Fast glacier flow Ice streams, surging, and ... - Wiley Online Library

JOURNAL

OF GEOPHYSICAL

RESEARCH,

VOL.

92, NO. B9, PAGES 8835-8841, AUGUST

10, 1987

Fast Glacier Flow' Ice Streams, Surging, and Tidewater Glaciers GARRY

K. C. CLARKE

Department of Geophysics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada

INTRODUCTION

for a potential instability known as drastic retreat, the rapid upstream migration of grounding position [Meier and Post,

Fast flowing glaciers are unusual but important. Jakobshavns

Glacier

in western

Greenland

illustrates

this

point (Figure 1). Consideredto be the world's fastest glacier, its maximum observed flow rate (measured in midsummer

near the floating terminus)is 8360 m yr-1 [Lingle et al., 1981]. Except for a tidal influence near the terminus, the flow is believed to be fairly steady [Bindschadler, 1984]. Annu-

ally, this glacierdischarges36.6 km3 of ice to Davis Strait [Carbonnell and Batter, 1968], an ice volume corresponding to roughly 6% of the net annual accumulation of the Greenland ice sheet [see Lingle et al., 1981; Bindschadler, 1984] and a substantial fraction of the iceberg volume calved to the North Atlantic Ocean. Jakobshavns Glacier is an example of an outlet glacier; it collects ice from a huge catchment on the Greenland ice sheet and channels this through a highvelocity outlet that, in its lower reaches, is laterally bounded by rock walls. Inland from its terminal zone, Jakobshavns Glacier can be regarded as an ice stream; fast flowing ice is laterally bounded by slower flowing ice, not by rock. Other fast ice streams are Rutlord Ice Stream, flowing to the Ronne Ice Shelf, and ice stream B, flowing to the Ross

this issue]. Post [1975] was the first to note that since 1794, when

Captain Vancouver mapped the coast, nine Alaskan tidewater glaciers have undergone drastic retreat. A tenth, Columbia Glacier, had maintained a stable terminus position over the same time span but appeared to be a candidate for instability. Disintegration of the lower part of Columbia Glacier would occur by iceberg calving and might interfere with marine traffic to Valdez, Alaska. Recognizing this possibility, the U.S. Geological Survey launched a wideranging field program and the imminence of drastic retreat was confirmed [Meier et al., 1980]. In subsequent years the timing of the retreat was correctly predicted [Rasmussen and Meier,

1982; Sikonia, 1982; Bindschadler and Rasmus-

sen, 1983], and in 1984 the anticipated retreat was underway [Meier et al, 1985a]. Upstream migration of the grounding position has greatly increased the flow rate in the lower reaches of the glacier [Meier et al., 1985b; Krimmel and Vattghn, this issue]. Drastic retreat will continue until the grounding line reaches a new stable position at the head of the t•jordsome 30 km upstream I¾omthe 1983grounding line. Ice Shelf, both in West Antarctica. The flow rate of Rutlord There is a possible analogy between the drastic retreat of Ice Streamin placesexceeds400 m yr-• [Doake et al., this tidewater glaciers and the disintegration of marine ice sheets issue],andthat of ice streamB reaches827m yr-1 [Whillans et al., this issue]. Not all ice streams flow fast. Ice stream C, adjacent to ice stream B and in a similar setting, flows at a

mere5 m yr-1 [Bindschadler et al., 1987;Whillanset al., this issue], but there is persuasive evidence that some 250 years ago it, like ice stream B, was fast flowing [Rose, 1979; Shabtaie and Bentley, 1987]. Ice streams, it appears, can switch between

slow and fast modes of flow. The transition

from slow to fast flow is spatial as well as temporal. The example of Slessor Glacier, an East Antarctic ice stream, shows a sharply delineated transition from slow sheet flow to rapid streaming flow (Figure 2). Unlike Jakobshavns Glacier, no obvious bedrock sidewalls control its path. Jakobshavns Glacier is also a tidewater glacier; it discharges ice to the ocean. The terminus of Jakobshavns Glacier is floating, but many tidewater glaciers are grounded

such as the West

Antarctic

on beds that lie below

ice sheet.

