of silt sized magnetite grains and were similar to sand grain orientations measured in thin sections strengths of fabrics formed by orientations of k1 indicate that most of the till sampled was deformed but to variable strains deformation averaged over the 20 km spanned by the study approximated horizontal simple shear however large differences in k1 fabric azimuths 45 over lateral distances of meters to tens of meters indicate highly heterogeneous bed deformation perhaps focused in anastomosing zones with associated divergent and convergent till shear this interpretation is supported by orientations of principal susceptibilities that imply in many cases either that shear planes were steeply dipping particularly transverse to the shearing direction or that there were major components of pure shear variations in k1 fabric azimuth with depth indicate that most of the till thickness did not shear simultaneously rather till accumulated at the bed as shear direction changed in response to temporally shifting zones of shallow deformation

multiphase flow of the late wisconsinan cordilleran ice sheet in western canada in central british columbia ice flow during the late wisconsinan fraser glaciation ca 25 10 ka occurred in three phases the ice expansion phase occurred during an extended period when glaciers flowed westward to the pacific ocean and east southeastward onto the nechako plateau from ice centers in the skeena hazelton coast and omineca mountains initially glacier flow was confined by topography along major valleys but eventually piedmont and montane glaciers coalesced to form an integrated glacier system the cordilleran ice sheet in the maximum phase a cordilleran ice divide developed over the nechako plateau to 300 km inland from the pacific coast at this time the surface of the ice sheet extended well above 2500 m above sea level and flowed westward over the skeena hazelton and coast mountains onto the continental shelf and eastward across the rocky mountains into alberta in the late glacial phase a rapid rise of the equilibrium line caused ice lobes to stagnate in valleys and restricted accumulation centers to high mountains discordant directions in ice flow are attributed to fluctuations of the ice divide representing changes in the location of accumulation centers and ice thickness ice centers probably shifted in response to climate irregular growth in the ice sheet rapid calving ice streaming and drainage of proglacial and subglacial water bodies crosscutting ice flow indicators and preservation of early valley parallel flow features in areas exposed to later cross valley glacier erosion indicate that the ice expansion phase was the most erosive and protracted event

thin film channelized drainage or sheetfloods beneath a portion of the laurentide ice sheet an examination of glacial erosion forms northern new york state usa abstract morphologic analyses of characteristic bedrock erosion forms found in northern new york state usa were used to reconstruct the subglacial conditions under which they may have been produced observations from sites on wellesley island n y at point salubrious n y and at an open pit mine near balmat n y were combined with regional mapping of a diverse suite of glacial erosion forms in order to establish the areal extent and significance of subglacial drainage in northern new york results of these site specific and regional studies are the basis for discussion of the probable role of subglacial meltwater in the region bedrock erosion forms preserved at various locations in northern new york are interpreted as products of flow separation of localized subglacial meltwater during ice bed decoupling the regional distribution of these forms indicates that they are largely restricted to pre existing subglacially modified bedrock valleys that served as subglacial tunnel channels regionally extensive deep subglacial meltwater flow characterized as sheetfloods is not required to account for the erosional features described from specific sites in northern new york state

glacier motion dominated by processes deep in underlying till black rapids glacier is a 40 km long surge type glacier in the central alaska range in spring 1997 a wireline drill rig was set up at a location where the measured surface velocities are high and seasonal and annual velocity variations are large the drilling revealed a layer of subglacial till up to 7 m thick that is believed to be water saturated at one location a string of instruments containing three dual axis tiltmeters and one piezometer was successfully introduced into the till the tiltmeters monitored the inclination of the borehole at the ice till interface and at 1 and 2 m into the till for 410 days they showed that no significant deformation occurred in the upper 2 m of the till layer and no significant amount of the basal motion was due to sliding of the ice over the till the measured surface velocity at the drill site is about 60 m a 1 of which 20 30 m a can be accounted for by ice deformation almost the entire amount of basal motion 30 40 m a 1 was taken up at a depth of 2 m in the till possibly in discrete shear layers or as sliding of till over the underlying bedrock we propose that the large scale mobilization of such till layers is a key factor in initiating glacier surges

recent drumlins flutes and lineations at vestari hagafellsjokull iceland in the foreland of the vestari hagafellsjokull glacier langajokull central iceland three scales of subglacial bed forms are found drumlins flutes and lineations the smaller bed forms are superimposed upon the larger these three bed forms probably formed as a result of the presence of a rigid subglacial core producing a lee side low pressure area into which sediment flowed and collected the cores of the drumlins are tumuli lava blisters whilst the cores of the flutes and lineations were large clasts differences between the three scales of bed forms are that a the flutes and lineations are far more mobile features as their cores could move within the deforming layer whilst the drumlin bedrock cores were fixed b the lineations formed in association with a clast pavement the change in scale of the bed forms may be related to changes in thickness of the deforming layer and size of the core at this site smaller bed forms were superimposed upon larger bed forms and these may have formed as the deforming layer progressively thinned in association with glacial retreat

sand intraclasts within a diamicton melange southern niagara peninsula ontario canada sand intraclasts found within diamicton units along the north shore of lake erie in the mohawk bay area of the niagara peninsula would appear to be part of a block in matrix melange the intraclasts are undeformed and many exhibit primary bedding structures numerous intraclasts have been rotated and or tilted and are in general subrounded in outline examination of the surrounding diamicton reveals that the diamicton clast fabrics exhibit a wide scatter and are not characteristic of any known till clast fabric around each intraclast exists an aureole of brecciated diamicton other evidence in the form of macro and microshear structures and banding within the diamicton indicate that the diamicton has been subject to high strain interpretation of the sand intraclasts seems to be intrinsically linked to the origin of the diamicton and together linked to the origin of the melange various hypotheses are suggested separately for the sand intraclasts diamicton and melange a subglacial deformable bed hypothesis is advanced as the most acceptable explanation for the complete sediment sequence in which diamicton and frozen sand intraclasts the latter mobilised from the substrate are moved as a melange below an active fast moving ice mass several implications from this study emerge with regard to glacial sedimentology and stratigraphic interpretations

