the internal sediment architecture of a drumlin port byron new york state usa abstract an exposure within the central portion of a large drumlin at port byron new york state usa part of the large new york drumlin field reveals a sequence of steeply dipping cemented sands and gravels of proglacial ice contact deltaic origin overlain by a thin till veneer the sands and gravels appear to have been deposited within the proximal proglacial environment during a late retreat phase of the laurentide ice sheet sometime prior to being overridden by subsequent ice and drumlinized during deposition of the ice contact delta escaping subglacial regelation meltwater permeated the proximal deltaic sediment pile and calcium carbonate was released in a series of pulses to form pore occluding calcite cement within the sand and gravel porespaces the calcium carbonate precipitated into the sands and gravels due to a reduction in hydrostatic pressure and co2 outgassing of the meltwater as it exited from beneath the ice sheet once cemented these deltaic sediments were considerably stronger and acted afterward as an obstacle around which the future ice advance streamed and in turn produced the characteristic drumlin shape in overriding the ice contact deltaic sediments the ice sheet emplaced a thin layer of till which exhibits syndepositional deformation features indicative of being emplaced as a deforming bed layer beneath the advancing ice sheet micromorphological analysis of the overlying till shows that no interstitial or intraclastic calcite occurs within the till

the relationship between drumlins and other forms of subglacial glaciotectonic deformation this paper brings together the field data from 33 drumlins to show the relationship between drumlin formation and other subglacial deforming bed processes it is shown that there is a drumlin structure continuum a depositional which are similar to flutes formed by sediment flowing into the low pressure area behind a large obstacle and these conditions are mostly found within rock cored drumlins b deformational which contain different styles of deformation associated with a relatively weak core including stoss side deformation including both small and large scale brittle and ductile deformation stoss and lee side deformation compressive deformation and extensional deformation c erosional which consist of either truncated stratified sequences or homogeneous tills with distinct ice flow patterns furthermore it is suggested that if more sediment enters the deforming layer than can be removed then there will be net subglacial deposition which will result in the build up of a deforming bed till but if more sediment leaves the deforming layer then net erosion will occur under net erosional conditions any obstacle to flow may be left behind as a drumlin thus although individual drumlins have different internal structures they are formed associated with subglacial net erosion 0 1997 elsevier science ltd all rights reserved

glaciohydraulic supercooling in iceland we present evidence of glaciohydraulic supercooling under jokulhlaup and ablation dominated conditions from two temperate icelandic glaciers observations show that freezing of sediment laden meltwater leads to intraglacial debris entrainment during normal and extreme hydrologic regimes intraglacial frazil ice propagation under normal ablation dominated conditions can trap copious volumes of sediment which forms anomalously thick sections of debris rich ice glaciohydraulic supercooling plays an important role in intraglacial debris entrainment and should be given more attention in models of basal ice development extreme jokulhlaup conditions can result in significant intraglacial sediment accretion by supercooling which may explain the concentration of englacial sediments deposited in heinrich layers in the north atlantic during the last glaciation

ring shear studies of till deformation coulomb plastic behavior and distributed strain in glacier beds a ring shear device was used to study the factors that control the ultimate steady strength of till at high shear strains tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26 3 and 18 6 for tills containing 4 and 30 clay sized particles respectively other tests at steady normal stresses and variable shear strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate this weakening may be due to small increases in till porosity these results provide no evidence of viscous behavior and suggest that a coulomb plastic idealization is reasonable for till deformation however viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till we hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure during a deformation event transient dilation ofdiscrete shear zones should cause a reduction in internal pore water pressure that should strengthen these zones relative to the surrounding till a process called dilatant hardening consequent changes in shear zone position when integrated over time may yield the observed distributed strain

a multiproxy study of a basal till a time transgressive accretion and deformation hypothesis a two part basal till at knud strand denmark reveals a uniform fabric pattern and strength petrographical composition and clay mineralogy the nature of the contact with the underlying sediments ductile deformation structures partly intact soft sediment clasts small meltwater channels and thin horizontal outwash stringers dispersed in the till indicate both bed deformation and basal decoupling by pressurised subglacial water a time transgressive model is suggested to explain the lack of vertical gradation in till properties in which debris released from the active ice sole is sheared in a thin zone moving upward as till accretion proceeds it is suggested that although strain indicators occur throughout the entire till thickness the deformation at any point of time encompassed the uppermost part of the till only allowing preservation of fragile clasts below the substantial thickness of the till up to 6 m coupled with a much smaller by more than one order of magnitude inferred thickness of the deforming bed suggests that the bulk of till material was transported englacially prior to deposition the lack of petrographical gradation in the till is attributed to effective mixing and homogenisation of material along the ice flow path copyright 2004 john wiley sons ltd

