a ploughing model for the origin of weak tills beneath ice streams: a qualitative treatment

Quaternary International 86 (2001) 59–70

A ploughing model for the origin of weak tills beneath ice streams: aqualitative treatment

Slawek M. Tulaczyka,*, Reed P. Schererb, Christopher D. Clarkc

aDepartment of Geological Sciences, 101 Slone Bldg., 0053, University of Kentucky, Lexington, KY 40506, USAbDepartment of Geology, Northern Illinois University, IL 61125, USA

cDepartment of Geography & Sheffield Centre for Earth Observation Science, University of Sheffield, S10 2TN, UK

Abstract

Glaciological studies of West Antarctic ice streams have shown that weak sub-ice-stream tills provide the basal lubrication thatmakes fast ice streaming possible under low driving stresses. Given the significant current interest in time-dependent ice streambehavior, there is a clear need for a conceptual model of weak sub-ice-stream tills that treats in a simple, but physically correct, way

the coupling between evolution of till properties and ice stream dynamics. As a possible alternative to the previous, viscous-bedmodel, we propose a ploughing model that is consistent with the experimentally determined Coulomb-plastic rheology of sub-ice-stream till. In the ploughing model, the till is a several-meters-thick layer of sedimentary material that is disturbed and transported

by ploughing that occurs during sliding of a bumpy ice base. The thickness of the till layer is determined in the ploughing model bythe amplitude of the largest roughness elements (‘‘ice keels’’). There is no direct proof for the existence of ice bumps and ice keelsbeneath the modern West Antarctic ice streams but bedforms (e.g. megalineations and bundle structures) left behind by Pleistocene

ice streams strongly support our assumption that an ice stream base is irregular. Generation of new till material occurs when icekeels protrude through the existing till layer and erode the top of the sub-till preglacial sediments. Based on a single tethered stakemeasurement of Engelhardt and Kamb (J. Glaciol. 44 (1998) 223) made at the UpB camp, Ice Stream B, West Antarctica (Fig. 1), weestimate that the till flux due to sliding with ploughing is thereo88m3 yr�1 per meter width. To balance the estimated till flux in the

UpB area, substrata erosion by ice keels would have to take place at a high, but not unreasonable, non-dimensional rate ofo1.7� 10�4 (assuming 1% contact area). In the case of the West Antarctic ice streams, erosion of sub-till materials by ice keels maybe particularly fast and unimpeded because these ice streams are overriding unlithified preglacial (Tertiary) sediments. The most

significant implication of the proposed ploughing model is that it permits treating basal resistance to ice motion as being velocityindependent (plastic till rheology) while allowing subglacial transport of till as in the viscous-bed model. Models of ice streams witha plastic bed exhibit a greater potential for unstable behavior than models of ice streams with viscous beds. r 2001 Elsevier Science

Ltd and INQUA. All rights reserved.

1. Introduction

Discovery of a high-porosity till layer beneath one ofthe fast-moving West Antarctic ice streams (Blanken-ship et al., 1986, 1987; Rooney et al., 1987; Engelhardtet al., 1990) provided an impetus for a significantparadigm shift in glaciology and glacial geology(Boulton, 1986). This finding demonstrated that fastice motion may be accommodated not only by sliding ofice over rigid bedrock, as it has been assumed in theclassical glaciological models, but also by shearing ofweak subglacial till (Weertman, 1957; Kamb, 1970;

Lliboutry, 1979; Alley et al., 1986, 1987; Boulton andHindmarsh, 1987). Over the last decade, an increasinginterest in stability of the West Antarctic ice sheet and inthe dynamic behavior of Pleistocene ice sheets hasfurther emphasized the need for a thorough under-standing of the role of weak tills in ice stream mechanicsand dynamics (MacAyeal, 1989, 1992; Hughes, 1992;Clark et al., 1996; Jenson et al., 1996; Marshall et al.,1996; Bindschadler, 1997, 1998). Ice motion associatedwith shearing of subglacial till has also becomeimportant in glacial geology where it is the favoredmechanism used to explain high fluxes of glacialsediments (Alley, 1991; Hooke and Elverh�i, 1996;Alley et al., 1998).The initial approach to modelling the role of

subglacial tills in fast ice streaming was based on

*Corresponding author. Department of Earth Sciences, University

of California, Santa Cruz, CA 95064, USA.

E-mail address: [email protected] (S.M. Tulaczyk).

1040-6182/01/$ - see front matter r 2001 Elsevier Science Ltd and INQUA. All rights reserved.

PII: S 1 0 4 0 - 6 1 8 2 ( 0 1 ) 0 0 0 5 0 - 7

application of the viscous till model developed byBoulton and Hindmarsh (1987) for the margin ofBreidamerkurjokull glacier in Iceland (Alley et al.,1986, 1987). In this approach, the sub-ice-stream till istreated as a viscous fluid that deforms pervasivelythroughout its thickness and replenishes itself by erosionof the underlying geologic substratum. The viscous tillmodel of ice streaming assumed that the gravitationalshear stress, which drives ice stream motion, is fullybalanced by the basal shear stress arising during viscoustill deformation (Alley et al., 1987). Influence of stressesdue to ice deformation in ice stream shear margins anddue to longitudinal extension or compression of an icestream was presumed to be negligible. Thus, ice streamvelocity was thought to be fully determined by themagnitude of the gravitational driving stress (assumedequal to the basal shear stress), till thickness, and tillviscosity.Recent studies of the West Antarctic ice streams paint

a more complex picture of ice stream mechanics. Firstly,samples of the sub-ice-stream till recovered in severallocations on Ice Stream B have shown practically noviscous behavior in laboratory tests (Kamb, 1991;Tulaczyk et al., 2000a). Rather, the tested till samplesbehave as a Coulomb-plastic material whose strength islinearly dependent on effective stress but practicallyindependent of strain rate. Moreover, analysis of icestream force balance indicate that rapid ice deformationin the shear margins contributes significantly moreresistive stress than deformation of subglacial till(Echelmeyer et al., 1994; Jackson and Kamb, 1997).This conclusion is consistent with direct measurementsof till strength (Kamb, 1991) which have demonstratedthat the strength of the sub-ice-stream material, and themagnitude of the basal shear stress that it can support, isseveral times smaller than the magnitude of thegravitational driving stress acting on the ice stream(B2 kPa vs. B14–20 kPa, Jackson and Kamb, 1997).The emerging picture of ice streams suggests that they

share some important characteristics of ice shelves andgrounded ice masses. Whereas ice sheets are firmlycoupled to their base, in the sense that the gravitationaldriving stress is equal to the basal shear stress, in thefloating ice shelves basal stresses are zero and thegravitational driving stress is supported by marginal andlongitudinal stresses (Paterson, 1994). Ice streams seemto fall in between these two cases. They do not float inwater but they do move over till so weak that it cannotsupport the gravitational driving stress. The weaker thetill is, the faster the ice stream motion because a greaterfraction of the driving stress must be supported by icedeformation in shear margins (Raymond, 1996,Eq. (39); Tulaczyk et al., 2000b).These new glaciological findings represent a signifi-

cant departure from the viscous till model, mainlybecause they imply a less active role of the till layer in

determining ice stream velocity. Similar conclusion canbe drawn from the results of recent studies that havefailed to find viscous deforming till beneath mountainglaciers (Blake, 1992; Iverson et al., 1995; Hooke et al.,1997; Iverson et al., 1997).If the sub-ice-stream till layers are not made of

viscous fluids that deform actively and erode thesubstrata to replenish themselves, then how should oneaccount for their existence and properties in the newframework of ice stream mechanics? Here, we propose anew, ploughing model for the origin of weak sub-ice-stream tills and discuss implications of this model forice-stream stability and for geologic models of glacialsediment fluxes.