subglacial hydrology in north western germany during the last glaciation groundwater flow tunnel valleys and hydrological cycles abstract numerical modelling of groundwater dynamics in an overpressured system of subglacial aquifers and aquitards under the marginal portion of the scandinavian ice sheet in northwestern germany has been coupled with calculations of basal meltwater production rates and with field evidence of meltwater erosional features largely fine grained subglacial sediments with relatively low hydraulic conductivities had a capacity to drain only about 25 of all basal meltwater produced in the relevant subglacial catchment area the rest of the meltwater was evacuated through tunnel valleys in spontaneous outburst events subglacial drainage cycles are proposed in which each cycle begins with meltwater drainage through the substratum groundwater flow followed by water ponding at the ice bed interface and ends in catastrophic meltwater releases through tunnel valleys

characteristic space time patterns in seismic activity in the northwest chubu district of honshu island japan and the 1984 nagano ken seibu earthquake abstract the spatial pattern of the epicentral distribution in the northwest chubu district of honshu island japan changed drastically after the occurrence of the 1948 fukui earthquake m 7 1 the change can be explained by invoking that the wnw ese trending seismic belt which contains the focal region of the fukui earthquake was activated and that the activity along another seismic belt trending ne sw which had been conspicuous before the fukui earthquake disappeared a notable feature is that large earthquakes occurred successively along the newly activated seismic belt migrating to the east the 1984 nagano ken seibu earthquake m 6 8 occurred on the extension of that migrational activity the focal region of the nagano ken seibu earthquake is situated at a distinct inflection point in the seismic zone which was activated after the fukui earthquake the spatial pattern of the seismic zone is very similar to the distribution of quaternary volcanoes in that region and the same pattern is also recognized in the bouguer anomaly these features suggest that the seismic belt is a weak zone in the crust corresponding to the boundary of the tectonic blocks the focal region of the nagano ken seibu earthquake had been very quiet over a period of 10 years with the seismic activity subsequently resuming several years before the nagano ken seibu earthquake presenting two active centers in the seismic gap near one of which a large destructive earthquake had occurred in 1906 it seems that the occurrence of the nagano ken seibu earthquake could have been predicted if the characteristics of the seismic activity after the fukui earthquake and the tectonic features in the region had been correctly interpreted

subglacial clast behaviour and its implication for till fabric development new results derived from wireless subglacial probe experiments this study has investigated the three dimensional movement of clasts within deformation till using embedded wireless probes these probes were part of an environmental sensor network which measured subglacial properties temperature water pressure resistivity case strain and tilt six times a day and relayed that data via radio to the glacier surface where they were forwarded and broadcast on line the systemwas installed at briksdalsbreen norway and operated from august 2004 until august 2006 approximately 2000 probe days worth of data were collected with an increase in performance 41 more readings during the second year the probes showed similar patterns of water pressure rises throughout the two years but with slightly different magnitudes and timings these changes in water pressure could be related to clast behaviour the probes decreased their dip over the year and the rate of change was related to an increase in glacier velocity after initial changes in dip the probes experienced changes in orientation followed by rotation about the a axis this continuous rotation was similar to the motion suggested by jeffery 1922 the motion of ellipsoidal particles immersed in a viscous fluid proceedings of the royal society of london series a 102 161 179 for the behaviour of clasts within a viscous material in addition some probes also showed short frequent dip oscillations in spring and autumn which were interpreted to reflect stick slip events similar to lodging and demonstrated how local conditions can interrupt the predicted rotation pattern overall it is demonstrated that when water pressures were high decoupling occurred associated with basal sliding and dip oscillations and when water pressures fell the ice and sediment were coupled and till deformation occurred these events happened during summer and autumn it is this combination of lodgement and deformation that builds up both a complex but predictable fabric and a resultant composite till sedimentology

characteristic patterns in the space time distribution of large shallow earthquakes in the japanese islands abstract characteristic patterns of intraplate large shallow earthquake occurrence in the japanese islands are investigated it is shown that in each district there are seismic belts or seismotectonic active zones where large shallow intraplate earthquakes frequently occur in some cases the activation of seismic belts seems to be related to the occurrence of large shallow earthquakes such as the 1891 nobi earthquake with a magnitude of 8 0 and the 1923 kanto earthquake with a magnitude of 7 9 also it is probably caused by stress increase due to interplate coupling such as in the chugoku district southwest japan before the 1946 nankaido earthquake with a magnitude of 8 1 in the kyushu district other unique tectonics seem to be involved in the intraplate seismic activity non overlapping of active areas and the concentration of earthquake occurrence within a relatively short period are usually observed for each seismotectonic zone further migration of seismic activity and synchronous activity as well as correlation of earthquake occurrences in distant parts along seismotectonic belts are recognized in most tectonic zones it is pointed out that the same concept of seismic gap of the first kind originally proposed for interplate large shallow earthquakes along the deep sea trench may also be applicable to the occurrence of intraplate large earthquakes along seismic belts these characteristic patterns are considered to give a promising clue to selecting the sites where large earthquakes are expected to occur in the near futue seismic belts or seismotectonic active zones probably play an important role in intraplate tectonics they are a kind of boundary of blocks along which change in stress field can be transferred easily though their tectonic origin may be different for each seismotectonic zone

the pre late wisconsin stratigraphy of southern simcoe county ontario implications for ice sheet buildup decay and great lakes drainage evolution1 recent three dimensional mapping investigations in southern simcoe county ontario allow refinement of the existing regional stratigraphic framework analysis of 25 continuously cored boreholes ha

fast glacier flow ice streams surging and tidewater glaciers fast flowing glaciers are unusual but important jakobshavns glacier in western greenland illustrates this point figure 1 considered to be the world s fastest glacier its maximum observed flow rate measured in midsummer near the floating terminus is 8360 m yr 1 lingle et al 1981 except for a tidal influence near the terminus the flow is believed to be fairly steady bindschadler 1984