facies characteristics of a proglacial turbiditic sand lobe at svalbard abstract a shallow marine turbiditic lobe inferred to be associated with an ice contact delta was deposited during the last glaciation weichselian at svalbard the norwegian arctic the postglacially raised sandy lobe is exposed in a 200 m long river section and exhibits six units that reflect an early phase of glaciomarine mud deposition followed by three episodes dominated by deposition from sandy turbidity currents the lobe is characterized by a complex pattern of turbidite filled erosional features from thin scour fills reflecting a highly mobile flow pattern to stacked chute fill deposits showing confined flow the sandy turbidites show rhythmic deposition of massive to graded laminae deposited from suspension alternating with faint laminae or ripple cross lamination deposited from tractional transport semi continuous turbidity currents are thought to have been generated by retrogressive slumping of ice proximal well sorted sandy deposits on the delta front and slope the lateral and vertical changes of lithofacies within the sandy lobe are mainly attributed to normal mechanisms of turbiditic lobe growth with only minor effect from synsedimentary glacier terminus movement and minor changes in the meltwater outflow position this detailed mapping of facies changes allows a fairly accurate reconstruction of the axis of the former discharge outlet

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

evidence from microstructures of deformable bed conditions within drumlins chimney bluffs new york state abstract discussion on the process of drumlin formation has been ongoing for over a century at present two dominant schools of thought prevail viz formation by a subglacial hydraulic mechanisms and b due to subglacial sediment deformation the latter mechanism depends on the existence of a subglacial deformable bed evidence from part of the large central new york drumlin field reveals evidence of syn depositional deformation during the emplacement of diamictons that constitute the major portion of drumlins exposed along shore bluffs in chimney bluffs state park these diamictons cannot be lodgement till facies members but appear to comprise facies indicative of melange style deposition under subglacial deformable bed conditions the evidence obtained from microstructures and plasma fabric within these diamictons supports the existence of a deformable bed during diamicton emplacement it is likely that formation of the drumlins occurred under these same subglacial bed conditions

glacial sedimentary evidence supporting stick slip basal ice flow last glacial late devensian subglacial sediments in the southern sperrin mountains north central ireland comprise tabular bedrock rafts which are interbedded with diamicton till and brecciated bedrock and separated by glaciotectonic shears bedrock rafts and diamicton beds alternate laterally and vertically in the profile suggesting the ice bed interface was chequered with both high strength rock rafts and low strength diamicton patches during accumulation of the sediment pile the overall facies arrangement is interpreted qualitatively as reflecting stick slip basal ice flow and is related to the redistribution of free subglacial meltwater at the ice bed interface it is inferred that stick phases occur over bedrock rafts as meltwater migrates to lower pressure regions infilling shallow cavities on the surrounding diamicton surface slip occurs when porewater pressure increases and the meltwater layer backfills across bedrock raft surfaces causing ice bed uncoupling the presented field evidence suggests a stick slip rather than pervasive subglacial deformation model may be a better approximation for the flow of parts of the late devensian mid latitude ice sheets

till as a glacial tectomict its internal architecture and the development of a typing method for till differentiation abstract till has been studied for over a century yet still a clear understanding of the mechanics of deposition and or emplacement of till eludes us tills in the past have been generally subdivided into sediments formed in various sub environments of the glacial system as such it has been generally agreed that tills can be separated in terms of these types based on various attributes such as clast fabric increasingly such separation into various sub types of till has become however highly debatable and tenuous based upon the use of micromorphology and a greater in depth appreciation of the internal architecture of till it has become apparent that previously established classifications of till are erroneous micromorphology reveals that deposition and or emplacement of tills is a complex tectonic process such that lodgement melt out and flow tills as previously defined do not appear to exist rather a new term better describes tills as glacial tectomicts by using micromorphology in establishing the presence of sets of microstructures a statistical method is introduced that permits differentiation of tills from other diamictons the implications for this new understanding of tills allied to statistical typing is considerable in terms of till formation glacial stratigraphy and understanding glacial environments in terms of stress histories porewater transmission subglacial deformation mechanics till rheological phase changes and the timeline of till formation