Marine

ice sheets rest

sea level and the conditions

for their

continued existence depend on ice thickness relative to sea level [e.g., Weertman, 1974; Hughes, 1975, Figure 27; Thomas, 1979]. It has been suggestedthat climatic warming caused by increased atmospheric CO2 could trigger the collapse of the West Antarctic ice sheet and raise sea level by 5 m [Mercer, 1978; Thomas et al., 1979]. The drastic retreat of Columbia Glacier presents the opportunity to observe the processes of grounding line migration and glacier disintegration as they might occur if the West Antarctic ice sheet became

unstable.

Ice streams and tidewater glaciers seem able to switch between

slow

and

fast

modes

of

flow.

In

the

case

of

Paper number 6B6330.

tidewater glaciers the switchover is externally triggered, either by ice thinning or by rising sea level, whereas for ice streams it is uncertain whether it is internally or externally triggered. Of special interest are glaciers that demonstrate intrinsically unstable flow or "surging" [Meier and Post, 1969]. Unlike normal valley glaciers that can maintain a fairly steady flow rate and adjust stably to external changes, surging glaciers experience extreme flow pulsations (Figure 3). Variegated Glacier in Alaska has become the type example of a surging glacier. Its most recent surge occurred in 1982-1983 and was closely monitored [Karnb et al., 1985; Httmphrey et al., 1986; Raymond et al., this issue]. The maximum velocity measured during the surge exceeded

0148-0227/87/006B-6330505.00

23,000m yr-• and was maintainedfor about2 hours[Kamb

on beds that lie below

sea level.

Whether

the terminus

is

floating or grounded, tidewater glaciers have complex interactions with the ocean. Tidal variations

can cause flexure of

floating tongues like that of Jakobshavns Glacier [Lingle et al., 1981] and oscillations in the flow rate of grounded glaciers like Columbia Glacier, Alaska [Walters and Dttnlap, this issue]. Secular changes in sea level and ice thickness affect the location of the grounding line and are the trigger Copyright 1987 by the American Geophysical Union.

8835

8836

CLARKE:

Fig. 1.

INTRODUCTION

Jakobshavns Glacier, western Greenland. This outlet glacier begins as an ice stream, forming within the

Greenlandice sheet,and terminatesas a tidewaterglacier.Owingto its very highvelocity, as greatas 8.4 km yr-•, it is sometimes regarded as a glacier that is permanently surging. The ice stream (A) can be traced from its head (B), roughly 100 km inland from the coast, to its calving terminus (C); beyond the terminus, floating icebergs (D) choke Jakobshavns Isfjord. Portion of Landsat image 22087-14293 provided courtesy of R. S. Williams, Jr., U.S. Geological Survey.

et al., 1985]. Surges of Variegated Glacier are known to have occurred in 1906, some years before 1933, about 1947, 1964-1965, and in 1982-1983, suggesting an approximate periodicity of 19 years. Between surgesthe glacier becomes nearly stagnant.Although surge-typeglaciers are uncommon (some 200 have been identified in North America), they have unique scientific importance. Their very existence suggests that different physical processescontrol slow and fast flow. Surging glaciers cross and recross the threshold between processes that dominate slow flow and those that dominate fast flow. By studying surgingglaciers it should be possible to identify these processes and learn the causes of flow instability. For this reason, glaciologists assign the highest priority to understanding the mechanics of surging. The paper by Raymond [this issue] is the most recent review of this important subject, and the only one that incorporates recently acquired insights from the study of the Variegated Glacier surge. How

FAST SHOULD

GLACIERS

FLOW.'?

Representative examples of glacier flow behavior are plotted in Figure 4 using a logarithmic velocity scale. Flow

ratesrangefrom 5 m yr-l for ice streamC to over20 km yr-l

for Variegated Glacier during its surges. Jakobshavns Glacier and White Glacier (a valley glacier in Arctic Canada, chosen to represent normal flow behavior) are believed to flow at a fairly steady rate, making gradual adjustments to external changes. Ice stream C, Columbia Glacier, and Variegated Glacier illustrate different kinds of unstable flow. Ice stream B is shown as having stable flow, but this has not been demonstrated.

At what rate should these glaciers be flowing? For a glacier to maintain a constant size, neither growing nor shrinking, mass accumulation and mass losses must exactly balance.