coupling between a glacier and a soft bed ii model results the relation between the local effective pressure and shear stress on till beneath storglaciaren sweden discussed in iverson and others 1999 provides an empirical basis for studying the processes that control the strength of the ice bed coupling particles in the bed that protrude into the glacier sole support shear stresses that are limited by either ploughing or the traditional sliding mechanisms model calculations based on studies of cone penetration through fine grained sediment and sliding theory agree with the observed relation between shear stress and effective pressure if the water layer at the ice bed interface is assumed to thicken rapidly as the effective pressure approaches zero studies of the hydraulics of linked cavities provide support for this assumption if the mean thickness of the water layer reflects the extent of microcavity development at the interface comparison of the calculated shear stress with the ultimate strength of till suggests that bed deformation limits the shear stress on till beneath storglaciaren only at intermediate effective pressures at very low effective pressures like those inferred at the site of the tiltmeter discussed in iverson and others 1999 and at sufficiently high effective pressures ploughing and sliding should focus motion near the glacier sole a calculation using parameter values appropriate for ice stream b west antarctica suggests that ploughing may occur there at shear stresses not sufficient to deform the bed at depth this conclusion is reinforced by the likelihood that pore pressures in excess of hydrostatic should develop down glacier from ploughing particles thereby weakening the bed near the glacier sole however given the apparent sensitivity of the ice bed coupling to basal conditions that may be highly variable any blanket assumption regarding the flow mechanism of ice masses on soft beds should probably be viewed with skepticism

the athabasca fluting field alberta canada implications for the formation of large scale fluting erosional lineations abstract current drumlin and fluting hypotheses centred on processes of formation erosion and deposition by subglacial deformation accretion by lodgement glacial erosion and meltwater erosion and deposition rarely receive comprehensive field testing this paper documents the morphology internal structure and landscape associations of the athabasca fluting field alberta and discusses how well these characteristics are explained by postulated fluting forming processes fluting hypotheses are comprehensively tested by this approach and their rejection or acceptance are firmly based on field observation and knowledge of processes using multiple criteria fluting is shown to be erosional and not to be a product of pervasive subglacial deformation or accretion by lodgement morphology landscape associations bed truncation plus glaciofluvial lags and gravel on fluting ridges are inferred to support erosion by meltwater intact primary bedding synsedimentary faults and injection structures very close to the landsurface showing no sign of shear deformation are used to reject erosion by subglacial deformation fluting is shown to be closely associated with tunnel channels and indicates that these channels were overtopped by broad floods that formed the fluting field and eroded anabranching tracts over the length of the alberta plains prior to fluting formation sequences with melt out and lodgement till conduit sediment and boulder lags record net deposition over an extended time period diapirism prior to fluting formation marks a period of extreme pressure gradients which are related to bed separation following fluting formation there was only minimal landscape modification dominated by glaciofluvial and glaciolacustrine deposition not glacial processes this progression is presented as evidence for two very different glacial regimes separated by a short period with diapirism and extensive landscape modification by meltwater erosion we note that the study area was glaciated only once during the late wisconsinian and it follows that glacial sedimentation and landscape modification were confirmed to this single glacial episode

on the formation of the tunnel valleys of the southern laurentide ice sheet abstract catastrophic releases of meltwater produced by basal melting and stored for decades in subglacial reservoirs at high pressure may have been responsible for eroding the broad deep tunnel valleys that are common along the margins of some lobes of the southern laurentide ice sheet we surmise that these releases began when the high water pressure was transmitted to the margin through the substrate the water pressure in the substrate at the margin would then have been significantly above the overburden pressure leading to sapping failure headward erosion of a conduit in the substrate piping could then tap the stored water resulting in the outburst in some situations development of a siphon may have lowered the reservoir below its overflow level thus tapping additional water following the flood the seal could have reformed and the reservoir refilled setting up conditions for another outburst order of magnitude calculations suggest that once emptied a subglacial reservoir could refill in a matter of decades the amount of water released during several outbursts appears to be sufficient to erode a tunnel valley we think that tunnel valleys are most likely to have formed in this way where and when the glacier margin was frozen to the bed and permafrost extended from the glacier forefield several kilometers back under the glacier as reservoirs would then have been larger and more common and the seal more robust and more likely to reform after an outburst

glacial geomorphology modeling processes and landforms abstract the primary goal of glacial geomorphology is to provide physically based explanations of the past present and future impacts of glaciers and ice sheets on landform and landscape development to achieve this requires the integration of studies of landform with studies of the processes responsible for form development over a wide range of spatial and temporal scales during the twentieth century significant improvements in approaches to recognizing and describing glacial landforms have been matched by impressive advances in understanding and modeling ice flow and glacial erosion and deposition processes at present process models are being tested explicitly in terms of predicting the development of known forms which also provides new insight into the controls on form development evaluations of the implications of deformable beds for process and form development are also being attempted finally we are reassessing long held beliefs about the significance of glacial action in landform development and sediment production as we head towards the twenty first century glacial geomorphology will advance through the use of three dimensional numerical models that include ice flow basal sliding with explicit consideration of deformable beds erosion and deposition processes and underlying material characteristics these models will be used to address form evolution and test process models and will include both the temporal and spatial aspects of form development space based landform recognition as well as the challenges presented by the concerns of global change will place heightened emphasis on large scale problems which will include reexamination of the basic significance of glacial versus nonglacial action in landform development and sediment production

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

precursory activity in seismic belts containing the focal region of forthcoming large intraplate earthquake abstract seismic activity in the region surrounding the foci is investigated for three severe earthquakes two with a magnitude of 6 1 and one with a magnitude of 5 3 which have occurred in japan in recent years the most conspicuous feature commonly noticed is precursory activation of seismic belts which include the focal regions of main shocks the repetition of the same pattern in the space time distribution of earthquake occurrence along the seismic belt is also observed for each case the precursory activity of seismic belts terminates in rather a short period and after that the area around the focus of the forthcoming large earthquake becomes quiescent which demonstrates the appearance of the seismic gap of the second kind mogi 1979 the periods of seismic quiescence for the cases investigated in this paper are longer than those which are given by the regression relationship between earthquake magnitude and precursor time proposed for example by sekiya 1977 however our definition of anomalous seismic activity is clear and it is possible to give a physical meaning to it as an increase in the local stress field in the seismic belt we propose that a kind of coupling between intraplate tectonic blocks analogous to interplate coupling in the subduction region is responsible for the formation of the stress field relevant to these earthquakes although this is at present only one of the possible viewpoints on the formation of the focal region of large intraplate earthquakes it may be worthwhile to study various precursory phenomena in connection with this hypothesis

on the formation of the tunnel valleys of the superior lobe central minnesota abstract when considering the formation of tunnel valleys most researchers have appealed to basal melting as the primary source of water erosion of the tunnel valleys can then be accomplished by steady state drainage over a long period or by storage and subsequent catastrophic release however field relationships in a large system of tunnel valleys formed by the superior lobe in central minnesota indicate that another source of water must be considered the minnesota tunnel valleys are composed of individual segments 10 20 km long and the segments commonly terminate in subaerial outwash fans at recessional ice margin positions eskers associated with the tunnel valleys are also composed of short segments frequently beginning at moulin kames and terminating at the head of outwash fans the dominant source of the water responsible for tunnel valley formation was seasonal meltwater from the glacier surface that reached the bed through moulins and crevasses the apparent continuity of the valleys resulted from the headward development of the englacial drainage system during ice retreat