till fabric associated with deformable beds this paper investigates the development of till fabric associated with glaciers overlying deformable beds first the till fabrics from both modern and ancient soft bed and hard bed subglacial environments are investigated and then soft bedded sites with specific strain histories are examined the results indicate that lodgement tills had a strong fabric whilst there is a wide range of much weaker fabric strengths associated with soft bed tills it is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development field sites were chosen to isolate these two factors and fabrics investigated in areas of known relative shear strain and known relative thickness of the deforming layer the results indicate that 1 for a fixed deforming layer fabric strength does increase with increasing deformation except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain and 2 lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength whilst soft bed tills associated with a thick deforming layer have a low fabric strength where the deforming layer is thin clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed however where the deforming layer is thicker and shear strains are high melt out processes encourage a parallel fabric but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric these processes combine to reduce the overall fabric strength till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till

the vendian succession of northeastern spitsbergen petrogenesis of a dolomite tillite association abstract the sedimentary record of late precambrian time is magnificently displayed in the highland snowfields of northeastern spitsbergen svalbard vendian strata are represented essentially by the polarisbreen group which consists mostly of dolostone and includes two dolomitic glacial units the oldest sediments in the polarisbreen group compose the elbobreen formation c 400 m which is divided into four laterally persistent members the lower carbonate member e1 125 m contains a distinctive basal dark grey limestone with microspar filled synaeresis cracks suggested to be of lagoonal origin and associated with minor dolostone shale and chert higher parts of the member are dominantly dolostone partly stromatolitic with some shale and sandstone shallow subtidal to intertidal deposition is indicated by the dominance of intraclastic lithologies and relics of anhydrite penecontemporaneous dolomite is partially overprinted by microsparry dolomite thought to be of groundwater origin the redefined petrovbreen member e2 consists of diamictite and other detrital dolostone pronounced thickness variations 2 40 m are thought to be original depositional features the member represents the deposits of a short glacial period in which the following depositional processes are inferred lodgement massive diamictite subaqueous meltout massive and bedded diamictite ice rafting lithologies bearing dropstones and possibly also diamictite redeposition by sediment gravity flows some diamictite and conglomerate rhythmite and shale current winnowing thin tabular conglomerate subaerial or subaqueous meltwater action channelled conglomerate and sandstone periglacial shrinkage diamictite wedge fillings the macdonaldryggen member e3 230 m is a monotonous succession of shaly dolostone of lagoonal origin it grades up into the slangen member e4 25 m which consists of subtidal to intertidal dolarenite with anhydrite relics succeeded by fenestral dolostone that was fractured and cemented by saline groundwaters in an emergent environment the wilsonbreen formation 160 m represents a return to glacial deposition but this time longer lasting and with substantial extra basinal material represented the gropbreen member w1 28 107 m and the ormen member w3 44 139 m consist dominantly of dolomitic diamictite with subordinate conglomerate and sandstone and are separated by a middle carbonate member w2 3 30 m which contains distinctive rhythmitic and stromatolitic limestone as well as sandstone the same depositional processes can be recognised as in the petrovbreen member but the wilsonbreen formation is overall of somewhat more continental aspect lower proportion of rhythmite and dropstone structures in addition there are basal breccia and crack fillings formed by frost shattering of the underlying cemented dolostone tabular sandstone thought to be formed by wave reworking of outwash a striated terrestrial cobble pavement supraglacially derived breccia horizons periglacial wedges filled by sand and the w2 assemblage of possible lacustrine origin the dracoisen formation 525 m represents an abrupt return to non glacial conditions an upward deepening wave dominated succession of pure dolostone d1 20 m and impure dolostone d2 105 m is succeeded by offshore black shale d3 150 m and then by a very shallow water succession of evaporite lacustrine aspect with a dolostone containing evaporite relics d5 10 m separating dolomitic sandstone and shale d4 80 m and d6 150 m the contact with the transgressive cambrian sandstones above is poorly exposed deposition of the succession dominantly under marine conditions is inferred but it is difficult to rule out a lacustrine environment at any particular horizon this dolomite tillite association can be explained by penecontemporaneous and minor secondary dolomite formation in marginal environments with warm climatic indicators at some levels being sharply interrupted because of rapid climatic changes by glacial sediments containing abundant detrital dolomite since the latter sediments make up only 17 of the 1080m thick succession glacial conditions only occupied a small proportion of vendian time