2. Characteristics of sub-ice-stream tills

In this section, we review the properties of sub-ice-stream tills to provide background for further discussionof our model of till generation.Considering the logistical challenges facing glaciolo-

gical research in West Antarctica, a significant effort hasbeen invested into studying the weak till layers under-lying the West Antarctic ice streams. This effort yieldeda number of important observations made using remotesensing combined with inverse modeling, geophysicalsurveys, borehole experiments, or direct analysis ofsubglacial till cores (Alley et al., 1986; Blankenship et al.,1986, 1987; Rooney et al., 1987; Engelhardt et al., 1990;Kamb, 1991; MacAyeal et al., 1995; Jacobel et al., 1996;Jackson and Kamb, 1997; Engelhardt and Kamb, 1997,1998; Scherer et al., 1998). Much of the existingknowledge comes from research on Ice Stream B,especially in the area of the so-called UpB camp (Fig. 1).

Fig. 1. Location map shows outlines of the ice streams flowing

through the Ross Sea Section of West Antarctica. Letters A through

E denote the individual ice streams. Ice elevation is indicated by

contour lines at 250m intervals. Major mountain ranges are shown in

black (after Tulaczyk et al., 1998, Fig. 1).

S.M. Tulaczyk et al. / Quaternary International 86 (2001) 59–7060

Both geophysical and sedimentological investigationssuggest strongly that the weak sub-ice-stream tills aredeveloped over sedimentary basins that are buriedbeneath parts of the West Antarctic ice sheet (Rooneyet al., 1991; Scherer, 1991; Anandakrishnan et al., 1998;Bell et al., 1998; Tulaczyk et al., 1998). These basinslikely contain Mid to Late Cenozoic sediments similar tothe ones forming the glacimarine sedimentary sequencethat fills the Ross Sea sedimentary basin (Barrett,1975; Hayes and Frakes, 1975). This inference isstrongly corroborated by three lines of evidence: (1)seismic properties of the sedimentary package under-lying the UpB area (Rooney et al., 1991); (2) texturalproperties of the till from beneath Ice Stream Bthat suggest glacial recycling from parent sedimentsanalogous to those found in the Ross Sea (Tulaczyket al., 1998); and (3) finding that the Ice Stream B tillsamples contain microfossil assemblages whose ageranges correspond to the age ranges of Ross Seasequences (Scherer, 1991; Scherer et al., 1998). Incontrast, subglacial presence of hard bedrock appearsto hinder development of efficient basal lubricationnecessary for ice streaming conditions (Anandakrishnanet al., 1998; Bell et al., 1998).The till layer beneath Ice Stream B is fairly

continuous and has thickness up to several meters(Blankenship et al., 1986, 1987; Rooney et al., 1987;Engelhardt et al., 1990; Tulaczyk et al., 1998). In places,it may be punctuated by ‘sticky spots’ (Rooney et al.,1987; Alley, 1993) although more abundant evidencefor their presence comes from studies on the stoppedIce Stream C (Anandakrishnan and Bentley, 1993;Anandakrishnan and Alley, 1997). Geophysical investi-gations in the UpB area yielded evidence fordrumlinization or large-scale fluting of the till layer(Rooney et al., 1987; Novick et al., 1994). Vertical andlateral variations in microfossil abundance, till porosityand composition, and in seismic properties, suggest thatthe several-meter-thick layers of sub-ice-stream till mayin fact represent packets of till units (Rooney, 1988 andthe next section of this paper).One of the most consistent physical characteristics of

the till samples recovered from beneath the WestAntarctic ice streams is their high porosity rangingfrom B39% to B45% (Tulaczyk et al., 2000a, 2001).This water-rich, muddy till has very low strength of onlya few kPa (Kamb, 1991). Mechanical behavior of thesub-ice-stream till samples has been examined usingseveral different apparatuses: shear box, triaxial device,ring shear device. The results of these tests have shownrepeatedly that the strength of this till is practicallyindependent of strain magnitude or shear strain rate butit depends linearly on effective stress (Kamb, 1991;Tulaczyk, 1998; Tulaczyk et al., 2000a).Very little is known about the character of deforma-

tion that the sub-ice-stream tills experience in situ. The

most relevant record available is that obtained with atethered stake employed at the bottom of a borehole inthe UpB area (Engelhardt and Kamb, 1998). With thetethered stake it is possible to determine what fraction ofice stream surface velocity is accommodated below andabove the depth of emplacement of this device in the till,in this case o0.3m. The record obtained in the UpBarea indicated that over 26 days only 31% of ice streamvelocity was accommodated below the shallow depth ofemplacement. This result favors basal sliding and/orshallow till deformation as the predominant mode of icestream motion. Perhaps the most surprising feature ofthe tethered stake record was its very high temporalvariability. The observed local contribution of slidingand shallow till deformation to ice stream motion rangesfrom ca. 5% to 100%.

3. The need for a new model

In addition to the general need for a new look at sub-ice-stream tills resulting from the recent shift in under-standing of ice-stream mechanics (as discussed in theintroduction), we consider three lines of evidencegathered through studies of the West Antarctic icestreams to be inconsistent with the viscous-bed model ofice stream mechanics (Alley et al., 1986, 1987; Alley,1989a, b): (1) results of geotechnical tests showing thatrheology of sub-ice-stream tills is not viscous butCoulomb-plastic (Kamb, 1991; Tulaczyk, 1998; Tulac-zyk et al., 2000a), (2) the tethered stake record from IceStream B demonstrating predominance of basal slidingand/or shallow deformation (Engelhardt and Kamb,1998), and (3) presence of intra-till inhomogeneities inporosity, composition, and microfossils. Since the firsttwo lines of evidence listed above have been discussed inprevious publications we will explain only briefly theirimportance to our current line of argumentation. Thethird point is developed here.One of the most straightforward ways to test the

viscous-till model of ice streaming is to verify whetherthe samples of sub-ice-stream till that have beenobtained by drilling, do indeed have the nearly linearlyviscous rheology assumed in this model (e.g. Alley et al.,1987). First such verification has been performed usingstrain-rate-controlled and stress-controlled shear-boxtests (Kamb, 1991). This test yielded a negative resultin that the examined till samples demonstrated nearlyplastic rheology with almost no dependence of strengthon strain rate. This result was criticized on the basis ofthe assumption that a relevant till rheology can only beobtained if one shears till samples to very high strains(100s or 1000s) whereas shear-box test is capable ofaccumulating only relatively small strains (B10). Asecond testing program was designed to examinethe potential influence of such variables like strain

S.M. Tulaczyk et al. / Quaternary International 86 (2001) 59–70 61

magnitude and effective stress on till rheology(Tulaczyk, 1998). This program involved tests in atriaxial device and in a ring-shear device. The results ofthese tests confirmed fully the conclusion reached on thebasis of the earlier shear box tests. At different levels ofstrain (1 to ca. 1000) and at different magnitudes ofeffective stress (B5–500 kPa), the samples of the UpBtill behave like a Coulomb-plastic material with nosignificant strain rate dependence of strength (Tulaczyk,1998; Tulaczyk et al., 2000a). These results are incon-sistent with the viscous-till model because the modelassumed that till viscosity is the fundamental parameterthat determines the velocity of ice stream flow and thatthe strain-rate dependence of till strength forces the tillto deform continuously throughout its thickness. Sinceit is not possible to observe this strain-rate dependenceof strength and to measure till viscosity, the viscous-tillmodel loses its appeal as a predictive and an explanatorytool.The tethered stake record from the UpB area

