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

morphodynamic modelling for a tidal inlet in the wadden sea abstract a dynamic model for morphological development in a tidal inlet het friesche zeegat in the wadden sea along the northern coast of the netherlands is set up to obtain a better understanding of the behaviour of the morphological system around tidal inlets the model is based on delmor a program package for morphological changes developed at delft hydraulics computations of the flow the sediment transport the initial bed level change rate as well as long term morphological development have been carried out the model has proven to be a useful research tool even though it does not yet reproduce reality in detail

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

discrete element modeling of subglacial sediment deformation 1 the discrete element method dem is used in this study to explore the highly nonlinear dynamics of a granular bed when exposed to stress conditions comparable to those at the bed of warm based glaciers complementary to analog experiments the numerical approach allows a detailed analysis of the material dynamics and the shear zone development during progressive shear strain the geometry of the heterogeneous stress network is visible in the form of force carrying grain bridges and adjacent volumetrically dominant inactive zones we demonstrate how the shear zone thickness and dilation depend on the level of normal overburden stress and we show how high normal stress can mobilize material to great depths the particle rotational axes tend to align with progressive shear strain with rotations both along and reverse to the shear direction the results from successive laboratory ring shear experiments on simple granular materials are compared to results from similar numerical experiments the simulated dem material and all tested laboratory materials deform by an elastoplastic rheology under the applied effective normal stress these results demonstrate that the dem is a viable alternative to continuum models for small scale analysis of sediment deformation it can be used to simulate the macromechanical behavior of simple granular sediments and it provides an opportunity to study how microstructures in subglacial sediments are formed during progressive shear strain

in search of ice stream sticky spots the form drag of large bedrock bumps sticking into the base of an ice stream can produce effective sticky spots supporting large basal shear stress bedrock regions surrounded by lubricating till at the same topographic level can cause sticky spots but tend to collect lubricating water and thus are unlikely to support a shear stress of more than a few tenths of a bar unless they contain abundant large bumps raised regions on the ice air surface also can cause moderate increases in the shear stress supported on the bed beneath surveys of large scale bedrock roughness strain grids across the margins of ice surface highs and possibly water pressure measurements in regions of thin or zero till would help identify and characterize sticky spots

control of barrier island shape by inlet sediment bypassing east frisian islands west germany abstract a study of the east frisian islands has shown that the plan form of these islands can be explained by processes of inlet sediment bypassing this island chain is located on a high wave energy high tide range shoreline where the average deep water significant wave height exceeds 1 0 m and the spring tidal range varies from 2 7 m at juist to 2 9 m at wangerooge an abundant sediment supply and a strong eastward component of wave power 4 4 103 w m 1 have caused a persistent eastward growth of the barrier islands the eastward extension of the barriers has been accommodated more by inlet narrowing than by inlet migration it is estimated from morphological evidence that a minimum of 2 7 105 m3 of sand is delivered to the inlets each year via the easterly longshore transport system much of this sand ultimately bypasses the inlets in the form of large migrating swash bars the location where the swash bars attach to the beach is controlled by the amount of overlap of the ebb tidal delta along the downdrift inlet shoreline the configuration of the ebbtidal delta in turn is a function of inlet size and position of the main ebb channel the swash bar welding process has caused preferential beach nourishment and historical shoreline progradation along the east frisian islands this process has produced barrier islands with humpbacked bulbous updrift and bulbous downdrift shapes the model of barrier island development presented in this paper not only explains well the configuration of the german barriers but also the morphology of barriers along many other mixed energy coasts

assessing subglacial processes from diatom fragmentation patterns reconstructing the size and glacial style of past ice sheet advances requires interpreting complex glacial sedimentary facies we use diatoms a major component of antarctic continental shelf deposits to infer the physical conditions under which these deposits were emplaced the degree of diatom fragmentation and the presence of diatoms of varying stratigraphic age in glacial sediments provide means to qualitatively gauge glacial mixing and transport here we report an experimentally calibrated index of diatom fragmentation that provides a simple but objective method of assessing the degree of subshearing imparted on marine glacial sedimentary deposits by using a ring shear device to subject diatomaceous sediment to stresses comparable to those beneath the ross ice streams we quantitatively assess patterns of diatom comminution resulting from compaction and from progressive shear stress elongate pennate diatoms are found to break disproportionately to discoid centric diatoms when subjected to shear stress thus a simple ratio of unbroken centric to pennate diatoms provides a reliable gauge of past shearing comparison of ring shear results with a suite of previously analyzed sediments that represent a variety of glacial glacial marine and hemipelagic settings of the ross sea and subglacial ross embayment demonstrates that this index can be employed for estimating relative subglacial stresses in this setting

