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Home , Esker, Hudson Bay Lowlands, Laurentide Ice Sheet, Till plain

Subglacial drainage, , and deforming beds beneath the Laurentide and Eurasian sheets

PETER U. CLARK Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506 JOSEPH S. WALDER U.S. Geological Survey, Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, 98661

ABSTRACT over a deforming bed and that developed over a rigid, nondeforming substrate (Rothlisberger, 1972; Shreve, 1972). In particular, Walder Glatiological theory predicts that the subglacial drainage network and Fowler showed that if subglacial water pressure is close to the at the base of gently sloping ice sheets resting on deforming ice-overburden pressure and the hydraulic gradient (largely controlled should consist of many wide, shallow, probably braided "canals" dis- by ice-surface slope) is low (such as at the base of a gently sloping ice tributed along the ice-sediment interface, rather than an arborescent sheet), then the drainage network over a deforming bed should consist network of relatively few large tunnels, as would develop over a rigid of numerous wide, shallow, braided channels along the ice-sediment substrate. A corollary prediction examined here is that eskers, which interface. Meltwater would flow relatively sluggishly through such a form in large subglacial tunnels, should be rare where subglacial bed drainage network. In contrast, meltwater flowing over a rigid sub- deformation occurred, but they may be relatively common where the strate or over a deformable bed at relatively high hydraulic gradient bed was rigid. Bed deformation would be most likely where subglacial would drain through an arborescent network of relatively few large was relatively continuous, fine-grained, and of low permeability— tunnels at substantially higher flow velocity. that is, in regions where till is derived primarily from underlying sed- The Walder-Fowler drainage theory implicitly predicts aspects imentary bedrock—but unlikely where discontinuous, coarse-grained, of the geomorphic and sedimentological record of the subglacial drain- high-permeability till was derived from underlying crystalline bedrock. age system. In this paper, we summarize salient aspects of the theory The observed distribution of eskers in areas covered by the Laurentide and show that one implication of the theory is that eskers, which form and Eurasian (British, Scandinavian, and Barents Sea) ice sheets during by sedimentation within large subglacial tunnels, should be favored the last glaciation shows that most eskers occur over crystalline bedrock where the bed is rigid and the subglacial drainage system comprises overlain by discontinuous, high-permeability till, but are rare or absent a network of arborescent tunnels, but unlikely to occur where there over sedimentary bedrock overlain by fine-grained, low-permeability is pervasive bed deformation and the drainage system consists of till, thus matching reasonably well our prediction. Glaciological theory shallow, braided channels. We then discuss characteristics of eskers and geologic evidence indicate that systems on a subcontinental and their mode of formation, and we examine the distribution of es- scale are time-transgressive. Sedimentological evidence for a "canal" kers from areas formerly covered by the Laurentide and Eurasian drainage system appears to be present in fine-grained where eskers (British, Scandinavian, and Barents Sea) ice sheets during the last are largely absent. glaciation. We find that eskers are indeed rare or absent where the ice-sheet bed would have been most susceptible to subglacial INTRODUCTION deformation.

Recognition that subglacial sediment deformation by pervasive SUBGLACIAL WATER FLOW AND THE ORIGIN OF ESKERS shear plays an important role in the behavior and dynamics of some modern (Boulton, 1979; Alley and others, 1986,1987; Boulton Glaciological Considerations and Hindmarsh, 1987; Engelhardt and others, 1990) has directed much attention to the potential contribution of this process to dynam- The traditional picture of subglacial (Rothlisberger, ics of the former Northern Hemisphere ice sheets (Boulton and Jones, 1972; Shreve, 1972) was built around the assumption that the 1979; Boulton and others, 1985; Fisher and others, 1985; Hughes, bed is rigid. Water pressurepw in a channel is less than the ice over- 1992; MacAyeal, 1993; Clark, 1994). Evidence of pervasive till de- burden pressure pt, and inward ice flow therefore tends to close the formation comes from direct observation beneath several glaciers channel. Counteracting this tendency is the melting caused by energy (Boulton, 1979; Boulton and Hindmarsh, 1987; Blake, 1992; Hum- dissipation within the flowing meltwater. Drainage is predicted to oc- phrey and others, 1993) and interpretation of geophysical data (Blan- cur in an arborescent network of channels (commonly called Roth- kenship and others, 1986, 1987). Criteria for identifying evidence of lisberger or R channels) cut into the basal ice. former subglacial sediment deformation remain elusive (Clayton and Walder and Fowler (in press) have shown how the classical view others, 1989; Alley, 1991) but are clearly needed in view of the po- of subglacial drainage must be modified for the case of a glacier or ice tential significance of this process to the behavior of former and sheet resting on deforming sediment. They demonstrated that the rhe- present ice sheets. ology of the sediment and the mechanics of by Theoretical analysis by Walder and Fowler (in press) indicates a subglacial streams strongly affect the shape of subglacial drainage fundamental difference between the subglacial hydrologic system conduits, and they concluded that two distinct types of drainage con-

Geological Society of America Bulletin, v. 106, p. 304-314, 8 figs.,Februar y 1994.

