the instability theory of drumlin formation and its explanation of their varied composition and internal structure despite their importance in understanding glaciological processes and constraining large scale flow patterns in palaeo glaciology there is little consensus as to how drumlins are formed attempts to solve the drumlin problem often fail to address how they are created from an initially flat surface in the absence of obvious cores or obstacles this is a key strength of the instability theory which has been described in a suite of physically based mathematical models and proposes that the coupled flow of ice and till causes spontaneous formation of relief in the till surface encouragingly model predictions of bedform height and length are consistent with observations and furthermore the theory has been applied to a range of subglacial bedforms and not just drumlins however it has yet to confront the myriad observations relating to the composition and internal structure of drumlins and this could be seen as a major deficiency this paper is a first attempt to assess whether the instability theory is compatible with the incredible diversity of sediments and structures found within drumlins we summarise the underlying principles of the theory and then describe and attempt to explain the main types of drumlin composition e g bedrock till glaciofluvial sediments and combinations thereof contrary to a view which suggests that the presence of some sedimentary sequences e g horizontally stratified cores is inconsistent with the theory we suggest that one would actually expect a diverse range of constituents depending on the inheritance of sediments that pre date drumlin formation the duration and variability of ice flow and the balance between erosion and deposition till continuity at the ice bed interface we conclude that the instability theory is compatible with and potentially strengthened by what is known about drumlin composition and as such offers the most complete and promising solution to the drumlin problem to date

till beneath ice stream b 4 a coupled ice till flow model a nonsteady model of an ice stream flowing on deforming till shows that the system responds rapidly and in a stable manner to reasonable marginal perturbations for the model we require one dimensional flow and continuity of ice and till and we assume linear viscous till rheology and balance between the driving stress for ice flow and the resistive stress at the bed this allows us to write coupled equations for the time rate of change of the ice thickness and the till thickness in terms of these thicknesses and the till viscosity an analytic steady state solution for ice stream b west antarctica shows that till viscosity decreases slowly downstream probably in response to decreasing effective pressure downstream nonsteady numerical experiments with fixed ice thicknesses at the ends show that a marginal perturbation causes a wave of adjustment to travel the length of the ice stream in about 50 years with a new steady state in about 200 years changes in till thickness tend to moderate perturbations in the ice and thus stabilize the system

the sensitivity of subglacial bedform size and distribution to substrate lithological control the varied nature of a glacial substrate has often been invoked to account for the varied distribution arrangement and morphological expression of subglacial bedforms the ease and rate of sediment production from the bedrock substrate and the rheological mechanical and hydraulic properties of the resultant subglacial sediment layer have been argued in certain samples or settings to account for the local to regional distribution of bedforms and bedform types and varying morphological expression such as size shape elongation or spatial arrangement from locally coherent observations and relationships it is tempting to extrapolate such patterns to express a more fundamental lithological control on subglacial bedform properties and formation here we use a large systematically compiled dataset 30 000 subglacial bedforms inscribed by the irish ice sheet to explore potential lithological controls upon subglacial bedform distribution and morphological expression we expect any lithological control upon form size or distribution to be expressed through a spatial correlation between bedform properties and those of the bed lithology we extract bedform morphometric and distribution statistics and underlying lithology data from datasets held in a geographic information system for comparative analyses at ice sheet scale i e considering our whole bedform population we find no apparent relationship between the lithology of either bedrock or till substrate and bedform occurrence density of arrangement or size regional to local examples do in contrast exhibit some coincident changes in substrate and bedform expression these are typically manifest as an abrupt shortening or lengthening of drumlins at a lithological boundary superimposed upon more gradual regional trends however not all local variability observed in the bedform population can be attributed to a lithological driver and where it can be inferred lithological control is often in tandem with or subordinate to other drivers such as topographic setting or ice dynamics we propose a hierarchy of controls upon bedform form and distribution strongly coupled to spatial scale lithological differences can modulate local scale form and expression but bedform incidence and properties are primarily governed by ice sheet scale glaciological patterns and drivers only under certain unknown conditions at a local scale is the bedforming process sensitive to the lithological properties of the bed