clast fabric development in a shearing granular material implications for subglacial till and fault gouge elongate clasts in subglacial till and in fault gouge align during shearing but the relation between clast fabric strength and cumulative shear strain for such materials is effectively unknown this relation was explored in experiments with a large ring shear device in which a till and a viscous putty that contained isolated clasts were sheared to high strains as expected rotation of clasts in the putty is closely approximated by the theory of g b jeffery who derived the orbits of rigid ellipsoids in a slowly shearing fluid clast rotation in the till however is strikingly different rather than orbiting through the shear plane as predicted by jeffery most clasts rotate into the shear plane and remain there resulting in strong fabrics regardless of the aspect ratios and initial orientations of clasts this divergent behavior is likely due to slip of the till matrix along the surfaces of clasts which is a natural expectation in a granular material but violates the no slip condition of jeffery9s model these results do not support the widespread belief that subglacial till deformation results in weak clast fabrics thus many tills with weak fabrics thought to have been sheared subglacially to high strains like many basal tills of the laurentide ice sheet may have been sheared only slightly with little effect on either ice sheet dynamics or sediment transport in addition these results indicate that in simple shear the rotation of clasts in till and in fault gouge is best analyzed with the model of a march who treated inclusions as passive markers

on the interpretation of subglacial till fabric abstract the modal distribution of stone long axis fabrics and their respective eigenvalues can be used to infer the genesis of subglacial till in this paper we offer a two axis diagram that compares fabric modality to fabric isotropy s3 s1 and addresses the problem of eigenvectors falling between the modes of some well developed till fabrics with low eigenvalues our simple five fold scheme of modality categories includes 1 unimodal clusters 2 spread unimodal 3 bimodal clusters 4 spread bimodal and 5 polymodal to girdle like fabrics and requires the analyst to study equal area lower hemisphere schmidt plots of the fabric data after assigning the fabric to a morality category isotropy is calculated and both results are plotted on the graph whic helps to separate two main fields of subglacial till 1 lodgement and subglacial meltout tills and 2 deformation fill on the basis of selected published fabrics from tills at modern glaciers as well as our own pleistocene till data lodgement and subglacial meltout tills tend to have unimodal or bimodal fabrics in contrast deformation tills and tills that experienced multiple processes tend to have polymodal to girdle like fabrics some overlap occurs between fields be cause of the complex nature of till formation i e because pure end member till facies are rare and most tills are hybrids we strongly recommend that schmidt plots be visually analyzed and used in conjunction with eigenvalues when studying till however fabric data alone is not enough multiple criteria including structural lithologic and stone morphologic data from the till must also be considered before drawing conclusions on till genesis furthermore if eigenvectors fall between fabric modes then they cannot be used to indicate former ice movement directions finally our new modality isotropy diagram may have wider applications

a decade of sedimentation in ice contact proglacial lakes bering glacier ak abstract bathymetric surveys during the 1991 2000 decade in two ice contact proglacial lakes on the eastern sector of bering piedmont lobe captured the buildup effects of the 1993 1995 surge following ice front advance of 1 0 1 5 km into tsivat and tsiu lakes the basins were significantly altered by surge related sedimentation including the impact of a subglacial outburst into tsivat lake the subsequent changes in basin shape size and morphology were monitored by six bathymetric surveys measured changes in water depth serve as a proxy for determining increments of sediment accumulation upwelling ice front vents fed by subglacial tunnels transported suspended fine sediment directly into the lake system the rate of suspension settling within both lakes varied from 0 6 to 1 2 m year 1 prior to the surge suspended load during surge years increased sixfold from 1 7 to 13 9 g l 1 accompanied by increased sediment accumulation of 2 2 3 1 m year 1 vent related aggradation and subsequent filling of tsivat lake caused sediment bypassing to tsiu lake where encroachment by delta growth contributed to a postsurge rate of bottomset accumulation of 3 0 m year 1 the total sediment influx from subglacial sources is represented by the sum of bathymetrically determined accumulation plus an estimated volume of sediment that remained suspended thus passing through the lake system total sediment flux along the eastern bering piedmont lobe from 1991 to 2000 is approximately 227 million cubic meters

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

what causes till thickness to change over distance answers from mýrdalsjokull iceland abstract many explanations have been provided for variations of the spatial distribution and thickness of till sheets this paper gives new insight into the architecture of a stratigraphically distinct till sheet as a function of the type of substratum and preadvance topography at a scale of 10 km this emphasises the sensitivity of the subglacial system to changes in the basal drainage conditions the study area is the forefield of slettjokull at the northern margin of the m rdalsjokull ice cap central south iceland here detailed lithostratigraphical and sedimentological investigations including mapping of the thickness for two till units sediment logging clast fabric and geotechnical measurements provide a basis for interpretations the results show that the thickness of a stratigraphically distinct till sheet is directly correlated to the type of substratum where the substratum consists of sorted sediments the till is thin conversely the till is thick where the substratum consists of till overlying sorted sediments a sedimentological model is presented in which till thickness is tied to the variable hydraulic conditions experienced in a deforming subglacial bed