Ice

must

be transferred

from

the

accumulation

region to the ablation region at a prescribed rate. To make this simple idea quantitative, let x be a longitudinal distance coordinate that increases from x = 0 at the upper boundary of the accumulation region to a maximum value x = L at the lower terminus. Suppose that w(x) is the width of the glacier basin or catchment as a function of downstream distance; for balance, the ice flux must satisfy

qb(X)-'

W(xo)b(xo) dxo

CLARKE:

INTRODUCTION

8837

ki.l:orneter..s

Fig. 2. Slessor Glacier, an ice stream in East Antarctica. The boundary between stream flow (A) and sheet flow (B) is sharply delineated. The ice stream flows to the east side of the Filchner Ice Shelf (C), East Antarctica. Landsat image 1476-17164provided courtesy of R. S. Williams, Jr., U.S. Geological Survey.

where q0 is referred to as the balance flux and b is the ice accumulation rate averaged across the width of the catchment; b is positive in the accumulationregion and negative in the ablation region. The balance flux can differ significantly from the actual ice flux qm(X).Similarly the balance velocity, defined as vb(x) = qb(x)/S(x), where S(x) is the crosssectionalarea of the channel, can differ significantlyfrom the actual velocity Vm(X)= qm(X)/S(x)averaged across the cross section of the channel. The difference between vt, and Vm distinguishesglaciers that are nearly in balance from those that are out of balance and hence have instability potential. Figure 5 shows some representative examples. JakobshavnsGlacier is nearly in balance, and the difference between vo and Vm is within the likely error limits [Bindschadler, 1984]. Ice stream B, according to the calculations of Shabtaie and Bentley [1987] and Whillans et al. [this issue], is flowing at a rate that cannot be sustained. To stabilize itself, it must eventually reduce its flow rate by 50%, or if this is impossible, it must switch off for a while. Ice stream C is in the opposite situation. It could sustain a

flowof approximately 600m yr-] butis nearlystagnant.(My estimate is based on data from Whillans et al. [this issue], Shabtaie et al. [this issue], and Fastook [this issue].) Eventually, it must increase its flow rate, either gradually or by switching to fast flow. White Glacier is a normal glacier that is approximately in balance. A typical value for its annually

averagedflow rate is 30 m yr-l, but short-termvelocity fluctuations can occur in response to changes in subglacial water pressure [MMler and Iken, 1973; Iken, 1977]. Although White Glacier is a valley glacier, its velocity is comparable to the sheet flow velocity of the Greenland and Antarctic ice' sheets. It therefore can serve as reference against which ice stream velocities can be compared. Variegated Glacier, in contrast to White Glacier,

lurched from one extreme of

imbalance to the other. In 1973 during the quiescent phase the flow rate was far too low to establish balance; in 1983 during the surge the flow was far too fast to be sustained.

Columbia Glacier illustrates a different kind of response to imbalance. In 1976 before the onset of drastic retreat, the lower part of Columbia Glacier was out of balance, flowing

8838

CLARKE'

[_-: •........._.,. .....:•:.:.. ..........ß .... .L ................ •-

.

...

INTRODUCTION

t..:. ..... •. .... •..

kilometers -

.•.•....?..

•::79. :•'•:..•:

Fig. 3. Dusty Glacier, Yukon Territory, Canada. A surgingglacier that shows characteristic1oopingof its medial moraine. The loop pattern demonstrates pulsating flow in one tributary channel (A) and steady flow in the other (B). Portion of Canadian Government photograph A24515-10.

too fast for its rate of massaccumulation.Tidewater glaciers have no stable responseto long-term balance deficits of this magnitude. Reducing ice flow to the tidewater portion of the glacier only exacerbates the problem becauseas ice thins, it becomes less firmly grounded and the bottom sliding rate

increases. Drastic retreat of the grounding position is the inevitable consequence. Thus in 1986, during Columbia Glacier's drastic retreat, the imbalance has grown even more severe, and retreat will continue irreversibly until the grounding position has migrated to the head of the fjord.