(Engelhardt and Kamb, 1998) is also inconsistent withthe predictions of the viscous-till model. Deep deforma-tion distributed throughout several meters of tillthickness should accommodate almost all of the icestream velocity (Fig. 2) (Alley et al., 1989, their Fig. 5).In the observed record, it is sliding and/or shallow(o0.3m) deformation that account for most of the

motion. Equally important is the fact that the tetheredstake record is so highly variable. Large variability ofstrain rates is characteristic for deformations takingplace in highly non-linear and plastic materials (Hind-marsh, 1997).The third line of evidence that we consider to be

inconsistent with the predictions of the viscous-tillmodel for ice stream beds is the presence of inhomo-geneities in porosity, composition, and microfossilcontent that we have detected in the till cores recoveredfrom beneath Ice Stream B near the UpB camp (Fig. 3).These inhomogeneities are real in the sense that theirmagnitudes are greater than the analytical uncertaintiesassociated with the applied data collection methods. Theviscous-till model predicted that in the UpB area thesub-ice-stream till layer should be deforming pervasivelyand continuously to the thickness of ca. 6m (Alley et al.,1989). Such continuous deformation persisting as longas the ice stream is active (at least 1000s of years,Bentley, 1997) should result in a homogenization of thetill layer (Piotrowski and Tulaczyk, 1999). The moststriking is the distribution of microfossils, which are

Fig. 2. Hypothetical vertical distribution of strain in the till beneath

the UpB camp, West Antarctica. The shaded area represents the best

estimate for strain distribution under the assumption of viscous till

rheology (Alley et al., 1989, Fig. 5 for a till of 6m thickness). The solid

square located near the x-axis represents the result of the tethered stake

experiment performed by Engelhardt and Kamb (1998) at the UpB

camp. Dashed thin lines give two possible scenarios for strain

distribution under ploughing conditions with an assumption of a

much thinner ‘active’ portion of till.

Fig. 3. Vertical changes in till properties observed in the three longest

cores recovered from the UpB area, 89-7, 92-1, 95-1, West Antarctica

(see Tulaczyk et al., 2000a, for core locations). Between depths 1.9–

2.3m, core 89-7 displayed significant changes in three different

properties: (1) mineralogical composition of the sand fraction (increase

in lithic fragments), (2) grain-size distribution (increase in sand

abundance), and (3) till porosity. The upper part of the core 92-1

had higher porosity than the till samples taken from below ca. 0.9m in

this core (this quantitative observation was corroborated by a more

‘wet’ appearance of the upper part of the core when it was handled in

the lab). Core 95–1 showed a significant drop in porosity below 2.9m.

The same core had also much greater abundance of diatom fragments

in the uppermost layer than in the main body of the core (data from

Scherer et al., 1998, Table 1). All porosities were determined by the

weight-loss method (Bowles, 1992). Mineralogical composition of the

sand fraction and diatom abundance were derived by point counting

(Tulaczyk et al., 1998). Combination of sieving and pipetting was used

to analyze the abundance of sand, silt, and clay in the till samples.


found in any appreciable abundance only near the top ofthe till cores from UpB (Fig. 3; Scherer et al., 1998;Tulaczyk et al., 1998). We interpret this observation as asign of preferential till transport in a thin layerimmediately adjacent to the ice base.UpB diatoms are too rare to perform accurate

estimates of whole fossils, but abundance estimates ofsmall diatom fragments (>2 mm) in the o250 mmfraction of these sediments are highly reproducible,using the method of Scherer (1994), despite the fact thatthe number of fragments generated from a single diatomcan vary from a few to more than several hundred. Thedata demonstrate a distinct difference between sedi-ments recovered by piston coring several cm to several mbeneath the ice, and sediment samples that include theparticles in closest proximity to the ice. The uppermostsediments contain a significantly higher concentration ofdiatoms and diatom fragments than those below. Coresamples from beneath the uppermost several cm containa mean concentration of 1.4� 105 fragments per gramdry sediment (fr/g) ðn ¼ 14Þ; whereas samples fromcloser to the ice base at the central part of the ice streamat UpB contain a mean of 1.8� 106 fr/g ðn ¼ 11Þ(Scherer and Tulaczyk, 1997). Furthermore, the samplesknown to contain Quaternary diatoms contain as muchas two orders of magnitude higher total concentration ofdiatom fragments, with a mean of 2.3� 108 fr/g ðn ¼ 4Þ(Scherer et al., 1998).Significant variation in diatom abundance and

assemblages is also noted normal to ice flow, across afew km, but diatom abundance and diatom speciesgroupings tend to be comparable parallel to flow, up totens of km downstream in the Upstream B region(Scherer et al., 1998). We interpret these observations asfurther evidence of ploughing with ice flow, which maycreate distinct lateral till packets. The highest concen-tration of diatom fragments, including whole, well-preserved diatoms, is in a region identified by seismicproperties as possessing an unusually thin or absent tilllayer (Rooney et al., 1987). We speculate that this maybe a region recently excavated by ice keel ploughing.Well-preserved Quaternary diatoms may have beencarried from upstream sedimentary deposits in suspen-sion, in the cavity formed in the lee of an ice keel(Fig. 6B).The distribution of porosity in the three longest cores

recovered at UpB also suggests that till deformationdoes not extend everywhere to depths of several meters.In these cores, there is a significant drop in till porositybetween B0.5 and B2.5m depth (Fig. 3). Since tillstrength decreases with increasing till porosity (Tulaczyket al., 2000a), the upper, more porous parts of the tillcores must be weaker than the lower, less poroussections. It is mechanically more favorable for any typeof deformation (viscous or plastic) to concentrate in theweaker material. Therefore, the thickness of the more

porous sections of the cores probably reflects the depthto which till deformation is distributed in situ. Seismicdata collected in the UpB area also suggest that there isa weaker, 0.5–1.5m thick layer on top of the several-meter-thick packet of till that was presumed to havebeen deforming pervasively in the viscous-till model(Rooney, 1988). This seismic subunit was detected overmost of the length of the seismic profiles. The seismicevidence indicates that the upper core sections withhigher porosity are not local aberrations but that theymay be representative of a continuous weak layer towhich most of till deformation should be confined.

4. Sliding and ploughing by an uneven ice base

To avoid the problematic assumptions of viscous tillrheology and continuous till deformation, we propose a‘ploughing’ model for a weak, sub-ice-stream till layer.In this model, the till is a few-meter-thick layer of sub-till material disturbed and transported by ploughingthat occurs during sliding of a bumpy ice base (Fig. 4).The model is similar to these presented before by Brownet al. (1987) and Beget (1986). However, those authorsemphasized the importance of clast ploughing and westress deformation of till around ‘bumps’ in the ice baseitself. This change in emphasis is driven by theobservation that basal ice in the West Antarctic icestreams is devoid of debris (Scherer et al., 1998) and theunderlying tills are remarkably fine-grained and clast-poor (Tulaczyk et al., 1998). Our treatment of tillformation by sliding and ploughing is similar toconceptual models for generation of fault gouge layers(Tchalenko, 1970; Eyles and Boyce, 1998). In thisanalogy the ice base represents a rigid upper fault platewith asperities. Ploughing by these asperities is assumedto be the most important process for generation of theweak, deformable till (i.e. fault gouge layer) from theunderlying strata (i.e. the lower fault plane in theanalogous fault gouge model).