hydrodynamics and sedimentation in wave dominated coastal environmentscontrol of barrier island shape by inlet sediment bypassing east frisian islands west germany a study of the east frisian islands has shown that the plan form of these islands can be explained by processes of inlet sediment bypassing this island chain is located on a high wave energy high tide range shoreline where the average deep water significant wave height exceeds 1 0 m and the spring tidal range varies from 2 7 m at juist to 2 9 m at wangerooge an abundant sediment supply and a strong eastward component of wave power 4 4 103 w m 1 have caused a persistent eastward growth of the barrier islands the eastward extension of the barriers has been accommodated more by inlet narrowing than by inlet migration it is estimated from morphological evidence that a minimum of 2 7 105 m3 of sand is delivered to the inlets each year via the easterly longshore transport system much of this sand ultimately bypasses the inlets in the form of large migrating swash bars the location where the swash bars attach to the beach is controlled by the amount of overlap of the ebb tidal delta along the downdrift inlet shoreline the configuration of the ebbtidal delta in turn is a function of inlet size and position of the main ebb channel the swash bar welding process has caused preferential beach nourishment and historical shoreline progradation along the east frisian islands this process has produced barrier islands with humpbacked bulbous updrift and bulbous downdrift shapes the model of barrier island development presented in this paper not only explains well the configuration of the german barriers but also the morphology of barriers along many other mixed energy coasts

tidal inlet stability in response to hydrodynamic and sediment dynamic conditions factors controlling tidal inlet stability are analysed and the general dynamic behaviour of an inlet in response to hydrodynamic and sediment dynamic conditions are discussed using an example from christchurch harbour southern england a technique is developed for the estimation of current speeds within the entrance channel on the basis of the definition of the apparent basin area the results show that 1 the image ratio as one of stability criteria can be extended to include the influences of the characteristics of tides and freshwater discharges 2 for inlet systems characterised by small flood and ebb tidal deltas and a pattern of by passing of sediments due to tidal currents the equilibrium cross sectional area can be determined analytically 3 the ratio of the difference between flood and ebb sediment discharges within the entrance to longshore sediment flux i e the parameter k defined in the present study can be used as a stability criterion and 4 the evolution of an inlet system in response to an increase in longshore sediment flux appears to be characterised by a decrease in ae and a constant k value in early stages but by a slow change in ae and a decrease in k in later stages hence these two stability criteria can be combined to describe the inlet behaviours in response to sediment movement for different stages of evolution

effects of alongshore morphology on groundwater flow and solute transport in a nearshore aquifer variations of beach morphology in both the cross shore and alongshore directions associated with tidal creeks are common at natural coasts as observed at a field site on the east coast of rarotonga cook islands field investigations and three dimensional 3 d numerical simulations were conducted to study the nearshore groundwater flow and solute transport in such a system the results show that the beach morphology combined with tides induced a significant alongshore flow and modified local pore water circulation and salt transport in the intertidal zone substantially the bathymetry and hydraulic head of the creek enabled further and more rapid landward intrusion of seawater along the creek than in the aquifer which created alongshore hydraulic gradient and solute concentration gradient to drive pore water flow and salt transport in the alongshore direction within the aquifer the effects of the creek led to the formation of a saltwater plume in groundwater at an intermediate depth between fresher water zones on a cross shore transect the 3 d pore water flow in the nearshore zone was also complicated by the landward hydraulic head condition resulting in freshwater drainage across the inland section of the creek while seawater infiltrating the seaward section these results provided new insights into the complexity intensity and time scales of mixing among fresh groundwater recirculating seawater and creek water in three dimensions the 3 d characteristics of nearshore pore water flow and solute transport have important implications for studies of submarine groundwater discharge and associated chemical input to the coastal sea and for evaluation of the beach habitat conditions