_

1650

1700

1750

1800

1850

i1_

1900

1950

000

Year Fig. 4. Flow speed for representative glacier examples. The velocity graphs in this figure draw on real data but are partly impressionistic.The labels represent JakobshavnsGlacier (JG), a fast flowing outlet glacier that forms as an ice stream and terminates as a tidewater glacier; Columbia Glacier (CG), an Alaskan tidewater glacier that in 1984 began retreating drastically; ice stream B (lB), a fast moving Antarctic ice stream; ice stream C (IC), a slow moving Antarctic ice stream that switched from fast to slow flow some 250 years ago; White Glacier (WG), a normal valley glacier in Arctic Canada; Variegated Glacier (VG), a periodically surgingglacier. Data are extracted from Ml'iller and Iken [ 1973], Ml'iller [1977], Lingle et al. [1981], Kamb et al. [1985], Blatter [1987], Whillans et al. [this issue], and Krimmel and Vaughn [this issue].

CLARKE'

INTRODUCTION

assumption isolates the mechanisms of regelation, enhanced creep, and bed separation as important to sliding and also guides assumptions about how water is stored subglacially and how it migrates along the bed. Boulton and Jones [1979] start from an alternative view of the subglacial material. They imagine it to be unlithified, deformable, and somewhat permeable. In this system, substrate deformation becomes the important "sliding" mechanism, and the manner of water storage and escape from the bed differs from the hard bed picture. Both situations exist in nature, so neither effort is misdirected, but which situation properly applies to surging is unsettled. Kamb et al. [1985] and Kamb [this issue] propose a hard bed mechanism to explain the surges of Variegated Glacier. Clarke et al. [1984, 1986] find evidence to support a soft bed mechanism. An obstacle to the adequate development of a soft bed surge mechanism has been the lack of suitable constitutive descriptions for deformable subglacial sediments. The contributions of Boulton and

VG83 lB

IC

JG

VG73

CG86

c)

(:3

Fig. 5. A comparison of balance velocities with observed velocities for representative glacier examples. The labels along the abscissa denote ice stream B (IB), ice stream C (IC), White Glacier (WG), Variegated Glacier in 1973 before its surge (VG73), Variegated Glacier in 1983 during its surge (VG83), Jakobshavns Glacier (JG), Columbia Glacier in 1978 before the onset of drastic retreat (CG78), and Columbia Glacier in 1986 during drastic retreat (CG86). Hatched bars indicate the balance velocity at some representative point on the glacier; solid bars indicate the measured or estimated velocity (averaged across the channel cross section) at the same point. The vertical line separates graphs plotted at different vertical scales. Data are extracted from Mii!!er and ll•en [1973], Miiller [1977], Linj,,le el al. !1981], Ra,•n•u,•,•en and Meier 11982], Bindsr'hadler 11984], Bindsr'hadh'r el al. 11977], Kamb el a/. 11985],

B!atter [1987], Fa•stool• lthis issue], Krimmel and Vauj4hn Ithis issue], Shabtaie el al. lthis issue], and Whillan•s el al. [this issue]. CAUSES OF FAST FLOW

AND

8839

SOURCES OF INSTABILITY

Fast flow results from last sliding, not fast creep. The creep flow of glaciers is fairly well understood and can be adequately described by the flow law ,•,: = A o-,:", where F,.: is the shear strain rate, A is a coefficient that depends on temperature and ice characteristics, o-•: is the shear stress, and n is the flow law exponent. In comparison to creep, the sliding component of glacier flow is incompletely understood and challenging to quantify. The switching flow of ice streams, the pulsating flow of surging glaciers, and the occurrence of ice avalanches as described by ROthlisberger [this issue] demonstrate the existence of one or more flow

instabilities. The quest for mechanisms to explain these phenomena is in part a search for a suitably comprehensive sliding theory. Recent contributions by Fo,vler [ 1986], Kamb [this issue], and Lliboutry [this issue] reflect the state of the art.

The correct visualization of subglacial conditions is a necessary step toward the construction of successfulsliding theories. The foregoing sliding theories build on the premise that the subglacial bed is rigid and impermeable. This

Hindmarsh [this issue] and Clarke [this issue] are aimed at removing some of these difficulties. A complementary effort has been the construction of theories that seek to explain the large-scale dynamics of surging without precisely detailing the underlying physical mechanisms. Theories of this type can predict behavior that matches observations, but they are incomplete in their sketching and parameterization of physical processes. Budd [1975] was the first to propose a surge theory of this sort. The new theory of Fowler [this issue] is a further step in this direction

and builds

on recent

observations.