approaches to the study of glacier bed deformation investigation of glacier bed conditions and subglacial processes is an obligatory requirement for the modelling and understanding of contemporary glacier behaviour past glacier behaviour glacier response to both spatial and temporal variations in bed material and glacier response to external forcing factors such as climate change the recognition of the importance of coupling between the glacier and its bed resulted in a paradigm shift in the subject and drew attention to the critical importance of subglacial deformation as opposed to simply subglacial deposition and sliding a number of methods have been adopted to measure glacier bed deformation ranging from direct observation and instrumentation the interpretation of macroscale and microscale sedimentary structures to the interpretation of glacier bedforms and glacier bed modelling this paper is concerned to describe the methods used by the two authors and co workers and attention is given to field methods the type and arrangement of sedimentary structures thin section micromorphology and microfabric and clast microfabric and glacier bedform morphology as evidence of subglacial deformation sedimentary structures macro and mesoscale are used to identify constructional deformation where deformational units are preserved one on top of another due to the formation of a thin deforming layer by low basal stresses and excavational deformation in which the deformational style is superimposed on pre existing styles due to increasing thicknesses of glacier deformation caused by increasing basal shear stresses thin section micromorphology has been used to define a range of glacial processes of which subglacial deformation is just one but has drawn attention to critical criteria such as discrete microscale shears orientated matrix kink structures and fractured grains as well as deformed intraclasts that can be used to recognise deformational processes more recently individual aggregates taking the form of rounded pebbles within till matrix have been shown to reflect rotational processes during glacial deformation and provide an additional mechanism to discrete shear within subglacial deformation and an explanation for the transport paths of subglacial materials finally measurements of sediment flow directions using clast macrofabric characteristics coupled with measurement of quantities of sediment transfered and detailed palaeoglacial properties have been used to determine the rates and scales of subglacial deformation associated with the formation of small scale glacier bedforms

evidence against pervasively deformed bed material beneath rapidly moving lobes of the southern laurentide ice sheet abstract during the wisconsin glaciation the ice lobes of the southern part of the laurentide ice sheet moved rapidly as the result of elevated subglacial water pressure the rapid movement was probably not accompanied by wide spread pervasive deformation of unfrozen material under the ice the till stratigraphy of much of this area is well known and it is clear that the stratigraphic sequence is generally intact therefore such deformation if it occurred would have been restricted to the till sheet that was currently being deposited however this seems unlikely as well because the till of the region commonly contains lenses and clasts of unlithified bedded sand that should have been destroyed by pervasive shearing if unfrozen deformed till layers were widely present they must have been confined to the thin upper parts of till sheets

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

a laboratory study of ploughing a new laboratory device is used to investigate the resistance to clast ploughing at the base of glaciers in experiments in which a ploughing tip is dragged at different velocities and effective normal stresses through water saturated sediment from unteraargletscher switzerland pore pressures above and below the hydrostatic level develop around the tip the absolute magnitude of these non hydrostatic pore pressures increases with the ploughing velocity but remains small compared to the sediment yield strength so that the pore pressures do not significantly weaken the sediment the shear stress on the tip is independent of the velocity but scales with the applied effective normal stress in agreement with a coulomb plastic behavior of the sediment the results indicate that depending upon position close to the object both sediment compaction and dilation can influence the pore pressure distribution and thus the sediment yield strength comparison with other studies of clast ploughing suggests that the significance of sediment weakening in front of ploughing clasts may depend on the relative magnitudes of the non hydrostatic pore pressures therefore depending on the dominant pore pressure response of the deforming sediment clast ploughing may have the potential to either trigger ice flow instabilities or stabilize glacier motion

subglacial till formation microscale processes within the subglacial shear zone abstract this was a study of subglacial deformation till genesis from a modern temperate glacier at skalafellsjokull iceland detailed microscale properties of till samples from scanning electron microscope sem and thin section analysis were examined from a glacial site with in situ subglacial process monitoring and an exposed subglacial surface in the foreland two lithofacies were examined a grey sandy till derived from the ash and basalt and a silty reddish brown till derived from oxidized paleosols and or tephra layers these also represented a clay content continuum from low 0 3 to high 22 3 the evolution from debris to subglacial till was investigated this included a reduction in grain size 21 for grey lithology 13 reddish brown lithology and reduction in rounding ra 32 for the grey lithology 26 for the reddish brown lithology and the quantification and analysis of the different grain erosion comminution processes in the resultant till it was shown that the microstructures within a till were dependent on shear strain and glaciological conditions deformation history the low clay content tills were dominated by linear structures lineations and boudins and anisotropic microfabric whilst the higher clay content tills were dominated by rotational structures turbates and plaster and isotropic microfabric these results are important in our understanding of the formation of both modern and quaternary tills and informs our reconstruction of past glacial dynamics

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

glacial erosion of the wash and fen basin and the deposition of the chalky till of eastern england abstract it is widely accepted that the low lying area of eastern england comprising the wash and the fens was excavated by the anglian ice sheet moving first from the northwest and then from the north the ice moved rapidly across the deformable bed on the jurassic mudrocks and rode up over the chalk escarpment eroding the scarp face and setting back the lowered escarpment by several kilometres the eroded material was deposited beyond the basin as the chalky till of the southeast midlands and east anglia in this paper reconstruction of the form of the preglacial landscape of the jurassic and cretaceous outcrops allows estimation of the excavated volume of both mudrock 312 km 3 and of chalk 243 km 3 a total of about 555 km 3 this volume and the lithological mix matches closely the reconstructed volume estimate 363 km 3 of the now dissected chalky till around the margins of the glacial lobe the amount of erosion achieved by the anglian ice sheet amounts to 3 4000 bubnoffs b i e mm ka 1 even if we allow 20 ka for a complex glaciation this far exceeds rates of erosion recorded anywhere with the british isles today even if averaged over the 440 000 yr since that event the rate is 167 b far greater than the erosion achieved on similar rocks within the adjacent unglaciated thames basin estimated at 45 b