sedimentary successions in ice marginal fans of the late saalian glaciation southwestern poland abstract the late saalian end moraines of southwestern poland contain several features that may be attributed to the formation that is predominantly controlled by the deposition of sorted waterlaid sediments these hills are essentially small alluvial fans with ice contact facies they have a semi conical form plano convex geometry their distal slopes are up to 5 and proximal slopes are between 5 and 20 the sedimentary sequences contain two units the lower unit comprises coarse grained material of gravel bed sheetflow facies and gravel bed channel facies with subordinate high energy channel flow facies and boulders up to 1 8 m in diameter the upper unit contains sand bed sheetflow facies and sand bed channel facies the sediment bedding is usually conformable with and parallel to the inclined fan surface these sequences represent typical proximal distal fan facies associations with a high energy and pulsatory water discharge in the proximal zone the fans in spite of their formation in the proglacial zone have geomorphological features and sedimentary sequences that are very similar to present day fans and are equivalent to sheetflow dominated or humid alluvial fans in non glacial environments

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

effects of construction induced stresses on dynamic soil parameters of bootlegger cove clays this paper presents the influence of construction induced stress changes on the dynamic soil parameters of bootlegger cove formation bcf clays at the site of the port of anchorage poa expansion project results are presented based on numerical simulations of the poa wharf construction and laboratory experimentation where both free field and postconstruction conditions are reproduced the postconstruction stability of the foundation clay is considered under both static and dynamic loadings for the dynamic case a cyclic triaxial loading equivalent to the contingency level earthquake cle established for design was used whereas for the static case undrained triaxial compression and extension loadings to failure were employed the results from numerical simulations and laboratory experiments are presented and compared with field measurements and performance data collected during the project construction the results obtained from specially designed cyclic triaxial probes show the large effects that the construction induced stresses can have on the dynamic soil parameters and illustrate the importance of adequately considering such effects testing protocols and procedures are proposed for quantifying postconstruction soil parameters to better reflect field values and values at small and otherwise appropriate strain levels

automated drumlin shape and volume estimation using high resolution lidar imagery curvature based relief separation a test from the wadena drumlin field minnesota abstract resolving the origin s of drumlins and related megaridges in areas of megascale glacial lineations msgl left by paleo ice sheets is critical to understanding how ancient ice sheets interacted with their sediment beds msgl is now linked with fast flowing ice streams but there is a broad range of erosional and depositional models further progress is reliant on constraining fluxes of subglacial sediment at the ice sheet base which in turn is dependent on morphological data such as landform shape and elongation and most importantly landform volume past practice in determining shape has employed a broad range of geomorphological methods from strictly visualisation techniques to more complex semi automated and automated drumlin extraction methods this paper reviews and builds on currently available visualisation semi automated and automated extraction methods and presents a new curvature based relief separation cbrs technique for drumlin mapping this uses curvature analysis to generate a base level from which topography can be normalized and drumlin volume can be derived this methodology is tested using a high resolution 3 m lidar elevation dataset from the wadena drumlin field minnesota usa which was constructed by the wadena lobe of the laurentide ice sheet ca 20 000 years ago and which as a whole contains 2000 drumlins across an area of 7500 km 2 this analysis demonstrates that cbrs provides an objective and robust procedure for automated drumlin extraction there is strong agreement with manually selected landforms but the method is also capable of resolving features that were not detectable manually thereby considerably expanding the known population of streamlined landforms cbrs provides an effective automatic method for visualisation of large areas of the streamlined beds of former ice sheets and for modelling sediment fluxes below ice sheets

drumlin formation by subglacial meltwater erosion some drumlins are shown to belong to a family of remnant erosional ridges associated with subglacial meltwater the lengths of these ridges vary in scale from a few tens of millimetres to over 5 km similar form elements are illustrated for three scales of forms it is concluded that some drumlins are composed of preexisting sediment or bedrock left upstanding in ridges as a result of the removal of surrounding materials this erosional theory is related to an earlier theory in which drumlins are held to result from the infilling of subglacial cavities the unifying factor for both theories is the role of sheets of subglacial meltwater