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

ice stream surface texture sticky spots waves and breathers the coupled flow of ice till and water this paper addresses the coupling of flows of ice till and water and the issue of whether such coupling provides mechanisms for meso scale kilometres to tens of kilometres variability in ice sheet flow and texture the question of whether effective pressures at the ice bed interface are statically or hydraulically controlled is examined in this paper the answer is scale dependent and has a significant effect on the relationship between ice surface and basal topography the consequences of these considerations on till flow coupled ice till flow and coupled ice till and water flow are examined an analysis of till flow kinematics and shock formation is carried out the linear stability of coupled long wavelength ice till flow is analysed and regions in parameter space where this flow is unstable with rather small rate constants are found upstream moving ice surface waves are predicted the linear stablity of coupled ice till water flow is examined where water flow is modelled using a basal flow system with effective pressure dependent properties again regions in parameter space where the system is linearly unstable are found this time with relatively rapid rate constants the water pressure exhibits breather modes these analyses assume that there is a substantial basal traction a problem with models of ice streams wholly restrained at the side is identified they seem to predict erosion rates which are unfeasibly large there appears to be sufficient variability in the ice till water system to potentially explain texture in ice stream surfaces variations in ice stream thickness of tens of metres not directly relatable to topography and waves moving upstream or downstream most importantly the ice stream bed system is shown to exhibit meso scale variability simply by coupling ice flow according to the shallow ice approximation till flow according to the hydrostatic thin till approximation and water flow according to an effective pressure dependent hydraulics

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

stick slip statistical mechanics at the bed of a glacier the multiple processes which control motion at the bed of glaciers are too heterogeneous to be modeled directly but a simple stick slip slider block model of a glacier bed allows a statistical mechanical treatment of the physics beneath thin and shallow sloped glaciers the technique may reconcile apparently contradictory field observations of the relationship between glacier sliding and basal water pressure simulations also predict hysteresis in the relationship between water pressure and basal velocities

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

the periodic topography of ice stream beds insights from the fourier spectra of mega scale glacial lineations ice stream bed topography contains key evidence for the ways ice streams interact with and are potentially controlled by their beds here we present the first application of two dimensional fourier analysis to 22 marine and terrestrial topographies from 5 regions in antarctica and canada with and without mega scale glacial lineations msgls we find that the topography of msgl rich ice stream sedimentary beds is characterized by multiple periodic wavelengths between 300 and 1200 m and amplitudes from decimeters to a few meters this periodic topography is consistent with the idea that instability is a key element to the formation of msgl bedforms dominant wavelengths vary among locations and on one paleo ice stream bed increase along the direction of ice flow by 1 7 0 52 km 1 we suggest that these changes are likely to reflect pattern evolution via downstream wavelength coarsening even under potentially steady ice stream geometry and flow conditions the amplitude of msgls is smaller than that of other fluvial and glacial topographies but within the same order of magnitude however msgls are a striking component of ice stream beds because the topographic amplitude of features not aligned with ice flow is reduced by an order of magnitude relative to those oriented with the flow direction this study represents the first attempt to automatically derive the spectral signatures of msgls it highlights the plausibility of identifying these landform assemblages using automated techniques and provides a benchmark for numerical models of ice stream flow and subglacial landscape evolution

looking through drumlins testing the application of ground penetrating radar ground penetrating radar gpr is becoming a commonly applied technique in geomorphology however its use in the study of subglacial bedforms has yet to be fully explored and exploited this paper presents the results of a gpr feasibility study conducted on a drumlinized terrain in cumbria uk where five drumlins were investigated using multiple radar antenna frequencies the site was selected for the presence of nearby bedrock outcrops suggesting a shallow drumlinized diamict bedrock contact and a permeable lithology despite the clayey sediment and unfavourable weather conditions a considerable penetration depth of 12m was achieved when using a 50mhz antenna with a separation of 1m trace spacing of 1m and 128 fold vertical stack results indicate that the drumlinized diamict is in direct erosional contact with the bedrock while the internal drumlin geometry is generally chaotic on the stoss side evidence of layering dipping downflow at an angle greater than the drumlin surface profile was found on the lee side the inter drumlin areas comprise 4m of infill sediment that masks part of the original drumlin profile overall this study indicates that gpr can be deployed successfully in the study of glacial bedform sedimentary architecture