barrier island breach evolution alongshore transport and bay ocean pressure gradient interactions physical processes controlling repeated openings and closures of a barrier island breach between a bay and the open ocean are studied using aerial photographs and atmospheric and hydrodynamic observations the breach site is located on pea island along the outer banks separating pamlico sound from the atlantic ocean wind direction was a major control on the pressure gradients between the bay and the ocean to drive flows that initiate or maintain the breach opening alongshore sediment flux was found to be a major contributor to breach closure during the analysis period from 2011 to 2016 three hurricanes had major impacts on the breach first hurricane irene opened the breach with wind driven flow from bay to ocean in august 2011 hurricane sandy in october 2012 quadrupled the channel width from pressure gradient flows due to water levels that were first higher on the ocean side and then higher on the bay side the breach closed sometime in spring 2013 most likely due to an event associated with strong alongshore sediment flux but minimal ocean bay pressure gradients then in july 2014 hurricane arthur briefly opened the breach again from the bay side in a similar fashion to irene in summary opening and closure of breaches are shown to follow a dynamic and episodic balance between along channel pressure gradient driven flows and alongshore sediment fluxes

a method to estimate the longshore sediment transport at ebb tidal deltas based on their volumetric growth application to the guadiana spain portugal border

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

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

massive sediment bypassing on the lower shoreface offshore of a wide tidal inlet cat island pass louisiana abstract analysis of a series of historical bathymetric and shoreline surveys along the louisiana coast west of the mississippi river mouth detected a large area of deposition in water depths of 2 0 8 5 m offshore of a 9 km wide tidal inlet the cat island pass wine island pass system a 59 9 106 m3 sandy deposit formed from the 1930s 1980s spanning 27 km in the alongshore direction delineating the transport pathway for sediment bypassing offshore of the inlet on the shoreface bypassing connected the shorefaces of two barrier island systems the isles dernieres and the bayou lafourche the processes responsible for formation of this deposit are not well understood but sediment transport modeling suggests that sediment is transported primarily by wind driven coastal currents during large storms and hurricanes deposition appears to be related to changes in shoreline orientation closing of transport pathways into a large bay to the east and the presence of tidal inlets this newly documented type of bypassing an offshore bypassing of the inlet system naturally nourished the immediate downdrift area the eastern isles dernieres where shoreface and shoreline erosion rates are about half of pre bypassing rates erosion rates remained the same farther downdrift where bypassing has not yet reached as this offshore bypassing continues the destruction of the isles dernieres will be slowed

instruments and methods direct measurement of sliding at the glacier bed sliding at the base of trapridge glacier yukon territory canada was measured using a drag spool we describe this simple and inexpensive instrument as well as its installation and operation from 1990 to 1992 seven sites were instrumented with drag spools at six of the sites basal sliding during the period of observation accounted for 50 70 of the total flow observed at the glacier surface the contribution from ice creep is known to be small so most of the remaining surface motion must be attributed to subglacial sediment deformation for the seventh site the observed sliding rate was 90 of the total flow an indication that the sliding contribution varies spatially across the bed diurnal variations in the response of one of our instruments appear to be correlated to subglacial water pressure fluctuations and are interpreted in terms of changes in sliding velocity rather than the opening and closing of basal cavities

basal friction of ice stream e west antarctica we use surface velocity derived from sequential landsat imagery and a control method to estimate the basal friction distribution of a major west antarctic ice stream the area averaged basal stress is approximately 1 4 10 4 pa or about 29 of the area averaged driving stress of 4 9 10 4 pa uncertainty of the derived area averaged basal stress is difficult to assess and depends primarily on spatial variation of the flow law rate factor in the constitutive law spatial variation associated with depth averaged temperature variation gives an uncertainty of approximately 10 3 pa approximately 60 of the ice stream has a basal stress magnitude less than 10 4 pa and approximately 30 has less than 10 3 pa these characteristics suggest the presence of a mechanically weak water charged subglacial till small scale sticky spots where basal friction exceeds the area averaged driving stress are scattered irregularly across the subglacial regime and comprise approximately 15 of the ice stream area sticky spots cluster in regions where landsat imagery suggests structural features in the underlying bedrock