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

estimating the sliding velocity of a pleistocene ice sheet from plowing structures in the geologic record 1 as an ice sheet slides over its sediment bed some clasts partly embedded in the glacier sole plow through the bed surface the size distribution of such clasts if it can be characterized from structures in the geologic record can be used to estimate the sliding velocity of a past ice sheet by combining a theory of glacier sliding with a geotechnical theory of cone penetration sliding velocity can be calculated in terms of clast size parameters a fluidity parameter for ice and the thermodynamic properties of ice and clasts if frictional properties of the bed are measured the effective normal stress on the bed and bed shear strength during glaciation can also be calculated we used this approach to estimate the sliding velocity of an illinoian ice sheet that left plowing structures in cemented outwash near peoria illinois fluidity parameters for normal and basal temperate ice yielded sliding velocities of 140 168 m yr 1 and 60 72 m yr 1 respectively these are overestimates if solutes impeded regelation of ice past clasts or if friction between debris laden ice and clasts retarded slip preconsolidation stresses determined in laboratory tests on silt from the bed agree with effective normal stresses calculated using clast size parameters the high shear strength of the bed 145 kpa and primary structures preserved within it indicate that additional movement due to pervasive shear of the bed was unlikely application of this method elsewhere would provide basal velocity data that are otherwise unavailable for testing and tuning of ice sheet models

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

subglacial hydrology for an ice sheet resting on a deformable aquifer glissements et deformation de la calotte de glace en relation avec le materiau de l aquifere et la densite des chenaux de drainage

modeling the subglacial hydrology of the james lobe of the laurentide ice sheet to investigate the drainage conditions that might be expected to develop beneath soft bedded ice sheets we modeled the subglacial hydrology of the james lobe of the laurentide ice sheet from hudson bay to the missouri river simulations suggest the james lobe had little effect on regional groundwater flow because the poorly conductive upper cretaceous shale that occupies the upper layer of the bedrock would have functioned as a regional aquitard this implies that general northward groundwater flow out of the williston basin has likely persisted throughout the quaternary moreover the simulations indicate that the regional aquifer system could not have drained even the minimum amount of basal meltwater that might have been produced from at the glacier bed therefore excess drainage must have occurred by some sort of channelized drainage network at the ice till interface using a regional groundwater model to determine the hydraulic conductivity for an equivalent porous medium in a 1 m thick zone between the ice and underlying sediment and assuming conduit dimensions from previous theoretical work we use a theoretical karst aquifer analog as a heuristic approach to estimate the spacing of subglacial conduits that would have been required at the ice till interface to evacuate the minimum water flux results suggest that for conduits assumed to be on the order of a tenth of a meter deep and up to a meter wide inter conduit spacing must be on the order of tens hundreds of meters apart to maintain basal water pressures below the ice overburden pressure while evacuating the hypothesized minimum meltwater flux r 2007 elsevier ltd all rights reserved

glacial deposits at wylfa head anglesey north wales evidence for late devensian deposition in a non marine environment the nature and origin of glacial sediments at wylfa head are described and their significance with regard to sedimentary environments during late devensian deglaciation of the irish sea basin is discussed recent models of deglaciation under glaciomarine conditions are challenged the quaternary sequence at wylfa consists of eroded and glaciotectonically deformed bedrock locally derived lodgement till calcareous silt rich lodgement till containing northern erratics discontinuous units of orange brown silty sand of possible aeolian origin and grey laminated freshwater silts filling a small kettle hole the till units thicken to the south where the surface is drumlinised it is concluded that the landforms and deposits result from a warm based irish sea glacier which moved towards the southwest spatial variation in basal water pressure resulted from localised drainage through zones of more heavily jointed bedrock rapid glacial erosion occurred in areas where subglacial water pressure was relatively high while deposition of the resulting basal sediment took place where water pressures were reduced the glacier also carried basal calcareous silty till onshore which was deposited by lodgement processes none of the deposits at wylfa are interpreted as glaciomarine in origin and there is no evidence at this site for an isostatically induced marine transgression prior to deglaciation

bed conditions of pine island glacier west antarctica although 90 of antarctica s discharge occurs via its fast flowing ice streams our ability to project future ice sheet response has been limited by poor observational constraints on the ice bed conditions used in numerical models to determine basal slip we have helped address this observational deficit by acquiring and analyzing a series of seismic reflection profiles to determine basal conditions beneath the main trunk and tributaries of pine island glacier pig west antarctica seismic profiles indicate large scale sedimentary deposits combined with seismic reflection images measured acoustic impedance values indicate relatively uniform bed conditions directly beneath the main trunk and tributaries comprising a widespread reworked sediment layer with a dilated sediment lid of minimum thickness 1 5 0 4 m beneath a slow moving intertributary region a discrete low porosity sediment layer of 7 3 m thickness is imaged despite considerable basal topography seismic observations indicate that a till layer at the ice base is ubiquitous beneath pig which requires a highly mobile sediment body to maintain an abundant supply these results are compatible with existing ice sheet models used to invert for basal shear stress existing basal conditions upstream will not inhibit further rapid retreat of pig if the high friction region currently restraining flow directly upstream of the grounding line is breached however small changes in the pressure regime at the bed as a result of stress reorganization following retreat may result in a less readily deformable bed and conditions which are less likely to maintain high ice flow rates

reflections on soft subglacial beds as a mosaic of deforming and stable spots abstract much discussion regarding subglacial processes is centred around the question of how soft beds respond to ice overriding in particular how widespread and penetrative is their deformation how does it affect the ice stability and what are the consequences for large scale sediment redistribution patterns contrasting views have been proposed which are difficult to verify and reconcile in the light of scarcity of agreed criteria to identify deforming beds here we suggest that soft beds under warm based glaciers can be envisaged as a mosaic of deforming and stable spots transient in time and space according to changes in the control parameters at the ice bed interface the subglacial mosaic model is consistent with field data showing heavily deformed beds interspersed with largely undisturbed beds it is suggested that deformation found in tills is a cumulative effect of strain within a relatively thin ephemeral deforming spots under the ice sole affecting the till as it accretes during ice overriding in accord with the coulomb plastic rheology the role of clast ploughing in bed deformation is emphasised