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

drumlinised glaciofluvial and glaciolacustrine sediments on the smaland peneplain south sweden new information on the growth and decay history of the fennoscandian ice sheets during mis 3 abstract dug trenches in sediment cored drumlins on the smaland peneplain southern sweden occurring intermixed with rock cored drumlins revealed sedimentary successions and facies suggesting that the core sediments were primarily deposited proglacially both in glaciolacustrine settings as delta front and more distal deeper water sediments and as glaciofluvial braidplain sediments from the relationship between the drumlin sorted sediment cores and their surficial drape by a till carapace it is concluded that erosion was the main drumlin forming process here reshaping the original sediment morphology into streamlined landforms after which a more or less thick traction till was deposited over the eroded and or deformed ice bed interface drumlin formation probably took place during the last deglaciation over the area the drumlin core sediments in 14 excavated trenches were dated by optically stimulated luminescence osl most retrieved ages fall into two distinct groups both within marine isotope stage 3 c 60 30 ka when plotted as mean osl ages for each site all but two sites fall either into an older age group c 48 56 ka seven sites or into a younger age group c 28 38 ka five sites these age intervals correlate well with the suggested time spans for the ristinge and klintholm ice advances in denmark by ice flowing from the baltic basin westwards terminating in fan shaped patterns these interstadial ice advances in the southwestern sector of the fennoscandian ice sheet give rise to enigmatic questions on the physics and mechanics of ice sheet build up and decay especially as palaeoenvironmental data for northern fennoscandia suggest widespread ice free conditions within roughly the same time fame as the recorded ice advances in the south

uncertainty in the breakup spreading history and velocity variations of gondwana abstract there are three major uncertainties in the derivation of plate velocities in general firstly the errors associated with the identification of isochrons and fitting conjugate isochrons secondly various geological choices made in the model that represent the model creator s view of when break up occurred and how tight the fit was and whether certain isochrons should be part of the reconstruction or not this i term model uncertainty finally the choice of timescale while much of the geoscientific community have adopted recent timescale standards such as gts16 the smaller plate kinematic community continue to use legacy timescales typically a composite of kg85 and cent94 in this paper i examine these second and third source of uncertainty and show that they can be significant sources of noise in the signal of plate velocities frequently over 20 of the signal and often as high as 60 for those seeking to connect plate kinematics vertical motions and the mantle dynamics knowing the uncertainties in the kinematic model used is essential

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

till fabric patterns and significance particle response to subglacial stress abstract the analysis of particle arrangements within glacial sediments fabric has long been considered to reflect the behaviour of the ice that transported and deposited or deformed the sediment particles can develop specific fabric arrangements as a response to bulk sediment strain driven by effective stress at the bed of a glacier as such fabric analysis has been used to try and understand the complex relationship between glacier and ice sheet dynamics and the geological evidence of former glaciation developments have focused on the use of stereonet patterns and statistical measures vector and eigenvector analysis of clast fabric to infer the process of sediment deposition deformation and consequent ice flow directions and dynamics however as this study identifies fundamental processes of particle orientation march jeffery and taylor rotation mechanisms would suggest more complex explanations than have been previously reported this study reports the results of a detailed field and laboratory investigation of macro scale clast and microfabrics from a till sequence in central scotland the results suggest a systematic behaviour of different particle size fractions which may be interpreted in terms of the strain response of sediment three fabric patterns are identified that illustrate consistent progressively shifting and more complex particle behaviour it is suggested these orientation patterns indicate the degree to which subglacial sediment has undergone strain and the nature of the deforming bed erosional or accretionary it is concluded that this approach may yield fundamental information about the strain conditions underlying different ice masses providing critical data linking glaciological modelling with the sedimentary evidence of glaciation

microstructures in diamictons evidence of subglacial bed conditions abstract examination of microstructures within diamictons from southern ontario canada and southern bavaria and baden wurttemburg germany reveal evidence illustrative of certain subglacial bed conditions the microstructures from ontario were obtained from a complex glacial melange unit that forms the internal structure of an isolated drumlin megaflute on the north shore of lake erie the evidence from germany was taken from individual drumlins within drumlin fields in the inn chiemsee iller rhein and salzach glacier areas the structures within the diamicton from ontario reveal clear evidence of massive shear associated with high externally applied strain there are kink band arrays high angle shear zones crenulation and sedimentary dyke and water escapes structures in the german diamictons there is a lack of evidence of high levels of bulk deformation there is however a very high carbonate content some evidence points to the possibility that these diamictons may contain intraclasts of paleosols interpretation of both sets of diamictons suggests that the ontario sediments were subject to conditions at the bed of an ice mass best typified as a deformable bed the german sediments may be interpreted as 1 being so saturated that any deformation must have been by inter grain slip pervasive deformation and thus no structures are recorded or 2 being already cemented by the carbonates and sufficiently rigid to cause strain to be taken up elsewhere or 3 were frozen prior to transposition and were again too rigid to suffer deformation or 4 suffered no deformation whatsoever the analyses of these two sets of sediments have important implications for subglacial conditions before and during drumlin and other subglacial bedform development