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

the transitional climate of the late miocene arctic winter dominated precipitation with high seasonal variability the late miocene 11 6 5 3 ma was an important transitional period following the greenhouse conditions of the eocene in order to gain insight into the arctic paleoclimate of the time we performed high resolution intraring 13 c analyses on fossil wood collected from the late miocene khapchansky sediments of northeastern siberia 69 n from these data we quantified the ratio of summer to winter precipitation ps pw and compared it to current values for the region determined from modern wood samples and instrumental records we observed much greater frequency of winter dominated precipitation ps pw 1 and much greater variability in ps pw during the miocene than today specifically years with ps pw 1 occurred three times more often and years with at least three times as much precipitation in summer or winter 0 33 ps pw 3 0 occurred approximately twice as often during the miocene than today we attribute the high interannual variability in precipitation to an inconsistent moisture source associated with the relatively unstable and incomplete ice cover in the arctic ocean during the late miocene our result highlights the potential for enhanced variability in arctic precipitation in response to arctic sea ice decline caused by anthropogenic co 2 induced warming

the planar shape of drumlins abstract the asymmetry of the planar shape of drumlins is an established paradigm in the literature and characterizes drumlins as resembling tear drops with a blunt bullet shaped stoss end and a tapering pointed lee end it is widely cited and never been seriously questioned in this paper the planar shape of 44 500 drumlins mapped in various regional settings from drumlin fields in north america and northern europe were objectively analysed by means of geographic information system tools two parameters were considered the first denoted here as as pl focuses on the relative position of the point of intersection between the axes of the maximum length and the maximum width it is defined as the distance between the upstream i e beginning of the drumlin and the intersection point measured along the longitudinal axis divided by the entire length of the long axis results indicate that the intersection point of the majority of drumlins 64 is very close to the longitudinal midpoint 0 33 as pl as pl a is defined as the ratio between the area of the upstream half of the drumlin to that of the entire drumlin results show that for most drumlins 81 the upper half area is almost as large as the down half 0 45 as pl a

coupled ice till dynamics and the seeding of drumlins and bedrock forms the geomorphological effects of ice sliding over till internal deformation of till and till sliding over bedrock are considered two questions are examined 1 is the till sheet flow unstable i e is a layer of uniform thickness maintained or not and 2 does the slip of till over bedrock cause amplification of relief of the bedrock such instabilities seem to be necessary to explain such features as drumlins and whaleback forms it is found that the answer to 1 and 2 depends on the position of the system in a parameter space defined by the till rheology and applied shear stress the effective pressure at the ice till interface the thickness of ice and till and the wavelength of the instability two configurations are considered one where the wavelength of the perturbation is much less than the the ice thickness which is related to the classical nye kamb solution for flow over bumps and one where the wavelength is much greater than the ice thickness where the mechanics are described by the shallow ice approximation in both cases substantial areas of parameter space where till sheet and bedrock modes are unstable are found the conceptually related smalley unwin bifurcation is re examined the physical mechanisms by which ice and till flows couple are examined at very short wavelengths 10 m the ice is so rigid that it forces till waves to move at the ice velocity while at long wavelengths 1000 m the flows become essentially uncoupled and till waves move at the kinematic velocity at intermediate wavelengths 100 m high growth rates occur this is postulated to be the scale of drumlin seeding

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

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

size shape and spatial arrangement of mega scale glacial lineations from a large and diverse dataset mega scale glacial lineations msgls are a characteristic landform on ice stream beds solving the puzzle of their formation is key to understanding how ice interacts with its bed and how this in turn influences the dynamics of ice streams however a comprehensive and detailed characterization of this landform s size shape and spatial arrangement which might serve to test and refine formational theories is largely lacking this paper presents a detailed morphometric analysis and comparison of 4043 msgls from eight palaeo ice stream settings three offshore norway and antarctica four onshore canada and one from under a modern ice stream in west antarctica the length of msgls is lower than previously suggested mode 1000 2000 m median 2892 m and they initiate and terminate at various locations on an ice stream bed their spatial arrangement reveals a pattern that is characterized by an exceptional parallel conformity 80 of all mapped msgls have an azimuth within 5 from the mean values and a fairly constant lateral spacing mode 200 300 m median 330 m which we interpret as an indication that msgls are a spatially self organized phenomenon results show that size shape and spatial arrangement of msgls are consistent both within and also generally between different ice stream beds we suggest this results from a common mechanism of formation which is largely insensitive to local factors although the elongation of msgls mode 6 8 median 12 2 is typically higher than features described as drumlins these values and those of their width mode 100 200 m median 268 m overlap which suggests the two landforms are part of a morphological continuum and may share a similar origin we compare their morphometry with explicit predictions made by the groove ploughing and rilling instability theories of msgl formation although the latter was most compatible neither is fully supported by observations 2014 the authors earth surface processes and landforms published by john wiley sons ltd