The contrast between the activity of ice stream B and the present stagnation of ice stream C remains unexplained. A similar pairing between active Rutford Ice Stream and its slow moving companion occupying the Carlson Inlet drainage basin was noted by Doa/,c et al. [this issue]. Rose [1979] was the first to comment on the enigma of ice stream C and suggested that it may surge or that ice stream B may pirate some of its catchmerit.

The

transition

from

sheet

flow

to

stream flow need not be controlled by bedrock topography [e.g., Bentley, this issue; Shabtaie eta!., this issue], and for this reason, ice stream boundaries could possibly be mobile rather than spatially fixed. The idea of drainage capture is therefore not farfetched and on-off switching between pairs of ice streams might result. Fastoo/, lthis issue] has used a numerical model to test the ideas that sudden capture of its catchment or sudden cessation of bottom sliding can explain the present geometry and slow flow of ice stream C. In his view the catchment explanation is unsupported, but the possibility that both mechanisms operate together cannot be ruled out. Other evidence also suggests that ice stream boundaries are fixed rather than mobile. Mcintyre [ 1985] has compared the surface and bed topographies at the transition between sheet and stream flow and finds in many cases that the position of the ice stream head coincides with a bedrock step. Inland migration of the sheet-stream boundary from this pinning point is deemed unlikely. The ice streams of Marie Byrd Land (including ice streams B and C) appear to be exceptions. For these the transition appears to be gradual, and there is no apparent bedrock control; thus the sheet-stream boundary could be mobile. The transition from sheet to stream flow is intriguing. It corresponds to a transition from slow (or zero) sliding to fast sliding and in this respect is a transition from the nonsurging to the surging state [Hughes, 1983]. Whillans et al. [this

8840

CLARKE:

INTRODUCTION

issue], Shabtaie et al. [this issue], and Bindschadler et al.

[this issue] present new evidence, suggestingthat ice stream B contains within it large blocks of theologically distinct ice that forms local topographic highs or "rafts" moving with the ice stream. If this interpretation proves correct, the sheet-to-stream transition is a ragged one involving the entrainment of entire blocks of inland ice by the active ice

entific resonance of these events will be felt for many years, and I hope that readers of this special JGR section will sense some of the excitement.

Acknou,ledg•nents. On behalf of the organizing committee and participants, I wish to thank our principal sponsor, the American Geophysical Union, and cosponsors, the Canadian Geophysical Union and the University of British Columbia, for their support.

stream.

Financial

Perhaps the most far-reaching discovery about ice streams is that ice stream B is underlain by a layer of material, typically 8 m thick, that appears to be highly porous, water-saturated, and weak enough to deform [Alley et al., 1986, this issue (a), (b)' Blankenship et al., 1986, this issue' Rooney et al., this issue]. At a stroke this would seem to provide an answer to one of the puzzles of ice stream flow: how fast sliding can be consistent with the very low driving stressestypical of ice streams. The traditional explanation is that ice streams are underlain by a film of water sufficiently thick to keep the bed well lubricated and the overlying ice sliding. The explanation that seemsto apply to ice stream B is that the subglacial material is deforming rapidly. Evidence for a water-saturated sediment layer beneath ice stream B comes from the surprising seismic properties of the

Foundation and the Natural Sciences and Engineering Research Council of Canada. The opportunity of working with Fred Spilhaus, Cynthia Bravo, Brenda Weaver, and Robin Albert of the AGU and with Charles Bentley, Will Harrison, Mark Meier, Charles Raymond, Hans R6thlisberger, Bob Thomas, and Hans Weertman of the Organizing Committee made the task of convening this conference a pleasant one. The Committee on Glaciology (NRC/NAS) and the Sub-Committee on Glaciers (NRC Canada) assisted with conference organization. I am grateful to JGR Editors Gerald Schubert and Bill Kaula, to Marsha Berkowitz and Bonnie Pierce for encouraging and amply supporting this special issue, and to the authors and referees for their cooperation. Richard S. Williams, Jr., of the U.S. Geological Survey at Reston, Virginia kindly provided the Landsat images that are used in Figures 1 and 2. Peter Adams, Charles Bentley, Bob Bindschadler, Mark Meier, A1 Rasmussen, Charles Raymond, and Ian Whillans provided informa-