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R-channel canal general, the drainage system over deforming sediment should consist of R channels only if the ice-surface slope is large, on the order of x-section x-section about 10_1, as for a glacier. Ice-surface slopes associated with ice sheets are much lower, say a few times 10~3, except near the margins. ICE Pervasive till deformation and thus canal drainage will also be I favored if the sediment is "soft"; that is, if it creeps readily under BED applied stress. Creep is facilitated by the presence of clay (Mitchell, 1976), and so the drainage system over deforming till containing mod- erate amounts of clay is likely to be canal-like, whereas drainage over clay-poor, "stiff" till is likely to be in R channels. The Walder-Fowler theory thus predicts that, in general, melt- water drainage over pervasively deforming sediment beneath ice sheets should be in a non-arborescent system of canals instead of an arborescent tunnel network. Such a canal system at the base of an ice plan plan sheet should manifest an effective pressure pe typically about 1 bar, in accord with geophysical inferences (Blankenship and others, 1986, Figure 1. Schematic illustrations (cross section and plan views) of 1987) and bore-hole measurements (Engelhardt and others, 1990) at drainage types developed over rigid (R channel) versus deforming (ca- B, West , which overlies weak, dilated sedi- nals) beds. Drainage through an R channel develops over a rigid bed ment thought to be deforming (Alley and others, 1986,1987). characterized by relatively high effective pressures (low water pressure). Drainage through a canal develops over a deforming bed where effective Characteristics and Formation of Eskers and Esker Systems pressures are low. Eskers are straight-to-sinuous of stratified sediment, up to several hundred kilometers long, that are widespread on formerly duits may exist over a deforming substrate (Fig. 1). One type is ba- glaciated landscapes of the Northern Hemisphere (Prest and others, sically like the classical R channel, and the predicted relationship (in 1968; UNESCO, 1967-1980). They have been extensively studied the steady state) between discharge Q and effective pressure pe (ice with respect to their sedimentology, morphology, and petrography pressure minus water pressure) is of the form (compare with Lli- (Flint, 1971; Embleton and King, 1975; Shreve, 1985; Drewry, 1986). boutry, 1983): Shreve (1985) elegantly demonstrated that esker paths are explicable in terms of the physics of subglacial water flow at the base of active ,1/15 ice, where "active" means (Shreve, 1985, p. 644) that the ice is flow- Pe <* Q (1) ing forward everywhere. Eskers form by sedimentation within an

Thus, as discharge increases, p(, increases; hence, water pressure pw arborescent network of drainage channels, which are incised into the decreases. Larger channels are therefore at lower pw than smaller base of active ice and follow hydraulic-potential lows on the glacier channels and tend to capture the drainage of those smaller channels, bed. Because the dominant control on the gradient of hydraulic equi- forming an arborescent network with relatively few main trunk chan- potential is the ice-surface slope (Rothlisberger, 1972; Shreve, 1972), nels, just as in subaerial stream networks. this means that channels generally trend in the same direction as the Distinct from R channels are what Walder and Fowler termed ice-surface slope. The effect of glacier-bed topography is small but in "canals," which are streams incised into the subglacial sediment with places noticeable, as shown by Shreve (1985) in his study of esker a more or less roof of ice. Unless the subglacial sediment is ex- trends in . tremely cohesive, these canals will be much wider than deep. This Eskers most commonly have fairly steep sides and a single, sharp geometiy is like that of subaerial streams over non-cohesive sedi- crest (Embleton and King, 1975), although variations on this form ment, for which Parker (1978a, 1978b) discussed the pertinent me- exist, for reasons discussed in particular by Shreve (1985). Eskers are chanics governing the cross-sectional shape. The relation between typically a few tens to hundreds of meters in width and height. Such discharge and pressure in subglacial canals is predicted to be of the dimensions are much greater than what is physically reasonable for an form: R channel—even channels associated with gigantic Icelandic outburst floods have diameters of no more than about 10 m (Nye, 1976). Thus a eskers cannot form wholly within any single conduit, but must be built Pe Q~ (2) up over time as sediment melts out of channel walls and is deposited where m ~ 1/3. Unlike R channels, the greater the discharge through and reworked on the channel floor (McDonald and Vincent, 1972; a canal, the greater the water pressure. Consequently, there is no Banerjee and McDonald, 1975; Gorrell and Shaw, 1991). It is unlikely tendency for flow to concentrate in trunk conduits, and the drainage that sedimentation within broad, canal-like conduits could produce system should consist of shallow, wide canals distributed more or less steep-sided, sharp-crested eskers, however. uniformly over the entire glacier bed, perhaps connected in a braided The sedimentology of many eskers is compatible with the notion fashion, but not forming an arborescent network. of deposition within R tunnels but is difficult to explain if subglacial The two types of subglacial drainage conduits cannot exist si- drainage occurs via wide, shallow canals. Sedimentary structures (or multaneously. Which one will exist in any given circumstance is a lack thereof) in sharp-crested eskers reflect relatively high water ve- function of the hydraulic gradient (primarily dependent on the ice- locity (McDonald and Vincent, 1972; Banerjee and McDonald, 1975; surface slope) and on the rheological properties of ice and till. In Saunderson, 1977; Shreve, 1985; Gorrell and Shaw, 1991), although

