instantaneous end moraine and sediment wedge formation during the 1890 glacier surge of bruarjokull iceland abstract contemporary understanding of the behaviour of surging glaciers and ice streams is hampered by the lack of data on landsystem evolution and sedimentary environments this study concerns the ice marginal environment of the surge type bruarjokull in iceland the sediment distribution in the glacier forefield as well as the morphology sedimentology and tectonic architecture of the 1890 end moraine is investigated for highlighting the interaction between very dynamic ice and sediment landform associations as a result of substrate bedrock decoupling during the 1890 surge subglacial sediment was dislocated across the bedrock surface and deformed compressively leading to gradual substrate thickening and the formation of a sediment wedge in the marginal zone a drop in subglacial porewater pressure at the very end of the surge led to substrate bedrock coupling and a stress transfer up into the sediment sequence causing brittle deformation of the substrate simultaneously the glacier toe ploughed into the topmost part of the marginal sediment wedge initiating the moraine ridge construction fine grained and incompetent sediment deformed in ductile manner resulting in a narrow moraine dominated by rooted folds while coarse grained and competent sediment deformed in brittle fashion resulting in a wider moraine dominated by thrust blocks a new sequential model of subglacial and ice marginal processes operating during a glacier surge is proposed illustrating the stepwise formation of a marginal sediment wedge and an end moraine a twofold inseparable marginal end product that formed during the last days of the 1890 surge

architecture and structural evolution of an early little ice age terminal moraine at the surge type glacier mulajokull iceland the internal architecture and structural evolution of the arnarfellsmular terminal moraine at mulajokull a surge type glacier in central iceland is described in order to demonstrate submarginal and proglacial glaciotectonic processes during glacier surging as well as constraining the age of the maximum extent of the glacier the moraine is 4 7m high 50 100m wide and composed of a highly deformed sequence of loess peat and tephra that is draped by till up to the crest the internal architecture is dominated by steep high amplitude overturned folds and thrusts in the crest zone but open low amplitude folds on the distal slope section balancing suggests a basal detachment decollement depth of 1 4m and a total horizontal shortening of around 59 this implies that the glacier coupled to the foreland about 70m up glacier from its terminal position to initiate the formation of the moraine the structural evolution is polyphase in that the formation commenced with low amplitude open folding of the foreland followed by overfolding and piggyback thrusting radiocarbon dating and analysis of tephra layers along with historical references indicate that the most likely time of moraine formation was between a d 1717 and 1760 which suggests that mulajokull had its little ice age maximum and most extensive surge earlier than many other surge type glaciers in iceland

distinct patterns of seasonal greenland glacier velocity predicting greenland ice sheet mass loss due to ice dynamics requires a complete understanding of spatiotemporal velocity fluctuations and related control mechanisms we present a 5 year record of seasonal velocity measurements for 55 marine terminating glaciers distributed around the ice sheet margin along with ice front position and runoff data sets for each glacier among glaciers with substantial speed variations we find three distinct seasonal velocity patterns one pattern indicates relatively high glacier sensitivity to ice front position the other two patterns are more prevalent and appear to be meltwater controlled these patterns reveal differences in which some subglacial systems likely transition seasonally from inefficient distributed hydrologic networks to efficient channelized drainage while others do not the difference may be determined by meltwater availability which in some regions may be influenced by perennial firn aquifers our results highlight the need to understand subglacial meltwater availability on an ice sheet wide scale to predict future dynamic changes

electromagnetic surveying in current middle eastern archaeology application and evaluation since 1977 the smithsonian institution has had a major research program on the human biological history of the near east as part of this program electromagnetic em surveying methods have been extensively used to identify anomalies of potential archaeological significance below the surface an em 31 noncontacting terrain conductivity meter was used in jordan bahrain kuwait and egypt with excellent results in addition to the successful application of em equipment to archaeological problems we investigated the equipment s response to changing environmental and climatic conditions we have developed efficient methods for recording em data the following results were obtained recording accuracy is better than previously reported 0 97 percent repeatability error between two observers the degree of accuracy is primarily a function of operator error and less a function of equipment reliability and the em 31 produces helpful highly reproducible results in the arid and semiarid environment of the mi