origin of the active drumlin field at mulajokull iceland new insights from till shear and consolidation patterns abstract stratigraphic and morphologic data previously collected from the forefield of mulajokull iceland suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them we have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier s forefield patterns of till strain indicated by the anisotropy of magnetic susceptibility ams of 2300 intact till samples indicate that till was deposited during shear deformation with shearing azimuths and planes that conform to the drumlin morphology thus till deposition occurred as drumlins grew in agreement with lidar data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges previously described unconformities on the drumlin flanks however indicate that drumlin relief at mulajokull has resulted in part from erosion given that the last surge deposited a till layer both on and between drumlins a reasonable hypothesis is that erosion between drumlins occurred during normal quiescent flow of the glacier between surges densities of till samples analyzed in conjunction with laboratory consolidation tests indicate that effective stresses on the bed during such periods were on the order of 100 kpa larger between drumlins than within them an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins the observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation moreover the lack of ams fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till flux gradients in a deforming bed

ribbed moraines in northern manitoba canada characteristics and preservation as part of a subglacial bed mosaic near the core regions of ice sheets abstract ribbed moraines are enigmatic glacial landforms for which different models with contrasting paleoglaciologic implications have been proposed to explain their formation despite the great deal of attention this type of landform has received over the last several decades ribbed moraine fields in northern manitoba canada are among the largest in the world but have been seldom studied ribbed moraines in this part of the world overlie the low relief canadian shield are not constrained by topography and are part of a spatial subglacial landform assemblage associated with drumlinoid ridges within palimpsest and relict type glacial terrain zones field observations herein provide new insights into the characteristics of these transverse to ice flow ridges at landscape mapping and spatial analysis and landform internal structure using high resolution shear wave s wave seismic reflection surveys sedimentological characteristics clast fabric analyses scales two main types of ribbed moraine are recognized pristine high amplitude straight crested ridges and secondarily modified subdued drumlinized ridges ribbed moraine in northeast manitoba consist of massive matrix supported till at surface which is similar in matrix texture and composition to the regional till sheet though pristine moraines show a higher concentration of boulders a seismic profile reveals subparallel to surface layered stratigraphy with only minor folding and no major unconformities stacking or faulting the demonstrated fragmentary nature of ribbed moraine fields the inherited signature of the till within these fields the secondary patchy drumlinization of the ridges and the more abundant granitoid outcrops in areas of intense modification all suggest that subglacial ribbed moraine formed from pre existing sediments and were later preserved and or partially reworked through a regional mosaic of shifting subglacial bed conditions we therefore make the case that pristine unmodified moraines were preserved beneath stable sticky spots we support a link between ribbed moraine and widespread cold based and or dewatered subglacial conditions in inner core regions of ice sheets but for reasons of preservation and not necessarily formation

sliding of till over bedrock scratching polishing comminution and kinematic wave theory the observed ratio of removal of material by i scratches gouged by large clasts and ii polishing by debris suggests that the latter is the dominant form of bedrock erosion we examine this with a multi stage breakdown model of glacial debris which includes active erosion by fines these models suggest that a mother clast can through the action of its daughter debris erode several times its own volume of bedrock till sliding over bedrock is likely to be a potent source of scouring by proposing a till bedrock sliding law we investigate the kinematics of sliding till bodies ice flow naturally tends to thin and extend till cover even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its base a lower sliding velocity and for larger thicknesses a decrease in sediment flux with thickness this implies backward moving kinematic waves and shocks it is suggested that this is related to the blunt upstream faces of drumlins and that drumlinization can be a consequence of debris sliding over bedrock

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

streamlined subglacial bedforms on the narke plain south central sweden areal distribution morphometrics internal architecture and formation abstract a flow set of close to 1000 drumlins has been mapped by means of lidar derived digital elevation models and investigated by trenching the area is situated on the sw part of the narke plain and its surrounding uplands in south central sweden which was deglaciated in the early preboreal in a glacioaquatic setting we find that there is considerable morphological difference in drumlin distribution patterns over crystalline basement areas compared to streamlined terrain over palaeozoic sedimentary rock basement the former area is characterized by thin quaternary drift and the drumlins are all of the rock cored type built due to active deposition of sediment around obstacles to glacier flow the latter area is characterized by deep quaternary drift and the drumlins are more elongate and also larger in all dimensions as compared to rock cored drumlins irrespective of these geomorphological differences on local landscape scale we find that drumlin morphometric values remain part of a morphological continuum at the regional scale based on the internal sediment architecture as revealed in two cross drumlin sections we find that the soft cored drumlins were formed by compressional constructive deformation along with excavational deformation along the flanks of the emerging drumlins which shaped the separating troughs intermediate type drumlins are those that demonstrate a coupling between underlying palaeozoic sediment strata in areas of shallow drift sheet these are the result of differing rheological response between incorporated sedimentary rock and a deforming bed below the ice bed interface an overall conclusion is that we find geomorphic and architectural compositional differences between the drumlins and the flowset they form we can closely relate these differences to contextual geological variations with respect to basement type and drift depth we argue that drumlin formation is better explained not by one single unifying process but rather a set of processes erosion deformation deposition by which the subglacial system finds an efficiency equilibrium whereby an obstacle is shaped so that it enhances flow with a minimum of drag i e the typical streamlined form is the result of a positive feedback cycle that tends towards efficiency this approach argues that equifinality is playing a significant role in drumlin formation with a plurality of initial conditions and processes which when combined with a uniformity of specific overriding processes and mechanical properties results in a diversity of morphology and core type at the meso scale and a continuum of landforms at the macro scale