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seasonal fluctuations may result in low velocity during winter (He- solid mass of ice" (Shilts, 1984, p. 218). Instead, they argued that the brand and Amark, 1989; Gorrell and Shaw, 1991). This bears on the arborescent system developed on the ice surface and proposed a con- nature of the drainage conduits, because an important distinction be- ceptual model of esker formation in which subglacial tunnels erode tween R channels and canals is that water velocity should be consid- headward, "following the drainage net on the surface of the ice" erably higher in the former than in the latter, on the order of 1-3 m/s (Aylsworth and Shilts, 1989b, p. 425). From a glaciological perspec- for R channels, but only about 0.1-0.3 m/s for canals (estimates from tive, we see no reason to appeal to this hypothesis, because theory the results of Walder and Fowler [in press]). This difference is due to (Rothlisberger, 1972; Shreve, 1972) and field investigations (for ex- the considerably greater cross-sectional area (for a given discharge) ample, Hodge, 1976, 1979; Hantz and Lliboutiy, 1983) provide com- for a canal network than for channels. pelling support for the existence of arborescent subglacial drainage Gross topological aspects of esker systems (Shilts and others, channels. 1987; Aylsworth and Shilts, 1989a, 1989b) are also compatible with the hypothesis that they form by sedimentation within an arborescent NATURE OF THE SUBGLACIAL SUBSTRATE R-channel network, but not within canals. Eskers, which are reason- ably continuous features on the landscape, typically form dendritic Relation of Till to Bedrock (that is, arborescent) systems, splitting into "" as one progresses up flow (Aylsworth and Shilts, 1989a, p. 15). Individual If till is derived directly from underlying bedrock, texture of till esker ridges are as long as 75 km (Aylsworth and Shilts, 1989a). The reflects the underlying source material: till derived from metamor- spacing of eskers transverse to the ice-flow direction is typically on phic, igneous, or sandstone bedrock will generally have a coarser the order of kilometers. These characteristics of esker networks matrix (<2 mm) than that of till derived from carbonate, shale, and mimic those of an arborescent R-channel network, but are not at all siltstone bedrock (Fig. 2) (Scott, 1976; Jorgensen, 1977; Mickelson what one would predict from formation within subglacial canals, and others, 1983; Haldorsen, 1983; Clayton and others, 1985; Dredge which would be more numerous and closely spaced, forming a and Cowan, 1989). Exceptions to this general rule occur as a result of braided, non-arborescent network. long-distance subglacial sediment transport, which may deliver a sig- Eskers are found over large areas formerly covered by ice sheets, nificant amount of debris of texture different than expected from un- and it is possible to pose (at least) two alternative interpretations of derlying bedrock (for example, fine-grained carbonate-rich till over esker-system topology and the timing of esker-system formation at a crystalline bedrock). Glacial reworking of pre-existing (for subcontinental scale. This topology might represent a snapshot of the example, lake clays) may also lead to till texture that does not reflect full-glacial subglacial drainage network, with all eskers having formed underlying bedrock. Finally, in areas of crystalline bedrock, some more or less contemporaneously under the during the last lithologies (slates) may produce fine-grained tills. glacial maximum. Alternatively, different parts of the esker network Thickness and continuity of till cover also reflect underlying bed- might have developed over time during as the subglacial rock, being generally thin and discontinuous over crystalline bedrock drainage network evolved. (Fig. 3) (Haldorsen, 1983; Dyke and others, 1989; Dredge and Cowan, We interpret field data and theory as indicating a time-transgres- sive origin for esker systems. In regard to the , for example, most reconstructions of ice-sheet decay (for example, CI CI CI CI Dyke and Prest, 1987) show shifting positions of ice divides and cor- responding shifts in direction of ice-surface slope and ice flow. Such /k\ A directional changes are identified by changes in orientation of drum- lins and flutings as well as of eskers (Dyke and Dredge, 1989), thus indicating time-transgressive esker-system formation. The physics of subglacial meltwater flow also supports time- SdSt transgressive formation of esker systems developed beneath ice sheets. Although basal ice in the interiors of the former ice sheets may SdZM/j>\ a ' a have been at the melting point over large areas (Sugden, 1977; Hughes and others, 1981), there would have been no melting at the ice surface