ice speed of a calving glacier modulated by small fluctuations in basal water pressure ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in alaska greenland and antarctica high frequency measurements of ice speed and basal water temperatures from a calving glacier in patagonia show that changes in basal water pressure by a few per cent can significantly affect ice flow speed ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in alaska1 2 greenland and antarctica3 4 glaciers that calve in water flow much faster than those that terminate on land as a result of enhanced basal ice motion where basal water pressure is high5 however a scarcity of subglacial observations in calving glaciers limits a mechanistic understanding here we present high frequency measurements of ice speed and basal water pressures from glaciar perito moreno a fast flowing calving glacier in patagonia we measured water pressure in boreholes drilled at a site where the glacier is 515 5 m thick and where more than 60 of the ice is below the level of proglacial lakes we found that the mean basal water pressure was about 95 of the pressure imposed by the weight of the overlying ice moreover changes in basal water pressure by a few per cent drove nearly 40 of the variations in ice flow speed the ice speed was strongly correlated to air temperature suggesting that glacier motion was modulated by water pressure changes as meltwater entered the system we conclude that basal water pressure in calving glaciers is important for glacier dynamics and closely connected to climate conditions

clastic dykes in over consolidated tills evidence for subglacial hydrofracturing at killiney bay eastern ireland abstract a swarm of vertical gravel filled dykes up to 6 m high and several decimetres wide cut through an over consolidated till at killiney bay the dykes are rooted in a gravel layer and many display plumes of clastic debris ejected into the overlying sediments burst out structures such features have not previously been described these clastic dykes are interpreted as the infillings of hydrofractures which formed when water pressures in the basal gravel layer exceeded the overburden pressure and tensile shear strength of the capping till the burst out structures extend up to 7 m from the tops of the dykes and provide strong evidence for forceful upward flow evidence suggests that the hydrofractures formed subglacially probably during a minor re advance their presence in late devensian 26 13 ka bp tills with an irish sea provenance irish sea till may have important implications for the subglacial hydrology of the last irish sea ice sheet these hydrofractures cannot form in unconsolidated glacimarine sediment and their presence precludes a glacimarine origin for these deposits they greatly affect the geotechnical properties of irish sea tills in particular providing very permeable routes through otherwise impermeable layers with important consequences in situations elsewhere in the irish sea basin where they have been used as aquicludes in landfill and low level nuclear waste disposal sites

the morphology structural evolution and significance of push moraines abstract push moraines glaciotectonic ice marginal moraines have a restricted distribution at modern glacier margins and consequently are of potential value in reconstructing pleistocene ice sheets providing data both on former glaciodynamics and on the palaeoenvironment of the glacial foreland to the wider earth science community push moraines are of interest as analogues for thin skin tectonics within orogenic belts this paper reviews the morphology structural geology formation and significance of push moraines the morphological and structural characteristics of small seasonal push moraines through to large multi crested examples produced by sustained glacier advances are reported before the primary controls on push moraine formation are examined these controls include the nature of the applied glacial stress field the presence and properties of decollement horizons and the shear strength and rheology of the glacial foreland a conceptual model of push moraine formation is introduced in which the range of observed morphological and structural forms are viewed within a matrix defined by the main variables which control their formation the absence of consensus over which of these variables is of greatest importance currently limits the significance of push moraines in palaeoglaciological research as a consequence this review emphasises the need for future research in order to realise the true potential of push moraine in the reconstruction of pleistocene environments

groundwater flow beneath ice sheets part ii its impact on glacier tectonic structures and moraine formation abstract meltwater flowing as groundwater from beneath the margin of an ice sheet determines the distribution of sub surface heads and effective pressures a subglacial groundwater flow model is used together with an ice sheet loading model to compute the magnitudes and directions of the principal effective stresses in the subsurface from which the distribution of different types of sub surface failure in the subglacial and proglacial zones are deduced zones of hydrofracturing shear fracture and pervasive shear failure are distinguished beneath the ice sheet divide area intact rocks of high tensile strength may fail hydrofracturing and liquefaction are two coupled processes which lead to the formation of upward filled and downward filled sediment dykes and till wedges quicksand conditions are developed where strong vertical seepage pressures occur producing sediment diapirism it is suggested that subglacial permeability magnitude may be the product of a self organising process certain types of moraine extrusion moraines are suggested to be a consequence of upward movement and surface extrusion of sediment driven by rising groundwater it is suggested that groundwater over pressure associated with narrow proglacial permafrost plates are conductive to the formation of large push moraines and that many large ancient and modern examples are produced in this setting