a case study in the new york drumlin field an investigation using microsedimentology resulting in the refinement of a theory of drumlin formation abstract the formation of drumlins remains a major enigma it is accepted that drumlins form under active temperate ice most likely within a soft sediment deforming bed at ephemeral sticky points these sticky points likely lead to the causative mechanism around which sediment nucleation occurs the critical question is under what conditions and where and how do sticky spots form a comparative investigation of a drumlin and mega flute in the new york drumlin field as a case study demonstrates that rheological and sedimentological tills from these different forms are similar it is not therefore rheological change alone that must account for drumlin shape and form but likely the advective subglacial basal sediment flux rate at the ice bed the rate of sediment motion between the upper interface at the ice sediment bed boundary and the lower immobile sediment at depth is crucial at the lower decollement between the mobile and immobile sediment units within the deforming sediment package proto drumlin nucleation is likely to occur and develop into a streamlined form the trigger mechanism for such a perturbation is a derivative of sediment rheology and sediment flux rate driven by the overlying ice stresses recent evidence from antarctica lends credence to this new hypothesis that can be related to all drumlins formed under temperate soft sediment deforming bed conditions

water pressure coupling of sliding and bed deformation ii velocity depth profilesr b alley 130 139water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica

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

from the search of precursors to the research on crustal transients abstract earthquake prediction research of the last decade has produced many observations of precursory phenomena but the reliability of their physical correlation to the subsequent earthquake is often debatable and in any case very little can be inferred concerning the underlying coupling processes as precursors are only a small subset of the large family of crustal transients earthquakes swarms fluid instabilities episodic fault creep and as the latter are generally very poorly understood it is clear that any progress in short term prediction requires to study and model these transients a significant advantage of this approach is that transients are much more common than precursors and are thus easier targets for well focussed experiments i propose to consider a broader conceptual framework of a generalized self organized critical model for the crust where the interacting instabilities leading to distribution power laws are not only earthquakes but also transients such a model implies a significant coupling and cross triggerring between both inertial and slow instabilities our future ability to detect transients and to model the transient to earthquake triggering processes will be the key for assessing the predictability of earthquakes

the stability of a viscous till sheet coupled with ice flow considered at wavelengths less than the ice thickness a turbation method is used to analyse the stability of a thin till layer overlain by a deep ice layer ice is modelled as a linearly viscous fluid while the till viscosity has power law dependence on stress and effective pressure a linearized set of equations yields descriptions of the coupling of the ice flow with the sediment flow and reveals parameter ranges where the till perturbation amplitude can grow this shect flow instability is an essential part of any theory of drumlin formation and shows that viscous models of till have the ability to explain typical deforming bed features this is of great significance for large scale ice shect modelling

drumlinization and drumlin forming instabilities viscous till mechanisms glacially induced flow naturally tends to thin and extended till cover through shock formation even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its surface and base a lower sliding velocity or deformation rate and above a critical thickness a decrease in wave velocity with thickness leading to reverse facing shocks moving downstream for sliding and for some theologies of internal deformation a decrease in sediment flux with thickness occurs implying backward moving kinematic waves and reverse facing reverse moving shock downstream facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally eventually shocks coalesce leaving an upstream lacing shock for sliding and a downstream facing shock for internal deformation it is observed that some drumlins have downstream blunt ends only fairly realistic three dimensional drumlin shapes can be produced from symmetric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice sheet flow the fact that viscous theories produce drumlinoid forms suggests that on this scale till behaves viscously and the the lower length scale for drumlins represents the plastic viscous transition scale