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

probing the till beneath black rapids glacier alaska usa a heavy down hole hammer actuated from the surface by a light composition rope was used to place instrumented probes into the active 7 m thick clast rich till underlying a site on black rapids glacier alaska usa where the ice is 500m thick a till penetration of about 2 5m was obtained and greater depths seem possible the probes measured pore water pressure and two axes of tilt which they broadcasted without wires to a receiver just above the ice till interface

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

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

dynamics of a wave dominated tidal inlet and influence on adjacent beaches currumbin creek gold coast australia currumbin creek on the australian gold coast is a wave dominated tidal inlet which exhibits a particularly active morphology the recent history of currumbin creek entrance has seen rapid growth of the entrance for access to the ocean by fishermen as a world class surfing site and as a recreational area before the construction of two groynes in the 70 s currumbin creek entrance was highly variable in terms of inlet location and sand bar characteristics due to a cyclical behaviour of spit migration nowadays the entrance is stabilised however natural processes continue with the entrance infilling causing flood and navigation issues resulting in a regular dredging program to maintain an open entrance and for regular beach nourishment plans this paper investigates the behaviour of the entrance and adjacent beaches from aerial photographs and numerical modelling before groyne construction sand by passing was intense resulting in channel migration and sometimes the closure of the mouth after training works the longshore drift is diverted further north from the mouth leading to new circulation patterns behind the headland during fair weather conditions the sand transported by the longshore current is trapped by currumbin rock groyne resulting in a negative sediment budget in palm beach for high energy conditions the diverted longshore current splits in the southern palm beach resulting in a circulation cell the sediment of the southern beaches is stirred up and transported both northward and toward the inlet

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

discrimination of the flow law for subglacial sediment using in situ measurements and an interpretation model 1 subglacial hydrological and mechanical processes play a critical role in determining the flow characteristics and stability of glaciers and ice sheets to study these processes we have measured simultaneously basal water pressure pore water pressure sediment deformation glacier sliding and sediment strength beneath trapridge glacier yukon territory canada to interpret these data we have developed a simple hydromechanical model of processes beneath a soft bedded alpine glacier the glacier bed is divided into soft bedded regions that are hydraulically connected to the subglacial drainage system soft bedded but hydraulically unconnected regions and hard bedded regions each region is represented as a one dimensional column the columns are coupled by a simple ice dynamics model that accounts for water pressure driven changes in basal shear stress distribution synthetic responses for subglacial instruments are calculated from the modeled basal conditions providing a framework for improving interpretation of field records the model is used to determine which of several till flow laws best represents conditions beneath trapridge glacier investigated are linear viscous nonlinear viscous nonlinear bingham and coulomb plastic tills pore water pressures sediment deformation profiles and sliding rates are calculated for each flow law comparison of synthetic and field instrument responses suggests that till behavior is best represented as coulomb plastic model results also suggest that the ploughmeter is the most diagnostic in situ indicator of till behavior currently available and that using long term observations of sediment deformation profiles in regions of varying pore water pressure can result in an underestimation of flow law nonlinearity

decadal morphological response of an ebb tidal delta and down drift beach to artificial breaching and inlet stabilisation abstract the morphodynamic response of a mixed energy ebb tidal delta guadiana estuary southern portugal and its down drift barrier island canela island to channel relocation and stabilisation by jetties is examined using a series of sequential bathymetric maps and vertical aerial photographs spanning five decades morphological analysis indicates that the ebb delta is in an immature state characterised by weak sediment bypassing landward migrating shoals on the swash platform have been produced by the jetty induced artificial bank breaching and by the collapse of the eastern portion of the delta the welding of these shoals has largely controlled the evolution of the coast with local accretion and erosion lasting for years and large amounts of regional accretion occurring over decades due to sand accumulation against jetties located further down drift these observations provide insights into the potential response of a coast to very large locally concentrated sand nourishment in the form of shoals the main effects of the jetties on the coast are observed at the centre of canela island with the production of an erosion hot spot associated with a temporally persistent and divergent longshore transport providing sand to the adjacent areas significant accretion is anticipated for the next decade along the entire island due to the ongoing attachment of the presently observed shoals after the depletion of this sediment source and in the context of weak sediment bypassing the most severe down drift erosion induced by the jetties is predicted to occur some decades after their construction this study demonstrates that the geomorphic response of an ebb tidal delta to jetty construction must be considered at multiple temporal and spatial scales when assessing the impacts of jetties on the down drift coast