Fig. 4. Schematic representation of the ploughing model.


Recent investigations of modern and fossil beds of icestreams provide abundant evidence for presence of basalice bumps. For instance, large bumps in the base of IceStream B have been inferred from radar data (Novicket al., 1994) and studies of sea-bottom topography in theRoss Sea indicated that the base of the Pleistocene WestAntarctic ice sheet left a very irregular imprint afterdeglaciation (Anderson et al., 1992; Shipp and Ander-son, 1997a, b; Shipp et al., 1999). Perhaps the bestinsight into the geometry of an ice stream base isprovided by the topography of bedforms left behind byPleistocene ice streams in North America and elsewhere(Clark, 1993, 1994; Canals et al., 2000; Clark andStokes, 2001). Analysis of geophysical data, satelliteimagery and air photos acquired over the areas offormer ice streams show elongated bedforms of varyingamplitude, wavelength, and width. Fig. 5 shows anexample of such bedforms generated by the M’ClintockChannel ice stream that is discussed in detail in Clark

and Stokes (2001). Rather than the ridges having formedby some relief amplification process based aroundviscous deformation of sediment (Boulton, 1987; Hind-marsh, 1997), in our model we view them as residualaccumulations of sediment whose form is a consequenceof the carving of intervening grooves. Whilst this doesnot explain the pattern of all bedforms such as drumlinsand rogen moraine, it may explain the observed form ofmegalineations (Clark, 1993) or bundle structures(Canals et al., 2000).Is it realistic to propose, as we do here, that an ice

base can be treated as the (more or less) rigid elementthat deforms and molds the underlying substratum? Inclassical hard-bed models of ice motion, it was thesubstratum that was the rigid element in a glacialsystem. Irregularities in the bed forced ice to deformand, thus, shaped the ice base geometry (e.g. Weertman,1957). The rigid-ice-base assumption seems realistic aslong as the substratum is made of very weak till. Icestrength at strain rates common in nature is of the orderof 100 kPa (Paterson, 1994) and the strength of the sub-ice-stream till is two orders of magnitude smaller(Kamb, 1991). For instance, elimination of an ice bumpwith some arbitrary amplitude A and wavelength B10Athrough ice shearing would require accumulation ofB0.1 shear strain. Since ice strain rates at the low stresslevel of 1 kPa are of the order of 10�7 yr�1 (Glen flowlaw with flow-law constant of 5.3� 10�15 (s kPa)�1,Paterson, 1994, Table 3.3), the time required toeliminate the ice bump (B106 yr) would be much longerthan the residence time of ice in an ice stream (B103 yr;ice residence time can be estimated by dividing thetypical length of an ice stream, 100s of km, by the typicalice stream velocity, 100s of m yr�1).If it exists, differential basal melting represents a more

potent way of eliminating ice bumps. Melting mayconcentrate on ploughing ice bumps because stressesacting on them during ploughing should be several timesgreater than stresses associated with a flat ice bed slidingover till or with shear on intra-till planes (Brown et al.,1987; Tulaczyk, 1999). The additional melting due tothis increased shear heating may be of the order of1mmyr�1 (taking ice stream velocity B100myr�1, tillstrength B1 kPa, and large stress concentration factorof B10). Under these conditions, ice bumps with initialamplitude smaller than ca. 1m can be destroyed bymelting during the estimated ice residence time in an icestream. However, the fast motion of the ice stream basemay help to minimize the influence of the localizedincreased shear heating, thus diminishing the impor-tance of the differential shear heating. Thorsteinssonand Raymond (2001) consider this problem for a wavyice base moving over till of viscous rheology. Theyconclude that differential melting should affect mainlythose roughness elements whose wavelength is relativelysmall (B1m or less). Thus, it is reasonable to expect

Fig. 5. Landsat TM image (band 3) positioned across the boundary of

the M’Clintock paleo ice stream, Arctic Canada, that is running here

approximately in the south–north direction. Image is centered on

721:140:2100N, 1061:110:2200W. Western third of the image is character-

ized by approximately equidimensional hummocks, presumably

formed beneath slow-moving ice, whereas the central and eastern

parts of the image are dominated by strongly elongated bedforms

thought to be indicative of paleo-ice streams (Clark and Stokes, 2001).

Amplitude and transverse spacing of the elongated bedforms have

been measured using 1 : 50,000 topographic maps with contour interval

of 10m and air photos from the same area as the one shown on the

satellite image.


that an ice stream base will be relatively smooth at smallscale and that it is more likely to have bumps whosewavelength is of the order of meters or more. Never-theless, the fact that bedforms left behind by paleo-icestreams are characterized by presence of very long,continuous megalineations (Clark, 1993, 1994; Shippet al., 1999; also bundle structure of Canals et al., 2000)indicates that at least the large-scale ice base irregula-rities survive over distances of 10–100s of kilometers.The main stage of ice bump generation takes place

before or when ice enters an ice stream, at the time whenit is still in contact with a rigid bed. If an ice stream iscovered with weak, continuous till, the last contact of icewith hard bedrock should largely determine the geome-try of the ice base. This speculation is consistent with thegeometry of the bundle structure from AntarcticPeninsula (Canals et al., 2000, Fig. 3). The exactgeometry of the basal ice bumps, thus also bedformgeometry, will then result from combination of theinitial ice base roughness (governed by the geometry ofthe rigid bed) and later melting and large-scale stretch-ing that ice experiences during and after entering an icestream. Unfortunately, basal geometry for the WestAntarctic ice streams has not been examined withsufficient resolution to provide the observational con-straints that would be necessary for a quantitativetreatment of our ploughing model of till generation.This fact forces us to stress qualitative aspects of ourploughing model and to invoke data from areas affectedby paleo-ice streams (e.g. Fig. 5) to justify our assump-tion that an ice stream base is not flat.If an ice stream base is uneven and bumpy, this fact

will cause distribution of deformation in sub-ice-streamtills. In the past, it has been explicitly or implicitlyassumed that only till of linear or mildly non-linearrheology could experience distributed deformation (e.g.Alley, 1993). However, this presumption is correct onlyif one is considering a perfectly flat ice base moving overa smooth fluid with no effective-stress dependence ofstrength (Iverson et al., 1998; Tulaczyk, 1999; Tulaczyket al., 2000a). In the presence of basal roughnesselements (clasts or ice bumps), deformation is distrib-uted because till must move from the zone of compres-sion in front of each roughness element to the zone ofextension behind it. This process of till flow aroundclasts and bumps generates a viscous-like distribution ofdeformation in a plastic till (Tulaczyk, 1999).In support of our ploughing model of sub-ice-stream

tills, we show qualitatively that interaction of basal icebumps with the UpB till provides a plausible explana-tion for the significant fluctuations of sliding velocityobserved in the tethered stake record obtained byEngelhardt and Kamb (1998). During an observationperiod lasting ca. 26 days, the sliding velocity measuredbeneath ISB experienced several significant fluctuations(Fig. 6A). Engelhardt and Kamb (1998) infer that at