subglacialmaterial.The P wave velocityUpis lessthan 1700 m s-• and the S wave velocityv, is lessthan 160m s-• By

comparison, in theice theyfind•,p• 3830m s-1 andv,•• 1940m s-•. The very low S wave velocityindicatesthat the subglacial layer has very low rigidity' a subglacial material that matches this description is water-saturated till. Not only do these results provide a new understandingof ice stream mechanics, they tantalize those who seek surge mechanisms and give fresh impetus to those concerned with the connection between glacier flow mechanics and subglacialdeposits [e.g., Brown et al., this issue]. CHAPMAN

CONFERENCE

Comprehending the dynamics of glaciers and ice sheets and identifying potential sources of flow instability have become persistent themes in glaciology. The proceedingsof a

"Seminar

on

the

Causes

and

Mechanics

of

Glacier

Surges," held at St. Hilaire, Quebec, in 1968, were published in volume 6 of the Canadian Journal of Earth S•'iences and still make compelling reading. A decade later, companion symposia on "Glacier Beds' The Ice-Rock Interface" and "Dynamics of Large Ice Masses" were held in Ottawa and the proceedings were published respectively as volumes 23 and 24 of the Journal of Glaciology. In 1985, a workshop was held at Interlaken to discuss "Hydraulic Effects at the Glacier Bed and Related Phenomena" [see Haeberli et al., 1986; Paterson, 1986]. Most recently a Chapman Conference on Fast Glacier Flow' Ice Streams, Surging and Tidewater Glaciers was held at Whistler Village, British Columbia, in May 1986. The following special section of the JGR contains many of the papers presented at that symposium. This highly successful conference benefited from an unusual conjunction of natural events and scientific discoveries. Chronologically, these were the 1982-1983 surge of Variegated Glacier, the most completely studied surge of any glacier' the onset in 1984 of drastic retreat of Columbia Glacier, the culmination of an ambitious research program launched by the U.S. Geological Survey' and the discovery in 1984 that ice stream B was underlain by water-saturated, presumably deforming sediment. The sci-

contributions

tion and valuable

were

received

from

the National

Science

criticisms.

REFERENCES

Alley, R. B., D. D. Blankenship, C. R. Bentley, and S. T. Rooney, Deformation of till beneath ice stream B, West Antarctica, Nature, 322, 57-59, 1986. Alley, R. B., D. D. Blankenship, C. R. Bentley, and S. T. Rooney, Till beneath ice stream B, 3, Till deformation: Evidence and implications, J. Geophys. Res., this issue (a). Alley, R. B., D. D. Blankenship, S. T. Rooney, and C. R. Bentley, Till beneath ice stream B, 4, A coupled ice till flow model, J. Geophys. Res., this issue (b) Bentley, C. R., Antarctic ice streams: A review, J. Geophys. Res., this issue.

Bindschadler, R. A., Jakobshavns Glacier drainage basin: A balance assessment, J. Geophys. Res., 89, 2066-2072, 1984. Bindschadler, R. A., and L. A. Ras•nussen, Finite-difference model predictions of the drastic retreat of Columbia Glacier, Alaska, U.S. Geol. Surv. Prof. Pap., 1258-D, 17 pp., 1983. Bindschadler, R. A., W. D. Harrison, C. F. Raymond, and R. Crosson, Geometry and dynamics of a surge-type glacier, J. Glaciol., 18, 181-194, 1977. Bindschadler, R. A., D. R. MacAyeal, and S. N. Stephenson, Ice stream-ice shelf interaction in West Antarctica, in Dynamics of the West Antarctic Ice Sheet, edited by C. J. van der Ween and J. Oerlemans, pp. 161-180, D. Reidel, Hingham, Mass., 1987. Bindschadler, R. A., S. N. Stephenson, D. R. MacAyeal, and S. Shabtaie, Ice dynamics at the mouth of ice stream B, Antarctica, J. Geophys. Res., this issue. Blankenship, D. D., C. R. Bentley, S. T. Rooney, and R. B. Alley, Seismic measurements reveal a saturated, porous layer beneath an active Antarctic ice stream, Nature, 322, 54-57, 1986. Blankenship, D. D., C. R. Bentley, S. T. Rooney, and R. B. Alley, Till beneath ice stream B, 1, Properties derived from seismic travel times, J. Geophys. Res., this issue. Blatter, H., On the thermal regime of an arctic valley glacier, a study of the White Glacier, Axel Heiberg Island, N. W. T., Canada, J. Glaciol., in press, 1987. Boulton, G. S., and R. C. A. Hindmarsh, Sediment deformation beneath glaciers: Rheology and geological consequences, J. Geophys. Res., this issue. Boulton, G. S., and A. S. Jones, Stability of temperate ice caps and ice sheets resting on beds of deformable sediment, J. Glaciol., 24, 29-43, 1979.