groundwater flow beneath late weichselian glacier ice in nordfjord norway basal water pressure and water flow patterns are significant factors in controlling the behavior of an ice sheet because they influence ice sheet thickness stability and extent water produced by basal melting may infiltrate the subsurface or occur as sheet or channelized flow at the ice bed interface we examine subglacial groundwater conditions along a flowline of the scandinavian ice sheet through nordfjord in the western fjords region of southern norway using a steady state two dimensional groundwater flow model meltwater input to the groundwater model is calculated by a two dimensional time dependent thermomechanically coupled ice flow model oriented along the same flowline model results show that the subglacial sediments could not have transmitted all the meltwater out of the fjord during times of ice advance and when the ice sheet was at its maximum position at the edge of the continental shelf in order for pore water pressures to remain below the overburden pressure of the overlying ice other paths of subglacial drainage are necessary to remove excess water during times of retreat the subglacial aquifer is incapable of transmitting all the meltwater that was probably generated pulses of meltwater reaching the bed could explain non climatically driven margin readvances during the overall retreat phase

assessing the paradigm shift deformable glacier beds abstract during the mid 1980s glaciologists recognised that many glaciers and ice masses overlie soft sediments rather than hard bedrock these sediments are typically weaker than glacier ice and may therefore deform this realisation was heralded as a paradigm shift in glaciology since this realisation a wide body of theoretical and field based research has been undertaken to assess the mechanical and hydrological significance of such substrates we review the techniques and results of this work and attempt to assess this paradigm shift in glaciology and in particular its importance for quaternary scientists we suggest areas of research that remain unanswered and identify those in which glacial geology in particular may be able to inform glaciologists

experiments on the dynamics and sedimentary products of glacier slip abstract experimental work in glacial geomorphology is focused almost entirely on processes in the thin shear zone at the beds of sliding glaciers where ice at its pressure melting temperature moves over either rigid rock or deformable till experiments with rotary shear devices illuminate constitutive behavior there central to the dynamics of fast moving glaciers and provide a foundation for interpreting the sedimentary record of glacier slip and associated sediment transport results from experiments designed to study ice sliding over a rigid wavy bed shear deformation of till and plowing of clasts across the surface of a till bed point to a common conclusion drag at the bed can decrease with increasing slip velocity thereby concentrating driving stress elsewhere and promoting rapid glacier flow this rate weakening behavior at glacier beds is in contrast to the viscous slip resistance assumed in ice sheet models and most efforts to determine distributions of basal drag from glacier surface velocities ring shear experiments in which various basal tills and more idealized materials are sheared to high strains provide quantitative insight into grain size evolution mixing at contacts between basal tills microstructure development particle fabric development and development of anisotropy of magnetic susceptibility preferred orientations of principal magnetic susceptibilities provide the most dependable and complete description of till shear patterns when applied to basal tills of the geologic record magnetic till fabrics measured along thick till sections and calibrated experimentally indicate that deformation of the bed by two lobes of the laurentide ice sheet was shallow

a slip law for glaciers on deformable beds slip of marine terminating ice streams over beds of deformable till is responsible for most of the contribution of the west antarctic ice sheet to sea level rise flow models of the ice sheet and till bedded glaciers elsewhere require a law that relates slip resistance slip velocity and water pressure at the bed we present results of experiments in which pressurized ice at its melting temperature is slid over a water saturated till bed steady state slip resistance increases with slip velocity owing to sliding of ice across the bed but above a threshold velocity till shears at its rate independent coulomb strength these results motivate a generalized slip law for glacier flow models that combines processes of hard bedded sliding and bed deformation

architectural element analysis applied to glacial deposits internal geometry of a late pleistocene till sheet ontario canada deposits left by continental ice sheets are characterized by sedimentological complexity and stratigraphic heterogeneity but stratigraphic descriptions of such deposits and resulting first generation facies models are still based primarily on one or two dimensional borehole or outcrop data reconstruction of depositional environments hydrogeological investigations of pleistocene glacial deposits and hydrocarbon exploration in pre pleistocene glaciated basin fills require a more detailed understanding of the form and heterogeneity of lithofacies sequences in three dimensions architectural element analysis is used widely by sedimentologists for categorizing internal stratigraphic heterogeneity in sandstones particularly those of fluvial origin this paper demonstrates the first application of architectural element analysis to glacial deposits such as tills outcrop borehole and a broad range of subsurface geophysical data were collected from a thick 60 m till sheet present across an 80 km 2 study area near toronto canada the till sheet is not homogeneous but is composed of three distinct architectural elements and associated lithofacies viz diamict elements interbeds of subglaciofluvial sediments and glaciotectonically deformed zones application of architectural element analysis to these subglacial strata provides insights into the origin of drumlin bedforms and subglacial processes below the laurentide ice sheet and creates a framework for understanding ground water and contaminant movement in underlying aquifers

the 3d microscopic signature of strain within glacial sediments revealed using x ray computed microtomography abstract x ray computed microtomography ct a non destructive analytical technique was used to create volumetric three dimensional 3d models representing the internal composition and structure of undisturbed pro and subglacial soft sediment sample plugs for the purposes of identifying and analysing kinematic indicators the technique is introduced and a methodology is presented addressing specific issues relating to the investigation of unlithified polymineralic sediments six samples were selected based on their proximity to type brittle and ductile deformation structures or because of their perceived suitability for successful application of the technique analysis of a proglacial ideal specimen permitted the 3d geometry of a suite of micro faults and folds to be investigated and the strain history of the sample reconstructed the poor contrast achieved in scanning a diamicton of glaciomarine origin is attributable to overconsolidation under normal loading the sediment demonstrated to have undergone subsequent subglacial deformation another overconsolidated diamicton contains an extensive small scale the subglacial sample evidence indicates that complex polyphase brittle ductile deformation histories are common in such diamictic soft sediments and that the local micro scale environment composition structure shear forces and effective pressure controls rheology the sediment void ratio is a key indicator of strain three of the samples are tentatively placed at different points on the strain cycle for subglacially deforming soft sediment based on their void ratio characteristics and distribution it has been demonstrated that ct offers significant potential for elucidating glacial soft sediment kinematics the ability of the technique to both test a variety of hypotheses pertaining to mechanisms operating within the subglacial environment and evaluate efforts to replicate those processes under controlled laboratory conditions is therefore discussed along with solutions to the problems encountered within this project ct also permits the seamless linking of analytical techniques applied at the hand specimen mm micromorphological m mm scanning electron microscopy and x ray diffraction fluorescence nm m scales and the archiving duplication and dissemination of sample volumetric 3d data