polyphase deformation at an oscillating ice margin following the loch lomond readvance central scotland uk the sequence of glacitectonic disturbance of an ice contact delta during the initial stages of deglaciation following the loch lomond readvance is examined an ice marginal gilbert type deltaic sequence exposed in drumbeg quarry drymen scotland displays a polyphase deformation history that is punctuated by periods of erosion and deposition progradation of the delta into ice dammed lake blane led to a temporary stabilisation of the loch lomond glacier during recession from its loch lomond readvance maximum position this was followed by a ne directed readvance into the ice contact slope of the delta resulting in proglacial deformation d1 and the formation of a thrust block ridge in the middle to outer parts of the delta complex sedimentation continued uninterrupted subsequent retreat of the ice was accompanied by erosion and deposition as the ice contact slope was re established a second phase of readvance resulted in subglacial ductile shearing d2 and deposition of a diamicton during ne directed over riding of the delta sediments by the ice the direction of ice flow subsequently changed towards the se this was followed by a further retreat of the ice and re establishment of the fan delta complex the complexity of the glacitectonic sequence at drumbeg records the impact of an oscillating ice margin responding to either relative deepening of the lake waters in contact with the receding snout or climatic controls it provides further evidence that many of the scottish highland glaciers were subject to active recession rather than stagnation at the end of the loch lomond readvance

palaeoenvironmental interpretation of an ice contact glacial lake succession an example from the late devensian of southwest wales uk abstract during the late devensian late weichselian glaciation a number of large proglacial lakes developed in dammed river valleys along the southwest coast of wales u k this paper presents sedimentological data together with a digital terrain model to establish the sedimentation history dynamics and evolution of the largest lake glacial llyn lake teifi buried valley fill sequences within the margins of the former lake basin reveal a thick succession of glaciolacustrine muds which coarsen upward into or are locally abruptly overlain by proximal deltaic subglacial and glaciofluvial deposits sediment delivery pathways represented in the lacustrine succession include gravity flows suspension settling deltaic aggradation and iceberg rafting the latter indicating ice contact conditions the lacustrine muds are variably deformed with a range of syn and post depositional structures some of which indicate subglacial deformation associated with overriding of the lacustrine succession syn depositional structures indicate high sedimentation rates which may explain an absence of bioturbation structures the overall coarsening upward succession and cap of subglacial and or glaciofluvial deposits support recent theories suggesting that glacial llyn teifi formed during glacial advance there is no evidence to support glaciomarine conditions of sedimentation in this area of the irish sea basin

reconstruction and dynamics of the late wisconsin ontario ice dome in the finger lakes region new york distribution of drumlins on the ontario lowlands and the northern appalachian plateau of new york state are utilized in a glaciological model that estimates thicknesses of the late wisconsin ice sheet along five glacial flowlines over the central section of the finger lakes dynamics of the model are based upon a force balance approach that incorporates variable width of a flowband bedslope radius of curvature and center of outflow location drumlin axes along five hydrostatic isograms were projected northward points of axes convergence define the center of outflow in the eastern ontario basin flowlines along the axial trace of the major seneca and cayuga troughs and bordering interfluves were constructed southward to the valley heads moraine system calculation of flowline geometry for any assumed basal shear stress requires neither information on ice temperature nor the flow law of ice ice thicknesses for each flow line were calculated for b values of 5 1 0 and 1 5 bar ice thicknesses at 1 bar range from a minimum of 1 932 m for the seneca trough flowline to a maximum of 2 093 m at the keuka seneca interfluve flowline raising or lowering b by 5 bar results in maximum thickness differences of 21 and 26 respectively simulating ice recession a series of ice marginal terminal positions were plotted for a basal shear stress of 1 bar these predicted terminal positions agree well with locations of mapped moraine deposits these probable values of basal shear stress support the presence of glacial flow representative of internal deformation rather than basal sliding although recent interpretations of the formation of the finger lakes focus on ice streams facilitated by large volumes of subglacial meltwater our evidence discounts the presence of ice streams and supports the existence of outlet glaciers in the troughs of the finger lakes