long term morphological modeling of a tidal inlet the arcachon basin france abstract the arcachon lagoon on the french atlantic coast is a triangular shaped lagoon of 20 km on a side connected to the ocean by a 3 km wide inlet between the mainland and an elongated sand spit this tidal inlet exhibits a particularly active morphology due to locally strong tidal currents and rough wave conditions during the past 300 years minimum and maximum spatial extents of the cap ferret sand spit have varied by 8 km while one or two channels have alternately allowed circulation between the lagoon and the ocean these impressive morphological changes have never prevented regular flushing of the lagoon eventhough the spit came as close as 300 m from the coast during the 18th century according to bruun s concept of tidal inlet stability theory and engineering 1978 510 pp the balance between longshore littoral transport and the tidal prism ensures the perpetuity of the inlet process modeling was believed to give better insight into the respective roles of tides and waves in driving the long term morphological changes of the inlet a two dimensional horizontal morphodynamic model was therefore developed combining modules for hydrodynamics waves sediment transport and bathymetry updates the use of process models at a scale of decades requires a schematization of the input conditions we defined representative mean annual wave and tide conditions with respect to sediment transport i e conditions that induce the same annual transport as measured in the field driven by these representative conditions simulations run from the 1993 bathymetry show that the tide is responsible for the opening of a new channel at the extremity of the sand spit where tidal currents are the strongest while waves induce a littoral transport responsible for the longshore drift of sand bodies across the inlet one particular simulation consisted in running the model from a hypothetical initial topography where the channels are filled with sand and the entire inlet is set to a constant depth 3 m the results show the reproduction of a channel and bar system comparable to historical observations which supports the idea that the lagoon is unlikely to be disconnected from the ocean provided tide and wave conditions remain fairly constant in the following decades

spatial and temporal patterns of shoreline change of a 280 km high energy disrupted sandy coast from 1950 to 2014 sw france abstract a dataset of 15 geo referenced orthomosaics photos was generated to address long term shoreline change along approximately 270 km of high energy sandy coast in sw france between 1950 and 2014 the coast consists of sandy beaches backed by coastal dunes which are only disrupted by two wide tidal inlets arcachon and maumusson a wide estuary mouth gironde and a few small wave dominated inlets and coastal towns a time and spatially averaged erosion trend of 1 12 m year is found over 1950 2014 with a local maximum of approximately 11 m year and a maximum local accretion of approximately 6 m year respectively maximum shoreline evolutions are observed along coasts adjacent to the inlets and to the estuary mouth with erosion and accretion alternating over time on the timescale of decades the two inlet sandspit systems of arcachon and maumusson show a quasi synchronous behaviour with the two updrift coasts accreting until the 1970s and subsequently eroding since then which suggests that shoreline change at these locations is controlled by allocyclic mechanisms despite sea level rise and the well established increase in winter wave height over the last decades there is no capture of significant increase in mean erosion rate this is hypothesized to be partly the result of relevant coastal dune management works from the 1960s to the 1980s after a long period of coastal dune disrepair during and after the second world war this study suggests that long term shoreline change of high energy sandy coasts disrupted by inlets and or estuaries is complex and needs to consider a wide range of parameters including non extensively waves tides inlet dynamics sea level rise coastal dune management and coastal defences which challenges the development of reliable long term coastal evolution numerical models