least at the initial stages of the experiment, the tetheredstake was located within centimeters of the ice base.They also propose that the biggest and longest lastingdip in the measured sliding velocity occurred because thetethered stake was dragged by a clast or an ice bumpprotruding down from the ice base. Here we hypothesizethat the other, smaller and shorter lasting velocity slow-downs may have occurred when the tethered stakefound itself in a deformation zone surrounding plough-ing bumps (Fig. 6B). In this zone, till is dragged in thedirection of ice motion and a tethered stake imbedded inthe till will record this as a slow-down. The recordedfluctuations in sliding velocity last typically one or twodays and have a similar repeat interval. Given an icebase velocity of ca. 1.2mday�1, the roughness elementsgenerating the sliding fluctuations should have awavelength of a few meters. According to our con-ceptual model (Fig. 6B), the magnitude of the apparentslow-downs is controlled by the relative amplitude ofbasal protuberances with respect to the depth at whichthe tethered stake is employed. If we accept the inferencethat this depth was o0.3m (Engelhardt and Kamb,1998), the amplitude of most of the basal bumps shouldhave been smaller than this because they have causedonly moderate apparent slow-downs of B20% of totalvelocity. Each slow-down in sliding velocity is matchedby a corresponding increase in distributed till deforma-tion (Fig. 6C).Just as in the case of the viscous-till model (Alley et al.,

1986), ploughing by ice bumps is associated withdistributed till deformation that produces net till fluxin the direction of ice stream motion. However, thetethered stake experiment and the evidence indicatingactive till layer thinner than 6m (Fig. 3; Rooney, 1988)suggest that till flux in the ploughing model is muchsmaller than that under the assumption of viscous till.For instance (Fig. 2), extrapolating linearly from 100%of deformation at the ice base and through average of31% at the depth of emplacement of the tethered stake(o0.3m, Engelhardt and Kamb, 1998), one obtains anestimate of H o0.4m as the thickness of the till layerthat deforms during ploughing in the UpB area. Thisassumed geometry gives o88m3 yr�1 per meter width ofan ice stream as the total till flux, Qt [obtained fromQt ¼ 0:5HUs; where Us is the ice stream velocity of ca.440myr�1 at UpB (Whillans and van der Veen, 1993)].Whereas this till flux is approximately an order ofmagnitude lower than the flux predicted by the viscous-till model (Alley et al., 1986, 1989), it is still a significantflux that requires relatively high till generation rates fora steady state. If there is a steady state and if till isgenerated by erosion of substrata beneath the ice streamitself, as we will argue below, the till flux estimatedabove would translate into an average till generationrate of o0.73mmyr�1 for the ca. 120 km long flowlineupstream of the UpB camp.


Our presumption that till generation takes placemainly beneath the ice stream itself is driven largely bythe observation that in the UpB area there was noevidence for significant debris in the basal ice (Schereret al., 1998; Tulaczyk et al., 1998). If this observation isrepresentative for the ice stream in general, influx ofdebris with ice entering the ice stream can be assumednegligible. As illustrated in Fig. 4, in our model tillgeneration takes place also through ploughing, but onlywhen and where a ploughing ice bump has an amplitudethat exceeds the thickness of the till itself and protrudesinto the underlying substrata. In this framework, tillgeneration is performed just by the largest bumps(several meters?) whereas the smaller bumps contributeonly to till transport. Since we hypothesize that tillgeneration is localized rather than aerially extensive (as

it would be for abrasion of substratum by sliding till,e.g. Cuffey and Alley, 1996), it should be expected thatwhen a localized, large ploughing bump does erode thesubstratum, it is doing it at a relatively fast rate.At this point, the specific geologic setting of the West

Antarctic ice streams over young sedimentary basinsmay become important. Weak glacial and glaciomarinediamictons may represent the material that is beneaththe currently ‘active’ till layer. Samples of this sub-tillmaterial have not been recovered, but Rooney et al.(1991) and Tulaczyk et al. (1998) suggested that thesediamictons are similar to those studied previously in theRoss Sea (e.g. Hayes and Frakes, 1975; Anderson et al.,1984). Microfossil ages suggest that the till is derivedmostly from upper Miocene sediments (Scherer, 1991;Scherer et al., 1998). Samples of Miocene sediments

Fig. 6. (A) Record of basal sliding obtained beneath Ice Stream B in the UpB area, West Antarctica, using a tethered stake; modified from

Engelhardt and Kamb (1998, Fig. 4). The non-dimensional sliding velocity is obtained by dividing the measured sliding rate by the ice surface

velocity Uice ¼ 440myr�1 observed in the UpB area by Whillans and van der Veen (1993). Vertical arrows point out major departures of the

measured sliding velocity from the surface velocity. (B) and (C) illustrate how such departures may result from clasts or ice protrusions ploughing the

underlying till. Panel (B) shows an example of the pattern of deformation that may be caused by till ploughing (modified from Tulaczyk, 1999,

Fig. 9). Taking a reference frame moving with the ice (at speed Uice), we can equivalently assume that the vertical grid markers in (B) represent

progressive changes in the position and shape of one originally vertical marker which experiences incremental deformation due to passage of a

ploughing clast or ice protuberance. Open symbols indicate consecutive positions of four simulated tethered stakes which were initially emplaced at

different depths on the perfectly vertical grid marker (initial position x ¼ 0). If these four simulated tethered stakes were to experience a passage of a

ploughing protrusion as shown in (B), they would yield the sliding records shown in (C). We determined these synthetic sliding records by measuring

off the horizontal distance of each symbol from x ¼ 0 in (B) and dividing it by the distance at which this symbol would be located if the initially

vertical grid marker had not deformed.


taken from the top several hundred meters of the RossSea sequence were unlithified and had high porosity of34.6% to 43.5% (Barrett and Froggatt, 1978, Table 3)comparable to that of the sub-ice-stream tills them-selves. Thus, the substratum-ploughing ice bumps thatare responsible for generation of till in our model mayindeed be efficient at generating new till because they donot have to erode competent bedrock but simplydislodge unlithified sediments.A snapshot of the sub-ice-stream zone beneath the

UpB camp obtained with seismic surveys indicated thatover most of the area (B98%) the weak till layerseparates the ice stream base from the underlyingsedimentary sequence (Rooney et al., 1987). If at anysingle moment the contact area between the largest icebumps and the sequence is indeed of the order of 1%,the local rate of till generation must be of the order ofo73mmyr�1 for the areally averaged till generationrate to reach the previously estimated steady-state valueof o0.73mmyr�1. Although the local erosion rate ofo73mmyr�1 is high, it is not unreasonably high giventhe fast ice stream velocity and (presumed) weak natureof the eroded substrata. If expressed in non-dimensionalterms, i.e. the dimensional erosion rate of o73mmyr�1

divided by the ice velocity of 440myr�1, the localerosion rate is o1.7� 10�4. This is comparable to thenon-dimensional erosion rate reported by Humphreyand Raymond (1994) for bedrock erosion by theVariegated glacier during surge, B10�4.It is important to note that, at least qualitatively,

there is a potential for a stabilizing feedback between thetill thickness and the till generation rate. For instance, iffor some arbitrary reason the till layer becomes thicker,then fewer ice bumps will be protruding through thelayer and eroding the substratum, which shouldcounteract the initial tendency of the till to thicken.Similarly, if the till becomes thinner due to somearbitrary disturbance, more ice bumps will span itswhole thickness and the areally averaged till generationrate should go up, pushing the system back toward asteady-state thickness of the till that is consistent withthe (as of yet unknown) roughness of the ice streambase.