black box modeling of the subglacial water system measurements of water pressure beneath trapridge glacier yukon territory canada yield the following generalizations about subglacial conditions in the studied region 1 even over short distances the subglacial water system is highly heterogeneous 2 the subglacial water system consists of at least two distinct components which we refer to as the connected and unconnected water systems 3 regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system 4 large spatial pressure gradients can exist within the unconnected water system and between the connected and unconnected systems 5 rapid pressure variations can occur in the unconnected water system 6 pressure variations in the unconnected water system do not match those in the connected system and can in fact be strongly anticorrelated with pressure variations in the connected system if the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing the input output relation between forcing and response can be efficiently represented as a low order nonlinear ordinary differential equation the response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of 1 7 hours and 7 4 hours that we believe are associated with process rates for substrate compression and pore water diffusion respectively

are drumlins a product of a thermo mechanical instability abstract of numerous theories of drumlin genesis none has been widely accepted it seems evident however that some form of positive feedback process is involved under certain circumstances perturbations are amplified herein we suggest that patchy areas of frozen bed provide the initial perturbation such frozen patches may occur in local areas underlain by material of lower thermal conductivity or on slight topographic highs drag exerted by the frozen patch deflects ice flow into its lee dragging with it mobile till eroded from the thawed area the energy balance is such that this till likely refreezes either producing a topographic perturbation or amplifying an existing one the resulting topography then deflects more of the geothermal heat away from the developing hill and into the adjacent trough resulting in a positive feedback once the thermal perturbation exceeds a critical though as yet undefined level melting may decouple the ice from the bed preventing further entrainment of till from thawed areas and thus limiting the height and length of the drumlin

using the size and position of drumlins to understand how they grow interact and evolve drumlins are subglacial bedforms streamlined in the direction of ice flow common in deglaciated landscapes they have been widely studied providing rich information on their internal geology size shape and spacing in contrast with bedform investigations elsewhere in geomorphology aeolian and fluvial dunes and ripples for example most drumlin studies derive observations from relict and thus static features this has made it difficult to gain information and insights about their evolution over time which likely hampers our understanding of the process es of drumlin formation here we take a morphological approach studying drumlin size and spacing metrics unlike previous studies which have focussed on databases derived from entire ice sheet beds we adopt a space for time substitution approach using individual drumlin flow sets distributed in space as proxies for different development times periods framed and assisted by insights from aeolian and fluvial geomorphology we use our metric data to explore possible scenarios of drumlin growth evolution and interaction we study the metrics of the size and spacing of 36 222 drumlins distributed amongst 71 flow sets left behind by the former british irish ice sheet and ask whether behaviour common to other bedform phenomena can be derived through statistical analysis through characterising and analysing the shape of the probability distribution functions of size and spacing metrics for each flow set we argue that drumlins grow and potentially migrate as they evolve leading to pattern coarsening furthermore our findings add support to the notion that no upper limit to drumlin size exists and to the idea that perpetual coarsening could occur if given sufficient time we propose that the framework of process and patterning commonly applied to non glacial bedforms is potentially powerful for understanding drumlin formation and for deciphering glacial landscapes

emergent drumlins and their clones from till dilatancy to flow instabilities subglacial bedforms are a range of landforms 10 105 m in length shaped mostly in glacial sediments and are abundant on ice sheet beds numerous explanations for their generation especially of drumlins have been proposed rather than viewing them as resulting from erosion or deposition directly by ice smalley and unwin 1968 proposed that both might occur simultaneously if beneath the glacier a layer of lubricating sediment existed and flowed conducting geomorphic work erosion deposition shaping of its own this idea appears to have steered the field leading perhaps to final resolution of the centuries old problem of drumlin genesis here i trace how the idea evolved leading to the conclusion that subglacial bedforms are emergent phenomena arising from self organisation in the coupled flow of ice sediment and water the ubiquity and patterning of bedforms appears to be well explained by a naturally arising flow instability in the subglacial system and which has been demonstrated by analytical and numerical modelling a problem for the instability theory is an apparent mismatch between predicted and observed sedimentological properties of bedforms a distinction is made between emergent drumlins drumlin clones and obstacle drumlins which helps explain some apparent contradictions and a conceptual framework is erected that might form a basis for confrontation of the theory with the wealth of observational evidence that exists

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

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

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

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

flow mechanism of glaciers on soft beds subhourly measurements of bed deformation bed shear strength subglacial water pressure and surface speed at storglaciaren a glacier in northern sweden showed that the shear strain rates of the bed decrease during periods of high water pressure and surface speed high water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing the instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation as has been commonly thought