brecciated diamictons from mohawk bay s ontario canada examination of sediments along the north shore of lake erie at mohawk bay reveals a relationship between the formation of intensely brecciated diamictons and the presence of sand block intraclasts it is postulated that the sand blocks were subglacially deposited within a meltwater environment and later frozen prior to being eroded and transported within a mobile subglacial debris layer on immobilization the frozen sand blocks encased within the diamicton acted as a heat sink creating cryostatic stresses within the surrounding diamicton as a result of the advance of a frost front and related frozen fringe the effect of these anisotropic stresses resulted in porewater migration to the frost front subsequent development of intense brecciation occurred as aureoles around the sand intraclasts due to localized high tensile stresses causing fracturing within the fine grained matrix of the diamicton

water throughflow and the physical effects of deformation on sedimentary glacier beds the sediment comprising a deformable bed beneath an ice mass is typically modeled as a homogeneous fluid without internal structure although clearly deformation must result from the spatial rearrangement of the particles that make up the sediment laboratory deformation experiments were used to simulate the effects of active deformation on subglacial sediments these experiments indicate that deformation results in the rearrangement of particles and changes in sediment structure scanning electron and optical microscopy of postdeformation sediments revealed both 1 particle alignment parallel or subparallel to the direction of principal strain and 2 dilation where active deformation had occurred both of these effects will alter the permeability and hence hydraulic conductivity of the sediment body increasing the rate of water flow through the bed parallel to deformation microscopic examination of these sediments allows estimation of postdeformation permeabilities these permeabilities may be anisotropic furthermore water throughflow by darcian processes enhanced by the effects of deformation may increase the proportion of basal water that can be evacuated by throughflow water will tend to be retained in the high permeability actively deforming layer and may destabilize this layer

coupling between a glacier and a soft bed i a relation between effective pressure and local shear stress determined from till elasticity to predict the distribution of motion beneath glaciers on soft beds the strength of the coupling between the ice and the bed and its variation with effective pressure must be known a record of shear strain acquired with a tiltmeter emplaced in till beneath storglaciaren sweden indicates that fluctuations in water pressure cause variations in the local shear stress on the bed and that the bed deforms elastically in response to these variations to estimate the shear stress from the elastic component of the total shear strain the shear modulus of the till was measured in relaxation tests conducted in the laboratory with a ring shear device after accounting for the elastic compliance of the device these tests yielded shear moduli of about 1000 and 1800 kpa at confining pressures of 85 and 280 kpa respectively these values are comparable to those of other granular materials undergoing recoverable shear strains of the same magnitude the local shear stress on the till calculated by applying the measured shear moduli to the tilt record scales with p e 1 7 where p e is the effective pressure this relation implies that as p e decreases at the ice till interface shear stresses on the till are reduced and concentrated elsewhere on the bed perhaps where the till is absent or the glacier is frozen to the bed when compared with the strength of the till determined from ring shear tests this relation also accounts for the lack of permanent deformation at depth in the bed during periods of low p e and indicates that most basal motion was by sliding or ploughing

glacial abrasion and sliding their dependence on the debris concentration in basal ice a model of bedrock abrasion by a temperate glacier is developed that takes into account the significant impeding effect of scattered rock fragments at the ice rock interface on glacier sliding an increase in the debris concentration above a relatively low value can reduce the abrasion rate by decreasing both the flux of rock fragments and the effective forces which press fragments against the bed abrasion of a simple wavy bed is shown to be most rapid for a relatively low debris concentration above which the abrasion varies inversely with the debris content a similar concentration dependence of abrasion is expected for beds of arbitrary geometry but low roughness sliding glaciers will tend to abrade until the basal ice is nearly stagnant at which point rocks of all sizes start to lodge against the bed

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

rapid soft bed sliding of the puget glacial lobe the puget lobe of the late pleis inferred to be lodgment till based on the tocene cordilleran ice sheet advanced into presence of oriented stones oriented stria western washington over a thick sequence of tions on stones overconsolidation and unlithified sediments the basal drag due to deformation features sliding over this surface was calculated by 1 this paper investigates the rapid motion of idealizing the sediment bed as a rigid planar the puget lobe over its bed of deformable sedi surface scattered with roughness elements cor ment studies of large vanished glaciers com responding to individual sedimentary particles plement basal motion studies of modern glaci and 2 taking the minimum reconstructed slid ers in which the glacier geometry and velocity ing velocity to be 500 m yr 1 the calculated are well constrained but the characteristics of basal drag minimized by choices of parameters the bed can only be inferred from observations is 400 kpa much higher than the reconstructed over very limited areas kamb et al 1985 gravitational driving stress of 40 kpa indi seismic measurements blankenship et al 1986 cating that a rigid bed and low water pressure or modelling lingle and brown 1987 obser are not consistent with the glacierfs rapid vations on glacier beds recently exposed by motion high subglacial water pressure aver glacial retreat can provide information about aging 90 of the ice overburden 10 mpa is conditions during a somewhat greater time span inferred from overconsolidation of subglacial but are restricted to the marginal environ clays the occurrence and deformation of ment in addition inferences about the bed of water deposited sediments within the till and modern glaciers are limited by the short dura the requirement to drain large quantities of tion and seasonality of the observations a water from the interface suggest that water notable benefit of studying the beds of pieis pressure reached 99 of the ice overburden tocene glaciers is that they record basal ploughing of till by ice entrained clasts per conditions for intervals spanning entire glaci vasive shearing and finally ice bed separa ations and accessible far from the glacier tion become possible as water pressure margins increases at the low inferred effective nor after a brief summary of data available from mal stress the basal drag is reduced from the previous reconstructions of the puget lobe the rigid bed case by water layers which decouple basal conditions of the lobe are explored along the smallest particles from the glacier sole two complementary lines we first present cal and ploughing which reduces the resistance culations of basal drag due to glacier sliding offered by the largest particles the limited for different models of till bed roughness that shear strain observed in much of the till indicate that rapid sliding requires very low implies pervasive shearing did not contribute effective stresses and extensive ice bed significantly to ice motion basal motion was decoupling or ploughing second we summarize confined to the ice bed interface or to dis geological and geotechnical data that indicate tinct faults within the substrate