defining archaeological features with electromagnetic surveys at the cahokia mounds state historic site electromagnetic em surveys have been used at the cahokia mounds state historic site in southwestern illinois to locate and define a number of buried archaeological features two instruments geonics em31 d and em38 conductivity meters were employed to locate portions of a wooden stockade known as the central palisade delineate a number of leveled earthen mounds and explore a broad flat area in the central portion of the site known as the grand plaza em surveys together with limited excavation provide a cost effective and nondestructive means of exploring a site as large and complex as cahokia archaeological excavations confirmed that em surveys were able to locate the central palisade and more importantly that they provided information on anthropogenically modified terrain within the grand plaza the em survey documented buried ridge and swale topography and borrow pits within this area this evidence of landscape modification challenges previous conceptions about the extent of earthmoving at

micromorphological evidence for polyphase deformation of glaciolacustrine sediments from strathspey scotland a combination of field investigation and micromorphological analysis has been applied to polydeformed late devensian rhythmites and glacigenic diamicton exposed in strathspey scotland this provided information on the geometry kinematics and relative ages of ductile and brittle structures and records a complex subglacial deformation history the deformation is interpreted as resulting from a single progressive event associated with over riding of proglacial lake sediments by wet based ice the earliest deformation d1 resulted from compaction loading pure shear and imposed a bedding parallel s1 fabric throughout the rhythmites s1 was subsequently deformed by kink bands and minor ductile shearing during d2 a later d3 event characterized by soft sediment deformation and fluidization of matrix poor sands was accompanied by an increase in pore water pressure this lead to hydrofracturing of the rhythmites the most intense deformation d4 which resulted from simple shear was partitioned into the upper part of the sequence it produced folding thrusting and brittle microfaulting in response to nnw directed ice push these findings indicate that in general subglacial deformation is not homogeneous and can extend to depths of 3 m below the presumed ice sediment interface

micromorphological evidence for the role of pressurised water in the formation of large scale thrust block moraines in melasveit western iceland pressurised meltwater has a major impact on ice dynamics as well as on sedimentary and deformational processes occurring below in front of glaciers and ice sheets but its role in glaciotectonic processes is yet to be fully understood this study explores micro and macroscale structures developed within decollements in two thrust block moraines of late weichselian age in melasveit western iceland the aim is to investigate how pressurised subglacial meltwater can aid the dislocation and transport of large unfrozen and unlithified sediment blocks by glaciers a detailed model is constructed for the development of the thrust block moraines and the microscale processes occurring along their detachments during thrusting the detachments are characterized by relatively thin zones of crosscutting hydrofractures which reflect fluctuating water pressures during glaciotectonism little evidence of shearing is observed along the leading edges of the thrusts in both moraines this is supported by high water pressures along the detachments and indicates that the thrust blocks were initially decoupled from the underlying deposits as the thrust moraines evolved an increased amount of shear occurred in between events of sediment liquefaction hydrofracturing and fluid escape this was followed by progressive locking up of the detachments and eventual cessation in the accretion of the thrust blocks