beach morphodynamics influenced by an ebb tidal delta on the north florida atlantic coast tidal inlets interrupt longshore sediment transport thereby exerting an influence on adjacent beach morphology to investigate the details and spatial extent of an inlet s influence we examine beach topographic change along a 1 5 km coastal reach adjacent to matanzas inlet on the florida atlantic coast analyses of beach morphology reveal a behavioral change between 0 64 and 0 86 km from the inlet channel centerline interpreted to represent the spatial extent of inlet influence beyond this boundary the beach is narrow exhibits a statistically significant inverse correlation of shoreline position with offshore wave conditions and has a uniform alongshore pattern in temporal behavior as determined from empirical orthogonal function eof analysis on the inlet side of the boundary the beach experiences monotonic widening with proximity to the inlet lacks spatial consistency in correlation between shoreline position and wave conditions and exhibits an irregular pattern in spatial eof modes we augment the field observations with numerical modeling that provides calculations of wave setup and nearshore current patterns near the inlet highlighting the effects of the ebb tidal delta on the assailing waves the modeling results are verified by a natural experiment that occurred during may 2009 when a storm produced sedimentary mass accreted to the lower beach then subsequently split into two oppositely directed waves of sediment that migrated away from the initial accretion site in the subsequent months our results suggest that the ebb tidal delta produces a pattern of wave setup that creates a pressure gradient driving an alongshore flow that opposes the longshore currents derived from breaking of obliquely oriented incident waves the resulting recirculation pattern on the margin of the ebb tidal delta provides a mechanism through which the inlet influences adjacent barrier island beach morphology copyright 2016 john wiley sons ltd

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

basal till fabric and deposition at burroughs glacier glacier bay alaska basal till and debris rich ice in the basal transport zone of burroughs glacier southeast alaska were studied to develop a model for till deposition and to acquire new pebble fabric measurements from till at a modern glacier studies of till deposition and fabric measurements from modern glacial deposits are important for studying the genesis of pleistocene and pre pleistocene glacial sediment basal till at burroughs glacier is deposited at the ice margin by the melting of basal debris rich ice that stagnated progressively from the bottom upward beneath moving ice during deglaciation the debris rich ice is typically 1 3 m thick and has very high sediment content ranging from 60 to 70 by volume the long axes of pebbles in the debris rich ice and in other parts of the basal transport zone of the glacier are typically well oriented parallel to ice flow the average s 1 eigenvalue for nine fabrics measured in the basal transport zone is 0 8092 0 067 the basal till is massive matrix supported silty sandy diamicton striated lodged boulders and crag and tails are common on the till surface s 1 eigenvalues calculated from 27 pebble fabrics measured in the till range from 0 4930 no fabric to 0 8261 the average value is 0 6757 0 092 the range of values is nearly identical to that reported for fabrics from undeformed lodgment till by dowdeswell and sharp 1986 although the final process of till deposition at burroughs glacier is passive melt out from stagnant debris rich ice the nature and pebble fabric of the till are inherited from active ice processes specifically the progressive stagnation of debris rich ice beneath active ice evidence of differential movement shear within the debris rich ice during deposition is reflected in a change in fabric azimuth with depth in the till beveled bullet boulders and shear surfaces these features are typically thought to be characteristic of lodgment till or brittle deformation of subglacial sediment the melt out process is passive because of the very high debris content of the debris rich ice if the same sequence of till deposition occurred in older glacial deposits and the debris content of the basal ice was high the final process of melt out would not be reflected in properties of the sediment

stick slip sliding behaviour at the base of a glacier measurement of basal sliding is an important component in studying the mechanical and hydrological coupling between a glacier and its bed during the 1992 summer field season we used a drag spool to measure sliding at the ice bed interface oftrapridge glacier a small surge type glacier in the st elias mount ains yukonterritory canada measured diurnal variations in sliding appear to be correlated to subglacial water pressure fluctuations in contrast to other observations where peak subglacial water pressure and glacier motion appear to coincide our data imply that maximum sliding rates coincide with rises in water pressure if the growth of water filled cavities at the glacier bed is associated with these pressure increases then our observations may corre spond to numerical results by iken 1981 which indicate that the largest sliding velocity occurs during cavity growth and not when the steady state size of cavitation is attained howe ver our data suggest the idea that a loca lized stick slip relaxation process is at work as the water pressure rises a local strain build up in the ice is released resulting in a momentary increase in sliding rate once the finite relaxation has occurred further rises in water pressure do not produce additional enhancement of basal sliding and the stick slip cycle begins again by accumulation of elastic strain we have developed a theoretical model for the sliding motion of ice over a surface having a basal drag that varies tem porally in response to changes in subglacial water pressure our model results support the proposed stick slip sliding process at the glacier base whereby accumulated elastic strain in the ice is released as the rising water pressure decouples the ice from the bed