sediment deformation beneath glaciers and its coupling to the subglacial hydraulic system abstract the extent and style of shear deformation in sediments beneath modern glaciers and the geological evidence for such deformation in deposited sediments are reviewed new evidence is presented from beneath a modern glacier of the spatial and temporal patterns of water pressure fluctuation and of time dependent patterns of deformation in sediments it is concluded that in most experimental sites beneath soft bedded modern glaciers deformation is a significant or major contributor to glacier movement and the resultant discharge of till is large enough to make sediment deformation a major till forming process particular modes of deformation facilitate incorporation of underlying material into the till whilst the capacity of a deforming till to absorb strain can protect the underlying strata from deformation leading to the commonly found relationship where till overlies other strata with a sharp planar interface it is argued that the almost ubiquitous occurrence of drumlins on the beds of former ice sheets is a reflection of the widespread occurrence of sediment deformation beneath them with important implications for the coupling of ice sheet flow and bed properties it is argued that the mechanical behaviour of the subglacial system is not simply determined by till properties but largely controlled by the subglacial water pressure regime determined by the nature of subglacial drainage results of field experiments show how the nature of the basal hydraulic system can play a vital role in controlling the coupling between the glacier and till deformation processes they show that rapid glacier advances can produce undrained loading of sediments that effective pressure may increase either upwards or downwards in a till according to the direction of drainage and that interstitial water pressures in subglacial sediments can show large and rapid variations producing strong variations in the rate and distribution of strain and in the partitioning of basal movement between sliding and deformation

ice thickness measurements of taku glacier alaska u s a and their relevance to its recent behavior using radio echo soundings and seismic reflections we measured cross sections of taku glacier near juneau alaska to resolve inconsistencies in previous measurements and to understand better the glacier s dynamics the maximum thickness is about 1477 m and the minimum bed elevation is about 600 m below sea level which establishes taku glacier as the thickest and deepest temperate glacier yet measured our data indicate that during the 19th century the terminus of taku glacier may have begun its rapid advance at a position where the ice bed was greater than 300 m below sea level and more than 25 km from the inland end of its submarine trough this behavior is uncharacteristic of temperate tide water glaciers the glacier which no longer calves has eroded a sediment layer 100 m thick since 1890 at an average rate of about 3 m a 1 since 1948 this high erosion rate retards advance by entrenching the glacier into the terminal moraine calculations based on ice deformation theory indicate significant basal ice motion near the terminus and high basal shear stress 140 220 kpa along much of its length estimated differences between ice flux and balance flux are consistent with observed thickening and positive net mass balance these data indicate that ice volume is increasing and that further advance is likely

microscopic evidence of subglacial deformation abstract subglacial deformation is by now a well established process however recognition of this process in pleistocene tills is not always clear studies of thin sections of pleistocene tills show that there are a variety of microscopic features caused by deformation this paper gives a review of microscopic evidence for subglacial deformation and starts with a case where deformation could be established macroscopically then traces the microscopic features through planar and rotational movements which are described and separated accordingly attention is also given to features caused by crushing of grains and sediment dewatering these studies indicate that almost all basal tills show evidence of subglacial deformation rather than sedimentary derivation this implies that if we want to understand subglacial processes such tills should be described in tectonic rather than sedimentary terminology

glacigenic clast fabrics genetic fingerprint or wishful thinking the interpretation of glacigenic diamictons is a subjective process for which quantitative support is frequently sought from parameters such as clast shape and fabric it has been widely suggested that different glacigenic diamicton facies possess distinct clast fabric signatures this paper examines this concept using a data set of 111 clast fabrics and a synthesis of published results eigenvalues are calculated and compared for a variety sedimentary facies it is concluded that clast fabric alone is not able to discriminate between different glacigenic facies and it is argued that clast fabric offers little quantitative support in the interpretation of glacigenic sediments it is suggested therefore that although clast fabric may continue to have a role as an indicator of relative strain at specific sites its use in the discrimination of glacigenic facies is limited consequently we should be much more selective in undertaking such analyses in the future copyright 1999 john wiley sons ltd

thin section production of diamicts problems and solutions abstract thin section production of diamict samples presents many problems primarily as a result of incomplete impregnation this problem is investigated in diamicts of known glacial glaciomarine and subaerial origin the inherent sedimentological characteristics rather than particular mode of deposition appears to influence resin impregnation of diamicts a technique of surface impregnation is presented to overcome this problem when samples fail to impregnate fully during initial preparation and is incorporated into current techniques within a flowchart in the majority of cases this technique results in successful production of a useful thin section

sediment glaciohydraulic supercooling and fast glacier flow glaciers often advance over proglacial sediments which then may enhance basal motion forglaciers with abundant meltwater thermodynamic considera tions indicate that the sediment ice contact in the direction of ice flow tends toward an angle opposed to and somewhat steeper than the surface slope by slightly more than 50 a simple model basedonthis hypothesisyieldstheextentof over ridden sediments as a function of sediment thickness and strength a result that may be useful in guiding additional fieldwork forhypothesistesting sediment floored as wellasrock floored over deepenings are common features along glacier flow paths and are expected based on theories of glacier erosion entrainment transport and deposition