micromorphological evidence of liquefaction injection and sediment deposition during basal sliding of glaciers the sliding of an ice mass over its bed represents one of the main mechanisms for the forward motion of glaciers and ice sheets facilitated by the periodic introduction of meltwater along the ice bed interface or regelation of the overriding ice in the geological record however physical evidence of this process having occurred beneath former ice masses is apparently limited this paper presents the results of a detailed micromorphological study of thinly stratified subglacial tills exposed at two sites i galmis in switzerland and ii plumpe farm near gretna in sw scotland the stratification within these tills comprises alternating layers of massive to weakly foliated diamicton and variably deformed folded faulted laminated silt and clay microtextural and microstructural evidence is interpreted in terms of repeated phases of basal sliding as the ice overrode a soft sediment bed elevated meltwater contents pressures encountered immediately prior to and during basal sliding promoted localised liquefaction within the bed decoupling of the ice mass from its bed enabled the injection of the liquefied diamicton along the ice bed interface and or into the laminated sediments immediately adjacent to this boundary the laminated silts and clays record the settling out of fines clay silt from meltwater trapped along the ice bed interface after an individual phase of basal sliding has ceased injection of till into the locally water saturated silts and clays resulted in partial liquefaction and incomplete mixing of these fine grained sediments with the diamicton density contrasts between the two liquefied sediments led to the development of a complex vinaigrette like texture comprising rounded to irregular till pebbles within a matrix of variably homogenised silty clay recoupling of the ice with its bed led to localised folding and thrusting within the laminated silts and clays hydrofracturing and injection of a network of sand filled veins and the imposition of a variably developed clast microfabric in the diamicton layers analysis of the clast microfabrics indicates that the intensity of these fabrics is highly variable reflecting the variation in the intensity of deformation imposed by the overriding ice

micromorphological evidence of warm based glacier deposition from the ricker hills tillite victoria land antarctica abstract thin sections of impregnated samples were used for micromorphological analysis of the ricker hills tillite in southern victoria land antarctica the tillite is composed of massive matrix supported diamicton with a porphyric coarse fine related distribution low sorting low rounding and medium to high angularity glacial deposits are completely represented by lodgement till phyllosilicate reorientation patterns plasmic fabrics are visible in most samples especially associated skelsepic and lattisepic patterns syndepositional pervasive till shearing glacially induced is evidenced by clast alignments shear planes rotational structures and pressure shadows boudinage ductile shear zones and rotational structures are also widespread in the deformed bedrock water escape structures are clearly evident in the tillite and in the bedrock shear planes infilled by injection veins in lodgement till and at the till bedrock interface testify to the presence of water during deposition secondary features like clay and silt coatings sparite calcite and oxidation products were found in all the examined outcrops they formed under phreatic conditions due to saturated water circulation a warm based ice sheet was responsible for the deposition of the tillite past ice flow directions were similar to present ones

subglacial decoupling at the sediment bedrock interface a new mechanism for rapid flowing ice abstract in undetermined on millennial or even centennial time scales the activity of rapid flowing ice can affect climate variability and global sea level through release of meltwater into the ocean and positive feedback loops to the climate system at the surge type glacier bruarjokull an outlet of the vatnajokull ice cap eastern iceland extremely rapid ice flow was sustained by overpressurized water causing decoupling beneath a thick sediment sequence that was coupled to the glacier this newly discovered mechanism has far reaching consequences for our understanding of fast flowing ice and its integration with sediment discharge and meltwater release c 2006 elsevier ltd all rights reserved

aligned glaciotectonic rafts on the central barents sea seafloor revealing extensive glacitectonic erosion during the last deglaciation 1 erosion rates on glaciated continental shelves are remarkably high especially within ice stream troughs although glaciotectonic erosion may have considerably contributed to enhanced glacial erosion of these landscapes entrainment mechanisms of glaciotectonically emplaced megablocks and rafts remain little understood here we report a northeast southwest trending chain of over 1300 glacial rafts observed on the seafloor in bjornoyrenna a paleo ice stream trough with particularly high erosion rates the landform assemblage adjacent to the glacial rafts includes streamlined bedforms and crevasse squeeze ridges and may represent a lateral shear zone where a paleo ice stream met cold stagnant ice during the last deglaciation individual glacial rafts along the chain are up to 30 m high tend to be elongated with lengths and widths on the order of tens of meters and are oriented from northeast southwest to north south the north southern rearrangement of rafts may reflect a compressional strain regime along the ice stream margin