glacial hydrology and erosion patterns a mechanism for carving glacial valleys article i nfo article history most glacial erosion models assume that erosion rates are proportional to ice sliding velocity while recent studies have shown that water plays a major role in modulating sliding velocities the impact it might have on erosion rates is still unclear here we incorporate subglacial hydrology into a glacial erosion model that is based on a sliding rule our results explicitly highlight that adding subglacial hydrology has profound impacts on the temporal and spatial patterns of glacial erosion first it suggests that erosion might mainly occur during melting seasons when subglacial water pressure is large and effective pressure is low i e before the channels fully develop and reduce the water pressure at the ice bedrock interface which promotes sliding and erosion second the distribution of erosion is bimodal with a peak at the equilibrium line altitude and as we demonstrate enhanced erosion at much lower altitudes within the ablation area where water due to melting abounds this has important implications because it explains why glacial erosion can simultaneously

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

coupling the flow of ice water and sediment in a glacial landscape evolution model abstract the processes of subglacial erosion depend not only on the sliding motion of glaciers over bedrock but also on the presence of meltwater and sediment in the subglacial environment in particular theoretical models for subglacial quarrying and abrasion as well as hypothesized erosion thresholds associated with subglacial sediment transport include both positive and negative effects of subglacial water and sediment on the rate of erosion in order to incorporate the existing theoretical models for subglacial erosion by quarrying and abrasion in a long term glacial landscape evolution model we here present a coupled computational framework for simulating the simultaneous flow processes of ice water and sediment we supplement a higher order ice sheet model with simple long term models for glacial hydrology and subglacial sediment transport as well as fluvial and hillslope related erosion processes an important strength of the modeling framework presented relates to the morphological detail of the landscapes produced which facilitates direct comparison with existing landforms this improves the connection between glacial landscape evolution models geomorphological observations and the existing knowledge of the physical processes that operate under glaciers we demonstrate with model examples how increased basal meltwater pressure and transport limited subglacial erosion lead to both positive and negative feedbacks related to glacial erosion and the formation of overdeepenings

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

the effects of entrained debris on the basal sliding stability of a glacier 1 new laboratory experiments exploring likely subglacial conditions reveal controls on the transition between stable sliding and stick slip motion of debris laden ice over rock with implications for glacier behavior friction between a rock substrate and clasts in ice generates heat which melts nearby ice to produce lubricating water an increase in sliding speed or an increase in entrained debris raises heat generation and thus meltwater production unstable sliding is favored by low initial lubrication followed by rapid meltwater production in response to a velocity increase low initial lubrication can result from cold or drained conditions whereas rapid increase in meltwater generation results from strong frictional heating caused by high sliding velocity or high debris loads strengthening of the interface healing during stick intervals between slip events occurs primarily through meltwater refreezing when healing and unstable sliding are taken together the experiments reported here suggest that stick slip behavior is common from motion of debris laden glacier ice over bedrock

morphodynamics of ebb tidal deltas a model approach abstract the results of 2dh numerical models of the frisian inlet located in the dutch wadden sea are discussed to gain further knowledge about the physical mechanisms causing the presence of both ebb tidal deltas and of channels and shoals in tide dominated inlet systems a hydrodynamic model extended with sediment transport formulations was used to verify earlier conceptual models that deal with ebb tidal delta characteristics the model does not confirm their hypothesis concerning the observed spatial asymmetry of ebb tidal deltas and suggests that long term morphological simulations are needed to understand this aspect furthermore the model indicates that the initial formation of the ebb tidal delta is mainly due to convergence of the tidally averaged sediment flux related to residual currents whilst the net sediment transport in the basin is mainly caused by tidal asymmetry a second model accounting for feedbacks between tidal motion and the erodible bottom was used to simulate the long term bathymetric evolution of the frisian inlet under fair weather conditions this model reproduces the gross characteristics of the observed morphology the presence of a double inlet system with two distinct ebb tidal deltas having different sizes and the presence of channels and shoals the role of the engelsmanplaat a consolidated shoal in the middle of the frisian inlet was not found to be crucial for the morphodynamic stability of this inlet system