5. Consequences for ice stream stability

Qualitatively, incorporation of plastic till rheologyinto models of ice stream motion introduces a greaterpotential for an unstable ice stream behavior ascompared to the past models that considered sub-ice-stream tills as nearly linearly viscous fluid (Alley,1989a, b vs. Kamb, 1991). In the plastic case, there issimply no direct feedback between ice stream velocityand the resistance provided by the subglacial till againstice stream motion because till strength does not depend

on shear rates. On the other hand, assumption ofviscous till implied greater ice stream stability becausevariations in ice stream velocity would face strong‘quenching’ due to the strain-rate-dependence of viscoustill strength.Recently, Hindmarsh (1998) argued that plastic till

rheology may not be appropriate for models of icemotion over deformable till beds because ice streamvelocity is finite and confined to a relatively narrowrange of values (o1000smyr�1). According to thisargument, such behavior is more consistent with viscoustill rheology, whose ‘quenching’ of velocity variationsprovides a simple explanation for this relatively lowvariability of ice velocities. However, the example of iceshelves clearly demonstrates that finite ice velocities witha relatively low range of variations can be achievedwithout any significant viscous basal control (Thomas,1973a, b). It is the requirement of ice continuity coupledwith the mildly non-linear rheology of ice itself thatforce ice velocities to fall within a finite and reasonablynarrow range. In ice streams, as in ice shelves, weak orno basal resistance against ice motion is compensated byshifting the support for the gravitational driving stressto the side margins and by expanding part of the drivingstress on ice stretching (Echelmeyer et al., 1994; Jacksonand Kamb, 1997; MacAyeal et al., 1995; Whillans andvan der Veen, 1997). Raymond (1996, Eq. (39)) andTulaczyk et al. (2000b) have shown that ice streammodels assuming plastic till beds can generally repro-duce the observed ice stream velocities.The plastic rheology of sub-ice-stream tills will have

especially far-reaching implications for modeling oftransient behavior of ice streams (e.g. initiation orcessation of ice streaming). Because the plastic tillstrength is highly sensitive to till water content (e.g.Tulaczyk et al., 2000a), transient behavior of ice streamsis likely to be caused by an increase or decrease in waterstorage in till (Tulaczyk et al., 2000b). Dynamics ofsubglacial water balance is thus of primary importancein considerations of changes in ice stream behavior.Advection of water in till transported during ploughingis likely to be a significant component of this waterbalance. Using our previous estimate of till flux at theUpB camp, o88m�3 yr�1 per meter width, we canestimate the magnitude of water advection ato36m�3 yr�1 (assuming 40% till porosity). In a steadystate, basal melting would have to be o0.3mmyr�1

along the ca. 120 km long flowline upstream of the UpBarea to compensate for the water loss due to advection.Given the fact that basal melting (or freezing) rate isexpected to be of the order of 1mmyr�1 or less in WestAntarctica (Tulaczyk et al., 2000b), water advection intill is likely to represent one of the primary componentsof sub-ice-stream water balance. This component willhelp determine the appropriate timescales for significantchanges in ice stream velocity.


6. Conclusions

Recent developments in research on ice-streammechanics and on till rheology call for a new modelfor generation of weak, sub-ice-stream tills whichpreviously have been thought to represent an activelydeforming and eroding layer of viscous till. Theploughing model proposed here represents a variationon a previous model (e.g. Beget, 1986; Brown et al.,1987) that invoked ploughing of till by clasts. In theframework of our ploughing model, weak sub-ice-stream tills represent simply a wet and slipperyboundary layer caught between bumps in the ice streambase. We conclude that such bumps exist based onseveral lines of evidence provided by geophysical remotesensing of modern and fossil ice stream beds. Till istransported subglacially as it deforms around the bumpsin a spatially and temporally variable fashion. Thecarving of numerous parallel grooves may explain theform of some commonly observed subglacial bedforms,particularly megalineations and bundle structures. Tilltransport rate should scale with the (as of yet unknown)amplitude and wavelength of these bumps. The largest ofthe basal ice bumps (referred to as ‘ice keels’) protrudethrough the whole till thickness and regenerate the weaktill by scrapping material off the top of the underlyingsequence of unlithified older glacial or glaciomarinesediments and by adding basal meltwater in the process.In spite of many uncertainties regarding the geometry ofthe basal ice bumps and the kinematics of till deforma-tion around them, we estimate the till flux at the UpBcamp to beo88m3 yr�1 per meter width of the ice stream(extrapolated from a single tethered stake experimentperformed by Engelhardt and Kamb, 1998). In a steadystate this flux would require that the ice keels erode thesub-till material at a reasonable non-dimensional rate ofo1.7� 10�4 (assuming B1% of contact area). A greatadvantage of the ploughing model for generation of sub-ice-stream tills is that it allows for significant till transportwhile dropping the problematic expectation that till hasviscous rheology. If sub-ice-stream tills behave plasticallyin situ, as they do in laboratory tests, this suggests that icestreams may behave in an unstable manner (e.g. turn onand off without significant changes in driving stress) overrelatively short time periods.

Acknowledgements

Partial financial support for this research wasprovided by the National Science Foundation throughgrant EAR-9819346 to S. T., and the Swedish NaturalSciences Research Council (NFR) to R.S. The principalauthor acknowledges also financial support from theOffice of Vice-Chancellor for Research and GraduateStudies at the University of Kentucky that subsidized

presentation of this research at the XV Congress ofINQUA in Durban, South Africa, August 1999.

References

Alley, R.B., 1989a. Water-pressure coupling of sliding and bed

deformation 1. Water system. Journal of Glaciology 35, 108–118.

Alley, R.B., 1989b. Water-pressure coupling of sliding and bed

deformation 2. Velocity–depth profiles. Journal of Glaciology 35,

119–129.

Alley, R.B., 1991. Deforming-bed origin for southern Laurentide till

sheets? Journal of Glaciology 37, 67–76.

Alley, R.B., 1993. How can low-pressure channels and deforming tills

coexist subglacially? Journal of Glaciology 38, 200–207.

Alley, R.B., Blankenship, D.D., Bentley, C.R., Rooney, S.T., 1986.

Deformation of till beneath Ice Stream B, West Antarctica. Nature

322, 57–59.

Alley, R.B., Blankenship, D.D., Bentley, C.R., Rooney, S.T., 1987. Till

beneath Ice Stream B, 4. A coupled ice-till flow model. Journal of

Geophysical Research 92, 8921–8929.

Alley, R.B., Blankenship, D.D., Rooney, S.T., Bentley, C.R., 1989.

Water-pressure coupling of sliding and bed deformation: III.

Application to Ice Stream B, Antarctica. Journal of Glaciology 35,

130–139.

Alley, R.B., Cuffey, K.M., Evenson, E.B., Strasser, J.C., Lawson,

D.E., Larson, G.J., 1998. How glaciers entrain and transport basal

sediments: physical constraints. Quaternary Science Reviews 16,

1017–1038.

Anandakrishnan, S., Alley, R.B., 1997. Tidal forcing of basal

seismicity of Ice Stream C, West Antarctica, observed far inland.

Journal of Geophysical Research 102, 15183–15196.