detection and characterisation of black death burials by multi proxy geophysical methods the construction of the new crossrail railway discovered 25 well preserved skeletons shallowly buried in central london in 2013 subsequent carbon dating and adna analysis confirmed the archaeological age and presence of the yersinia pestis black death plague epidemic strain here we present the non invasive multi proxy geophysical survey of the adjacent charterhouse square rapidly undertaken to detect any further burials and characterise the site historical records suggested the area was a burial ground for black death plague victims before subsequent cemetery and urban land use following initial trial surveys surveys imaged 200 isolated and similar sized burials in the south west of the site there were also two contrasting burial orientations present at various depths which suggested a series of controlled phased burials a well defined eastern burial boundary taking the form of a ditch and bank was also discovered geophysical surveys also identified a subsequent complex site history with multiple aged features this study revises knowledge of black death aged burials and provides important implications for successful geophysical burial detection with significant time and space limited site constraints

glacio tectonic structures a mesoscale model of thin skinned thrust sheets abstract a series of seven groups of push ridges ranging from 7 to 40 m in height 50 to 280 m in length and occupying a total width of more than 2 km mark the marginal zone of the a d 1890 maximum of eyjabakkajokull an outlet glacier of the vatnajokull ice cap iceland the internal structure of one ridge complex comprises two distinct elements a proglacial part which has been subject to compressional stresses resulting in the development of imbricate thrust sheets and a subglacial part which comprises low angle normal fault structures the two sub systems appear to be linked via a floor thrust and to have evolved together as the glacier reached the limit of its rapid advance in a d 1890

subglacial conditions under the last ice sheet in northwest germany ice bed separation and enhanced basal sliding abstract theoretical reconstruction of ice thickness during the weichselian glaciation in a portion of northwest germany yields maximum thicknesses of about 280 m some 50 km into the ice sheet from the margin this corresponds to averaged driving stress at the glacier bed in the order of 17 kpa the increase in ice thickness away from the margin was non uniform caused by undulations in pre existing topography steady state subglacial water pressure heads were high up in the ice sheet reaching the flotation point in places hydraulic lifting of the ice sheet probably occurred causing non deforming bed conditions where transformation of basal shear stresses to soft subglacial sediments was prevented by a water film thin stringers of undeformed sorted sediments occurring in tills are attributed to basal meltwater washing during phases of ice bed separation the basal water film was over 5 mm thick using a quantitative approach coupled with field data we demonstrate that neither internal ice deformation nor bed deformation contributed much to the ice motion instead the rapid movement of the weichselian ice sheet was caused by basal sliding with ploughing the ice sheet was probably characterized by an unstable behaviour

under pressure clastic dykes in glacial settings abstract clastic dykes are widespread in glacial settings and we provide examples from switzerland patagonia iceland and the antarctic ranging in age from tertiary to recent on the basis of these examples we establish the general characteristics of clastic dykes and proceed to establish the direction of propagation on sedimentological grounds micromorphological analysis reveals that sedimentation in the dykes is ruled by pressure gradients and that ordinary sedimentological rules do not apply clastic dykes act like safety valves in the subglacial hydraulic system their development depends on specific subglacial conditions like water volume and pressure the nature of the bed sediments or bedrock and the hydraulic properties of the bed a number of scenarios for clastic dyke development are presented as there is not just one set of conditions under which they form development of clastic dykes affects glaciodynamics like velocity and surging and landform formation like till thickness and bedrock disruption

new perspectives on the fossilization of faunal remains and the formation of the florisbad archaeozoological site south africa abstract new perspectives on the hydrology salinization of organic clay layers fossilization of faunal remains and the formation of the florisbad spring site are presented here these are based on a critique of previous hypotheses as well as new evidence and subsequent research it is contended that the fossilization of faunal remains could not as was previously postulated have taken place in areas of spring activity due to the low ca content of the spring water and a high eh resulting from spring flow it is further contended that fossils occurring in areas of spring activity were demineralized evidence suggests that fossilization of faunal remains at florisbad took place in the environs associated with organic clay layers which had high ca concentrations and a low eh a new hypothesis is put forward to account for the depositional history of the florisbad spring site this accommodates the formation of the site in light of its stratigraphy geomorphology salinization processes and the fossilization of faunal remains this hypothesis also explains a number of observed geomorphic anomalies