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

borehole water level variations and the structure of the subglacial hydrological system of haut glacier d arolla valais switzerland late summer subglacial water pressures have been measured in a dense array of boreholes in the ablation area of haut glacier d arolla switzerland interpolated surfaces of minimum diurnal water pressure and diurnal water pressure variation suggest the presence of a subglacial channel within a more widespread distributed drainage system the channel flows along the centre of a variable pressure axis vpa some tens of metres wide that is characterized by low minimum diurnal water pressures frequently atmospheric and high diurnal water pressure variations these characteristics are transitional over a lateral distance of c 70 m to higher and more stable subglacial water pressures in the adjacent distributed system water pressure variations recorded in boreholes located close to the centre of the vpa reflect the delivery of surface derived meltwater to the glacier bed and result in a diurnally reversing transverse hydraulic gradient that drives water out from the channel into the distributed system during the afternoon and back to the channel overnight subglacial observations suggest that such flow occurs through a vertically confined sediment layer borehole turbidity records indicate that the resulting diurnal water flows are responsible for the mobilization and transport of fine debris in suspension analysis of the propagation velocity and amplitude attenuation of the diurnal pressure waves suggests that the hydraulic conductivity of the sediment layer decreases exponentially with distance from the channel falling from c 10 4 m s 1 at the channel boundary to c 10 7 ms 1 70m away these apparent hydraulic conductivities are consistent with darcian flow through clean sand and typical glacial till respectively we suggest that fine material is systematically flushed from basal sediments located adjacent to large melt season drainage channels beneath warm based glaciers this process may have important implications for patterns of glacier erosion hydrochemistry and dynamics

basal mechanics of ice streams insights from the stick slip motion of whillans ice stream west antarctica the downstream portion of whillans ice stream west antarctica moves primarily by stick slip motion the observation of stick slip motion suggests that the bed is governed by velocity weakening physics and that the basal physics is more unstable than suggested by laboratory studies the stick slip cycle of whilla s ice plain exhibits substantial variability in both the duration of sticky periods and in slip magnitude to understand this variability we modeled the forces acting on the ice stream during the stick phase of the stick slip cycle the ocean tides introduce changes in the rate at which stress is applied to the ice plain increased loading rates promote earlier failure and vice versa results show that the bed of whillans ice stream strengthens over time healing during the quiescent intervals in the stick slip cycle with the bed weakening during slip events the time dependent strengthening of the ice plain bed following termination of slip events indicates that the strength of the bed may vary by up to 0 35 kpa during the course of a single day copyright 2009 by the american geophysical union

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

diffusive mixing between shearing granular layers constraints on bed deformation from till contacts shearing of subglacial till has been invoked widely as a mechanism of glacier motion and sediment transport but standard indicators for determining shear strain from the geologic record are not adequate for estimating the very high strains required of the bed deformation model here we describe a laboratory study of mixing between shearing granular layers that allows an upper limit to be placed on bed shear strain in the vicinity of till contacts owing to random vertical motions of particles induced by shearing mixing can be modeled as a linearly diffusive process and so can be characterized with a single mixing coefficient d ring shear experiments with equigranular beads and lithologically distinct tills provide the value of d although in experiments with till d decreases systematically with strain to a minimum value of 0 0045 mm 2 kinetic gas theory provides an estimate of the dimensionless mixing coefficient which is within an order of magnitude of laboratory values knowing the minimum value of d the distribution of index lithologies measured across till contacts in the geologic record can be used to estimate the maximum shear strain that has occurred across till contacts application of this technique to the contact between the des moines and superior lobe tills in east central minnesota u s a indicates that shear strain did not exceed 15 000 at the depth of the contact