Anandakrishnan, S., Bentley, C.R., 1993. Micro-earthquakes beneath

Ice Stream B and Ice Stream C, West Antarctica. Journal of

Glaciology 39, 455–462.

Anandakrishnan, S., Blankenship, D.D., Alley, R.B., Stoffa, P.L.,

1998. Influence of subglacial geology on the position of a West

Antarctic ice stream from seismic observations. Nature 394, 62–65.

Anderson, J.B., Brake, C.F., Myers, N.C., 1984. Sedimentation on the

Ross Sea continental shelf, Antarctica. Marine Geology 57,

295–333.

Anderson, J.B., Shipp, S.S., Bartek, L.R., Reid, D.E., 1992. Evidence

for a grounded ice sheet on the Ross Sea continental shelf during

the late Pleistocene and preliminary paleodrainage reconstructions.

In: Elliot, D.H. (Ed.), Contribution to Antarctic Research III.

Antarctic Research Series 57, American Geophysical union,

Washington D.C., pp. 39–62.

Barrett, P.J., 1975. Textural characteristics of Cenozoic preglacial and

glacial sediments at site 270, Ross Sea, Antarctica. In: Hayes, D.E.,

Frakes, L.A. (Eds.), Initial Reports of the Deep Sea Drilling

Project, 28. US Government Printing Office, Washington, DC, pp.

757–767.

Barrett, P.J., Froggatt, P.C., 1978. Densities, porosities, and seismic

velocities of some rocks from Victoria Land, Antarctica. New

Zealand Journal of Geology and Geophysics 21, 175–187.

Beget, J., 1986. Influence of till rheology on Pleistocene glacier flow in

the southern Great Lakes area. USA Journal of Glaciology 32,

235–241.

Bell, R.E., Blankenship, D.D., Finn, C.A., Morse, D.L., Scambos,

T.A., 1998. Influence of subglacial geology on the onset of a West

Antarctic ice stream from aerogeophysical observations. Nature

394, 58–62.

Bentley, C.R., 1997. Rapid sea-level rise soon from West Antarctic Ice

Sheet collapse. Science 275, 1077–1078.

Bindschadler, R., 1997. West Antarctic Ice-Sheet collapse. Science 276,

662–663.


Bindschadler, R., 1998. Monitoring ice sheet behavior from space.

Reviews of Geophysics 36, 79–104.

Blake, E.W., 1992. The deforming bed beneath a surge-type glacier:

measurement of mechanical and electrical properties. Unpublished

Ph.D. Thesis, University of British Columbia, Vancouver, 179pp.

Blankenship, D.D., Bentley, C.R., Rooney, S.T., Alley, R.B., 1986.

Seismic measurements reveal a saturated porous layer beneath an

active Antarctic ice stream. Nature 322, 54–57.

Blankenship, D.D., Bentley, C.R., Rooney, S.T., Alley, R.B., 1987. Till

beneath Ice Stream B, 1. Properties derived from seismic travel

times. Journal of Geophysical Research 92, 8903–8911.

Boulton, G.S., 1986. GeophysicsFa paradigm shift in glaciology.

Nature 322, 18.

Boulton, G.S., 1987. A theory of drumlin formation by subglacial

sediment deformation: In: Menzies, J., Rose, J. (Eds.), Drumlin

Symposium. A.A. Balkema, Rotterdam, pp. 25–80.

Boulton, G.S., Hindmarsh, R.C.A., 1987. Sediment deformation

beneath glaciersFrheology and geological consequences. Journal

of Geophysical Research 92, 9059–9082.

Bowles, J.E., 1992. Engineering Properties of Soils and their

Measurement. McGraw-Hill, New York, 291pp.

Brown, N.E., Hallet, B., Booth, D.B., 1987. Rapid soft bed sliding

of the Puget glacial lobe. Journal of Geophysical Research 92,

8985–8997.

Canals, M., Urgeles, R., Calafat, A.M., 2000. Deep sea-floor

evidence of past ice streams off the Antarctic Peninsula. Geology

28, 31–34.

Clark, C.D., 1993. Mega-scale glacial lineations and cross-cutting ice-

flow landforms. Earth Surface Processes and Landforms 18, 1–29.

Clark, C.D., 1994. Large scale ice-molded landforms and their

glaciological significance. Sedimentary Geology 91, 253–268.

Clark, C.D., Stokes, C.R., 2001. Extent and basal characteristics of the

M’Clintock Channel Ice Stream. Quaternary International, this

issue.

Clark, P.U., Licciardi, J.M., MacAyeal, D.R., Jenson, J.W., 1996.

Numerical reconstruction of a soft-bedded Laurentide ice-sheet

during the last glacial maximum. Geology 24, 679–682.

Cuffey, K., Alley, R.B., 1996. Is erosion by deforming subglacial

sediments significant? (toward till continuity). Annals of Glaciol-

ogy 22, 17–24.

Echelmeyer, K.A., Harrison, W.D., Larsen, C., Mitchell, J.E., 1994.

The role of the margins in the dynamics of an active ice stream.

Journal of Glaciology 40, 527–538.

Engelhardt, H.F., Kamb, B., 1997. Basal hydraulic system of a West

Antarctic ice stream: constraints from borehole observations.


Engelhardt, H.F., Kamb, B., 1998. Sliding velocity of Ice Stream B.


Engelhardt, H.F., Humphrey, N., Kamb, B., Fahnestock, M., 1990.

Physical conditions at the base of a fast moving Antarctic ice

stream. Science 248, 57–59.

Eyles, N., Boyce, J.J., 1998. Kinematics indicators in fault gouge:

tectonic analog for soft-bedded ice sheets. Sedimentary Geology

116, 1–12.

Hayes, D.E., Frakes, L.A., 1975. General synthesis. In: Hayes, D.E.,

Frakes, L.A. (Eds.), Initial Reports of the Deep Sea Drilling

Project, 28. US Government Printing Office, Washington, DC,

pp. 919–942.

Hindmarsh, R.C.A., 1997. Deforming beds: viscous and plastic scales

of deformation. Quaternary Science Reviews 16, 1039–1056.

Hindmarsh, R.C.A., 1998. Drumlinisation and drumlin-forming

instabilities: viscous till mechanics. Journal of Glaciology 44,

293–314.

Hooke, R.L., Elverh�i, A., 1996. Sediment flux from a fjord during

glacial periods, Isfjorden, Spitsbergen. Global and Planetary

Change 12, 237–249.

Hooke, R.L., Hanson, B., Iverson, N.R., Jansson, P., Fischer, U.H.,

1997. Rheology of till beneath Storglaci.aren, Sweden. Journal of


Hughes, T., 1992. Abrupt climatic change related to unstable ice-sheet

dynamics: towards a new paradigm. Palaeogeography Palaeocli-

matology Palaeoecology 97, 203–234.

Humphrey, N., Raymond, C.F., 1994. Hydrology, erosion and

sediment production in a surging glacier, Variegated Glacier,

Alaska, 1982–1983. Journal of Glaciology 40, 539–552.

Iverson, N.R., Hanson, B., Hooke, R.L., Jansson, P., 1995. Flow

mechanics of glaciers on soft beds. Science 267, 80–81.

Iverson, N.R., Baker, R.W., Hooyer, T.S., 1997. A ring shear device

for the study of till deformation: tests on tills with contrasting clay

content. Quaternary Science Reviews 16, 1057–1066.