the search for graves over the decades grave markers in old cemeteries have been lost geophysical exploration can sometimes locate the unmarked burials the two techniques which may be best for this search are a ground penetrating radar survey and a soil conductivity survey a ground penetrating radar survey with its capability for estimating the depth and shape of buried objects is particularly suitable with an electromagnetic induction survey the disturbed soil in the grave shaft can sometimes be detected as a change in electrical conductivity both of these surveys also can locate large metal objects these surveys have limitations at some sites the radar cannot profile deeply enough at others the soil strata are so complex that graves cannot be distinguished a conductivity survey can be degraded by metallic trash and other small objects in the topsoil it can give the best results where the earth is distinctly stratified results from nine surveys are illustrated here the sites are all in the u s a and the gra

clastic injection dynamics during ice front oscillations a case example from solheimajokull iceland abstract soft sediment deformation structures are being increasingly used as a tool for reconstructing palaeoenvironments and porewater pressure conditions in glacial settings however the potential of hydrofractures and clastic injections in the reconstruction of ice dynamics remains poorly constrained this paper presents the results of a detailed study of a clastic injection network outcropping in the solheimajokull forefield south iceland sedimentological descriptions are combined with microscopic to macroscopic analyses of clastic injection geometries sediment fills and cross cutting relationships the 250 m long and 20 m high exposure observed along the east flank of the proglacial braid plain displays alternating glaciofluvial sediments and subglacial tills illustrating oscillations of the ice margins these sediments are cross cut by a dense network of injection composed of dykes propagating upward or downward sills and stepped sills these clastic injections result from processes of hydrofracturing and the sediment fills in these hydrofractures are generally laminated with an increase of grain size towards the centre of the injections these fracture fill characteristics suggest multiple injection phases within the hydrofractures and an increase of porewater pressure over time five main generations of clastic injections showing different senses of propagation and dip directions are determined and are interpreted as forming in different environments per descensum clastic dykes dipping down ice demonstrate subglacial hydrofracturing underneath flowing ice while sills and per ascensum clastic dykes form in submarginal to marginal environments due to the decrease of ice overburden pressure the integration of these results with the sedimentological characteristics allows the holocene ice front oscillations of the solheimajokull to be reconstructed this study demonstrates the importance of hydrofracture systems and their sediment fills in the reconstruction of palaeo ice dynamics

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

the 1890 surge end moraine at eyjabakkajokull iceland a re assessment of a classic glaciotectonic locality abstract the glaciotectonic architecture and sequential evolution of the eyjabakkajokull 1890 surge end moraines in iceland were studied for understanding better the formation and evolution of glaciotectonic end moraines and their relation to glacier dynamics based on morphological geological and geophysical data from terrain cross profiles cross sections and ground penetrating radar profiles we demonstrate that three different qualitative and conceptual models are required to explain the genesis of the eyjabakkajokull moraines firstly a narrow single crested moraine ridge at the distal end of a marginal sediment wedge formed in response to decoupling of the subglacial sediment from the bedrock and associated downglacier sediment transport secondly large lobate end moraine ridges with multiple closely spaced narrow asymmetric crests formed by proglacial piggy back thrusting thirdly a new model shows that moraine ridges with different morphologies may reflect different members of an end moraine continuum this is true for the eastern and western parts of the eyjabakkajokull moraines as they show similar morphological and structural styles which developed to different degrees the former represents an intermediate member with decollement at 4 5 m depth and 27 33 shortening through multiple open anticlines that are reflected as moderately spaced symmetric crests on the surface the latter represents an end member with decollement at about 27 m depth and 39 horizontal shortening through multiple high amplitude overturned and overthrusted anticlines appearing as broadly spaced symmetric crests we propose that the opposite end member would be a moraine of multiple symmetric wide open anticlinal crests of low amplitude our data suggest that the glacier coupled to the foreland to initiate the end moraine formation when it had surged to within 70 190 m of its terminal position this indicates a time frame of 2 6 days for the formation of the end moraines