ice streams of the late wisconsin cordilleran ice sheet in western north america abstract the late wisconsin cordilleran ice sheet cis of western north america is thought to have reached its maximum extent 2 5 106 km2 as late at c 14 5 ka most 80 of the ice sheet s bed consists of high mountains but its core zone sited on plateaux of the intermontane belt of british columbia and coterminous parts of the usa shows broad swaths of subglacially streamlined rock and sediment broad scale mapping from new digital imagery data identifies three subglacial bed types 1 hard beds of variably streamlined bedrock 2 drumlinized soft beds of deformation till reworked from antecedent sediment and 3 mixed beds of variably streamlined bedrock protruding through drumlinized sediment drumlins on soft beds appear to be erosional features cut into till and antecedent sediments and identify the catchment areas of paleo ice streams expressed downglacier as flow sets of megascale glacial lineations msgls grooved and cloned drumlins appear to record the transition from drumlins to msgls the location of paleo ice streams reflects topographic funneling of ice from plateau surfaces through outlet valleys and a soft bed that sustained fast flow rock cut msgls are also present locally on the floors of outlet valleys cis disintegrated in

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

drumlins products of controlled or uncontrolled glaciodynamic response abstract two schools of thought on the process of drumlin formation have recently developed that have created a controversy within this ara of glacial geology one hypothesis contends that drumlins are in the main developed within a soft mobile subglacial debris zone the other school argues that for those drumlins that contain primary deposited stratified sediment formation during catastrophic subglacial flood events is necessary a classification of subglacial bed types is presented that allows for the accommodation of both schools it is proposed that a new paradigm for subglacial research may be needed in order that a closer relationship between glaciodynamics and sedimentological processes be established in future attempts at explaining glacial processes and froms

subglacial till formation sedimentary characteristics and classification we review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members processes of deformation flow sliding lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton the dominance of any one subglacial process varies both spatially and temporally giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation deposition at the ice bed interface we recommend that while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes genetic fingerprinting of subglacial tills should be less process specific and till classification must reflect the range of products encompassed by the subglacial till production continuum glacial geologists can presently unequivocally identify a glacitectonite rock or sediment that has been deformed by subglacial shearing deformation but retains some of the structural characteristics of the parent material b subglacial traction till sediment deposited by a glacier sole either sliding over and or deforming its bed the sediment having been released directly from the ice by pressure melting and or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing and support the theoretical case for c melt out till sediment released by the melting of stagnant or slowly moving debris rich glacier ice and directly deposited without subsequent transport or deformation because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions the patterns of which change temporally and spatially it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone

mass balance and sliding velocity of the puget lobe of the cordilleran ice sheet during the last glaciation an estimate of the sliding velocity and basal meltwater discharge of the puget lobe of the cordilleran ice sheet can be calculated from its reconstructed extent altitude and mass balance lobe dimensions and surface altitudes are inferred from ice limits and flow direction indicators net annual mass balance and total ablation are calculated from relations empirically derived from modern maritime glaciers an equilibrium line altitude between 1200 and 1250 m is calculated for the maximum glacial advance ca 15 000 yr b p during the vashon stade of the fraser glaciation this estimate is in accord with geologic data and is insensitive to plausible variability in the parameters used in the reconstruction resultant sliding velocities are as much as 650 m a at the equilibrium line decreasing both up and downglacier such velocities for an ice sheet of this size are consistent with nonsurging behavior average meltwater discharge increases monotonically downglacier to 3000 m3 sec at the terminus and is of a comparable magnitude to ice discharge over much of the glacier s ablation area palcoclimatic inferences derived from this reconstruction are consistent with previous independently derived studies of late pleistocene temperature and precipitation in the pacific northwest

meltwater discharge through the subglacial bed and its land forming consequences from numerical experiments in the polish lowland during the last glaciation numerical experiments suggest that the last glaciation severely affected the upper lithosphere groundwater system in nw poland primarily its flow pattern velocities and fluxes we have simulated subglacial groundwater flow in two and three spatial dimensions using finite difference codes for steady state and transient conditions the results show how profoundly the ice sheet modifies groundwater pressure heads beneath and some distance beyond the ice margin all model runs show water discharge at the ice forefield driven by ice sheet thickness modulated down ice decreasing hydraulic heads in relation to non glacial times the transient 3d model shows significant changes in the groundwater flow directions in a regionally extensive aquifer ca 90 m below the ice bed interface and up to 40 km in front of the glacier comparison with empirical data suggests that depending on the model run only between 5 and 24 of the meltwater formed at the ice sole drained through the bed as groundwater this is consistent with field observations documenting abundant occurrence of tunnel valleys indicating that the remaining portion of basal meltwater was evacuated through a channelized subglacial drainage system groundwater flow simulation suggests that in areas of very low hydraulic conductivity and adverse subglacial slopes water ponding at the ice sole was likely in these areas the relief shows distinct palaeo ice lobes indicating fast ice flow possibly triggered by the undrained water at the ice bed interface owing to the abundance of low permeability strata in the bed the simulated groundwater flow depth is less than ca 200 m copyright 2009 john wiley sons ltd

formation and deformation of basal till during a glacier surge elisebreen svalbard elisebreen a retreating valley glacier in nw svalbard experienced a surge in the past possibly during the little ice age when ice was thicker and the bed warmer the surge produced an assemblage of subglacial landforms and sediments now appearing in front of the downwasting ice margin we have analysed an array of sedimentary properties of basal till from glacial flutings ploughing marks an interflute ground moraine and meandering ridges in order to infer processes of till formation and deformation during the unrecorded surge till in flutings and ploughing marks reveals higher mechanical abrasion than in other landforms but grain size distribution and mineralogy are largely the same everywhere micromorphological characteristics are also very similar and comprise ductile and brittle deformation structures indicating fluctuating porewater pressures frequent micrograin fracturing suggests stress concentration along grain bridges during the surge till was formed by a combination of lodgement and deformation caused mainly by clast ploughing meandering ridges containing till with water escape structures indicate that surge termination was facilitated by rapid water evacuation from the ice bed interface through r type channels micromorphological similarity of till found in flutings ploughing marks and interflute areas suggests that the resolution of the micromorphological signature is insufficient to constrain the different processes that led to the formation of those landforms

modeling the influence of till rheology on the flow and profile of the lake michigan lobe southern laurentide ice sheet u s a discussion