Iverson, N.R., Hooyer, T.S., Baker, R.W., 1998. Ring-shear studies of

till deformation and an hypothesis for reconciling Coulomb-plastic

behavior with distributed strain in glacier beds. Journal of


Jackson, M., Kamb, B., 1997. The marginal shear stress of Ice Stream

B. Journal of Glaciology 43, 415–426.

Jacobel, R.W., Scambos, T.A., Raymond, C.F., Gades, A.M., 1996.

Changes in the configuration of ice stream flow in the West

Antarctic Ice Sheet. Journal of Geophysical Research 101,

5499–5504.

Jenson, J.W., MacAyeal, D.R., Clark, P.U., Ho, C.L., Vela, J.C., 1996.

Numerical modeling of subglacial sediment deformationFimplica-

tions for the behavior of the Lake-Michigan lobe, Laurentide Ice-

Sheet. Journal of Geophysical Research 101, 8717–8728.

Kamb, B., 1970. Sliding motion of glaciers: theory and observations.

Reviews of Geophysics and Space Physics 8, 673–728.

Kamb, B., 1991. Rheological nonlinearity and flow instability in the

deforming-bed mechanism of ice stream motion. Journal of


Lliboutry, L., 1979. Local friction laws for glaciers: a critical review

and new openings. Journal of Glaciology 23, 67–95.

MacAyeal, D.R., 1989. Large-scale ice flow over a viscous basal

sedimentFtheory and application to Ice Stream B, Antarctica.

Journal of Geophysical Research 94, 4071–4087.

MacAyeal, D.R., 1992. Irregular oscillations of the West Antarctic ice

sheet. Nature 359, 29–32.

MacAyeal, D.R., Bindschadler, R., Scambos, T.A., 1995. Basal

friction of Ice Stream E, West Antarctic. Journal of Glaciology

41, 247–262.

Marshall, S.J., Clarke, G.K.C., Dyke, A.S., Fisher, D.A., 1996.

Geologic and topographic controls on fast flow in the Laurentide

and Cordilleran ice sheets. Journal of Geophysical Research 101,

17827–17839.

Novick, A.N., Bentley, C.R., Lord, N., 1994. Ice thickness, bed

topography and basal-reflection stregths from radar sounding,

Upstream B, West Antarctica. Annals of Glaciology 20, 128–152.

Paterson, W.S.B., 1994. The Physics of Glaciers. Pergamon, Oxford,

UK, 480pp.

Piotrowski, J.A., Tulaczyk, S., 1999. Subglacial conditions under

the last ice sheet in northwest Germany: ice-bed separation

and enhanced basal sliding? Quaternary Science Reviews 18,

737–751.

Raymond, C., 1996. Shear margins in glaciers and ice sheets. Journal

of Glaciology 42, 90–102.

Rooney, S.T., 1988. Subglacial geology of Ice Stream B, West

Antarctica. Unpublished Ph.D. Thesis, University of Wisconsin-

Madison, Madison, 188pp.

Rooney, S.T., Blankenship, D.D., Alley, R.B., Bentley, C.R., 1987. Till

beneath Ice Stream B, 2. Structure and continuity. Journal of


Rooney, S.T., Blankenship, D.D., Alley, R.B., Bentley, C.R.,

1991. Seismic reflection profiling of a sediment-filled graben


beneath Ice Stream B, West Antarctica. In: Thomson, M.R.,

Crame, J.A., Thomson, J.W. (Eds.), Geological Evolution

of Antarctica. British Antarctic Survey, Cambridge, UK,

pp. 261–265.

Scherer, R.P., 1991. Quaternary and tertiary microfossils from beneath

Ice Stream B: evidence for a dynamic West Antarctic Ice Sheet

history. Palaeogeography, Palaeoclimatology, Palaeoceanography

90, 395–412.

Scherer, R.P., 1994. A new method for the determination of absolute

abundance of diatoms and other silt-sized sedimentary particles.

Journal of Paleolimnology 12, 171–180.

Scherer, R.P., Tulaczyk, S., 1997. Diatoms in subglacial sediments

yield clues regarding West Antarctic ice-sheet history and ice-

stream processes. Antarctic Journal of the United States 32,

32–34.

Scherer, R.P., Aldahan, A., Tulaczyk, S., Kamb, B., Engelhardt, H.,

Possnert, G., 1998. Pleistocene collapse of the West Antarctic ice

sheet. Science 281, 82–85.

Shipp, S., Anderson, J.B., 1997a. Drumlin field of the Ross Sea

continental shelf, Antarctica. In: Davis, T.A., et al. (Ed.), Glaciated

Continental Margins: an Atlas of Acoustic Images. Chapman &

Hall, London, pp. 54–55.

Shipp, S., Anderson, J.B., 1997b. Lineations on the Ross Sea

continental shelf, Antarctica. In: Davis, T.A., et al. (Ed.), Glaciated

continental margins: an atlas of acoustic images. Chapman & Hall,

London, pp. 378–381.

Shipp, S., Anderson, J.B., Domack, E., 1999. Late Pleistocene-

Holocene retreat of the West Antarctic ice-sheet system in the

Ross Sea: Part IFgeophysical results. Geological Society of

America Bulletin 111, 1486–1516.

Tchalenko, J.S., 1970. Similarities between shear zones of different

magnitudes. Geological Society of America Bulletin 81,

1625–1640.

Thomas, R.H., 1973a. The creep of ice shelves; theory. Journal of


Thomas, R.H., 1973b. The creep of ice shelves; interpretation of

observed behavior. Journal of Glaciology 12, 55–70.

Thorsteinsson, T., Raymond C. F., 2001. Sliding versus till deforma-

tion in the motion of an ice stream over deformable till. Journal of


Tulaczyk, S., 1998. Basal mechanics and geologic record of ice

streaming, West Antarctica. Unpublished Ph.D. thesis, California

Institute of Technology, Pasadena, California, 359pp.

Tulaczyk, S., 1999. Ice sliding over weak, fine-grained tills: dependence

of ice-till interactions on till granulometry. In: Mickelson, D.M.,

Attig J. (Eds.), Glacial Processes: Past and Modern, Geological

Society of America Special Paper 337, Geological Society of

America, Boulder, Colorado, pp. 159–177.

Tulaczyk, S., Kamb, B., Scherer, R., Engelhardt, H.F., 1998.

Sedimentary processes at the base of a West Antarctic ice stream:

constraints from textural and compositional properties of sub-

glacial debris. Journal of Sedimentary Research 68, 487–496.

Tulaczyk, S., Kamb, B., Engelhardt, H., 2000a. Basal mechanics of Ice

Stream B. I. Till mechanics. Journal of Geophysical Research 105,

463–481.

Tulaczyk, S., Kamb, B., Engelhardt, H., 2000b. Basal mechanics of Ice

Stream B. II Plastic-undrained-bed model. Journal of Geophysical

Research 105, 483–494.

Weertman, J., 1957. On sliding of glaciers. Journal of Glaciology 3,

33–38.

Whillans, I.M., van der Veen, C.J., 1993. New and improved

determinations of velocity of Ice Stream B and C, West Antarctica.


Whillans, I.M., van der Veen, C.J., 1997. The role of lateral drag in the

dynamics of Ice Stream B, Antarctica. Journal of Glaciology 43,

231–237.


a ploughing model for the origin of weak tills beneath ice streams: a qualitative treatment

Documents