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

a new microstructural mapping methodology for the identification analysis and interpretation of polyphase deformation within subglacial sediments although integrated clast macro and microfabric studies have been used to provide evidence of palaeo ice flow directions to discriminate between diamictons deposited in differing sedimentary environments and to model the response of tills to subglacial deformation such studies have typically only examined the bulk fabric developed within the diamicton largely ignoring the potential for several phases of fabric development within these typically polydeformed deposits we present a new graphical method for the identification description and analysis of clast microfabrics in polydeformed tills this method utilises high resolution digital scans of orientated thin sections imported into a standard computer graphics package to measure the orientation of long axes of detrital skeleton grains included within the till these data are then used to identify the main clast microfabrics present within the till allowing the delineation of clast microfabric domains in which elongate grains share a similar preferred orientation this process leads to the identification characterisation and most importantly the interrelationships between the various generations of microfabrics the relationships between these clast microfabrics and other microstructures e g plasmic fabrics turbate structures shears folds faults etc can also be examined providing a detailed microfabric microstructural map of the thin section clast long axis orientation data collected for different parts of a thin section are plotted on a series of rose diagrams allowing the variation in clast microfabric orientation to be displayed graphically a new set of terminology for the description of clast microfabrics in tills which is based upon the system used by metamorphic petrologists is also presented three case studies of the fabrics present within subglacial sediments of pleistocene age as well as the present are used as examples of how to apply this methodology

criteria to distinguish between subglacial glaciotectonic and glaciomarine sedimentation i deformation styles and sedimentology abstract this paper compares and contrasts the sedimentology and structural geology of a quaternary glaciomarine site melabakkar asbakkar iceland and a quaternary subglacial glaciotectonic site west runton uk research suggests that the glaciomarine environment is dominated by sedimentary processes related to sediment supply water content and distance from the glacier margin glaciotectonic deformation however is dominated by deformational processes related to effective pressure shear strain nature of the subglacial material and distance from the glacier margin from this study a series of criteria are suggested to distinguish both quaternary and contemporary glaciotectonic and glaciomarine environments in addition the paper also discusses the subsequent effects of glaciotectonic deformation on a primary glaciomarine site due to a glacial readvance

glacigenic rafting of weathered granite charlie lake new brunswick glaciation in the area of charlie lake new brunswick has produced a granitic till very similar in appearance to weathered bedrock portions of weathered granite were moved intact and redeposited as imbricated rafts under basal till the rafts vary from 0 3 to 2 m in thickness over a 10 m wide exposure and show deep weathering zones and brittle deformation despite the fact that they are highly friable the rafts are separated by glacigenic fault planes defined by lenses of diamict and rare clasts of distant origin they are recognizable only in excavations where fault planes can be seen to delineate slabs of rock imbricated mainly upglacier surface weathering features including core stones were preserved during movement over short distances

review of subglacial hydro mechanical coupling trapridge glacier yukon territory canada abstract the interaction of basal processes with the subglacial drainage system is a critical issue in understanding glacier dynamics since the recognition that many glaciers and ice masses overlie soft sediments rather than hard bedrock much research has been undertaken to investigate how mechanical and hydrological conditions of a deformable substrate control the coupling at the ice bed interface and thus affect fast ice flow and glacier surging in research undertaken on trapridge glacier a small surge type glacier in the st elias mountains yukon territory canada we have combined extensive field investigations using novel measurement techniques and theoretical modelling to study hydro mechanical coupling processes measurements of subglacial water pressure indicate that the basal water system can be dramatically inhomogeneous both spatially and temporally since ice bed coupling is strongly influenced by subglacial water pressure the stresses at the bed are also markedly heterogeneous and are expected to form a patchwork distribution which mimics the pressure distribution of the basal water system this heterogeneity in the stress field at the ice bed interface introduces a pronounced variability to the basal motion mechanics as such basal sliding and subglacial sediment deformation are not steady and continuous processes instead the variability of the subglacial water system leads to a spatial and temporal interplay of increased ice bed coupling at low water pressures at one site or time with ice bed decoupling during rising water pressures at other sites or times thus on the one hand there is downglacier shear deformation of the bed and accumulation of elastic strain in ice and sediment while on the other hand there is enhanced slip sliding of the glacier and upglacier shear motion of the bed due to an elastic relaxation of the sediment