Brown, N. E., B. Hallet, and D. B. Booth, Rapid soft bed sliding of the Puget glacial lobe, J. Geophys. Res., this issue. Budd, W. F., A first simple model for periodically self-surging glaciers, J. Glaciol., 14, 3-21, 1975. Carbonnell, M., and A. Bauer, Exploitation des couvertures photographiques a•riennes r•p•t•es du front des glaciers v0,1ant

CLARKE: INTRODUCTION

8841

dans Diske Bugt en Umanak Fjord, Juin-Juillet, 1964, Medd.

in 1984: Disintegrationunderway, U.S. Geol. Surv. Open File

Groenl., 173(5), 1-78, 1968.

Rep., 85-51, 17 pp., 1985a.

Clarke, G. K. C., Subglacialtill: A physical framework for its propertiesand processes,J. Geophys.Res., this issue. Clarke, G. K. C., S. G. Collins,and D. E. Thompson,Flow thermal structure, and subglacialconditionsof a surge-typeglacier, Can. J. Earth Sci., 21,232-240,

1984.

Clarke, G. K. C., J.P. Schmok,C. S. L. Ommanney,and S. G. Collins,Characteristicsof surge-typeglaciers,J. Geophys.Res., 91, 7165-7180, 1986.

Doake, C. S. M., R. M. Frohlich, D. R. Mantripp, A.M. Smith, and D. G. Vaughan, Geological studies on Rutford Ice Stream, Antarctica, J. Geophys. Res., this issue. Fastook, J. L., Use of a new finite element continuity model to study the transient behavior of ice stream C and the causes of its present low velocity, J. Geophys. Res., this issue.

Fowler, A. C., A slidinglaw for glaciersof constantviscosityin the presenceof subglacialcavitation, Proc. R. Soc. London Ser., A, 407, 147-170, 1986.

Fowler, A. C., A theory of glacier surges,J. Geophys.Res., this issue.

Haeberli, W., A. Iken, and H. R6thlisberger,Summary of the workshop:Hydraulic effectsat the glacierbed and related phenomena, International Workshop, 16-19 September 1985, Interlaken, Switzerland,Mitt. Versuchsanst.WasserbauHydrol. Glaziol. , 90, 15-22, 1986.

Meier, M. F., L. A. Rasmussen,R. M. Krimmel, R. W. Olsen, and D. Frank, Photogrammetricdeterminationof surface altitude, terminusposition,and ice velocity of ColumbiaGlacier, Alaska, U.S. Geol. Surv. Prof. Pap., 1258-F, F1-F41, 1985b. Mercer, J. H., West Antarctic ice sheet and CO2 greenhouseeffect: A threat of disaster, Nature, 271, 321-325, 1978.

Mtiller, F. (Ed.), Fluctuationsof Glaciers1970-75,vol. 3,269 pp., International Commission on Snow and Ice of the International

Association of Hydrological Sciences, Paris, 1977.

Milllet, F., and A. Iken, Velocity fluctuationsand water regimeof arctic valley glaciers, 1ASH Publ., 95, 165-182, 1973.

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G. K. C. Clarke, Department of Geophysicsand Astronomy, University of British Columbia, Vancouver, British Columbia, Canada

V6T

lWS.

(Received November 26, 1986; accepted February 27, 1987.)