except relatively near the ice-sheet margins. Except in those areas, 1 the only meltwater flowing at the glacier bed would have been derived LJU ' - — St from basal melting. Such meltwater would tend to channelize CRYSTALLINE SEDIMENTARY (Walder, 1982), but any channels would be very small; the subglacial M, drainage system would more closely resemble a very thin water film Figure 2. Ternary grain-size diagrams (-silt-clay) for till sam- over much of the bed (Alley, 1989). Only where surface meltwater ples from areas of crystalline bedrock (A-D) and sedimentary bedrock reached the bed would channelized flow have been significant. The (E-H). A. Till samples fromKeewati n District, (Scott, width of this zone of surface melting ( zone) is unknown, but 1976). B. Till samples from southern and (Vincent, it was likely on the order of hundreds of kilometers (subcontinental) 1989). C. Till samples fromsoutheaster n Canadian Shield (Scott, 1976). and would not have included ice divides at maximum extent. D. Till samples primarily fromPrecambria n shield bedrock of Norway Shilts (1984), Shilts and others (1987), and Aylsworth and Shilts (Jorgensen, 1977). E. Till samples from British Lowlands (Sladen and (1989a, 1989b) also proposed that eskers are time-transgressive, but Wrigley, 1983). F. Till samples from the southern Canadian Prairies they argued against the topology of esker systems as reflecting that of (Scott, 1976). G. Till samples from the and subglacial streams, stating that "it is hard to imagine how the Horton lowlands (Scott, 1976). H. Till samples fromth e St. Lawrence Lowlands [that is, arborescent] system could have developed so fully within a (Scott, 1976).

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1989) versus relatively thick and continuous over sedimentary bed- rock (Flint, 1971). Where fine-grained tills that were derived from long-distance transport occur over crystalline bedrock, they are gen- erally discontinuous, and a large fraction of the bed is exposed rock (Dyke and others, 1989). The Laurentide and Eurasian ice sheets covered a wide variety of bedrock types. Given the aforementioned influence of source bed- rock on till grain size and continuity, however, we can simplify the varied bedrock into two types: sedimentary and metamor- phic/igneous (Fig. 4). Throughout glaciated North America and Eur- asia, this distinguishes areas of discontinuous sandy tills derived from underlying crystalline bedrock of Precambrian shield areas and folded belts (Appalachians, Caledonians) and sandstone from ar- eas with continuous clayey-silty tills derived from underlying sedi- mentary bedrock (excluding sandstone) (Figs. 2, 3).

Physical Properties of Till

Sandy tills will have considerably higher hydraulic permeability than finer-grained tills—Freeze and Cherry (1979) show a variation in till permeability of six orders of magnitude—and thus (see Fig. 4) we expect tills derived from crystalline rock or sandstone generally to be more permeable than tills derived from fine-grained sedimentary rock. This consideration is important when we consider the mechanics of subglacial sediment deformation. Subglacial sediment will begin to deform if the stress applied by the ice exceeds the sediment's shear Figure 3. Area of predominantly exposed bedrock (stippled pat- strength Ts, which empirically depends upon effective pressure by the tern) over Canadian Precambrian shield (from Dyke and others, 1989). relationship

Figure 4. Generalized bedrock geologic maps of areas underlying the Laurentide (A) and Eurasian (B) ice sheets. Individual ice sheets comprising the Eurasian ice sheet are identified as Br, British Ice Sheet; Sc, Scandi navian Ice Sheet; and Ba, Barents Ice Sheet Margins of ice sheets are shown by heavy line Crystalline bedrock is shown by arrow pat tern. Sandstone bedrock underlying Scandina vian Ice Sheet is shown by stippled pattern Other sedimentary bedrock is shown by no pattern.

Sheet

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= c + vpe (3) nario applied to those areas where fine-grained, discontinuous tills overlie crystalline bedrock. Although the tills may have been weak where c is the cohesive strength, |x is a coefficient of internal friction, and deforming when ice-covered, the fraction of exposed rock was andpe is again the effective pressure, the difference between ice over- the primary control on basal resistance to ice motion. burden pressureand pore-water pressurepw. The pore pressure in These considerations of the physical properties of tills and their a sandy, high-permeability till will be lower than in a finer-grained till lateral continuity lead us to conclude that pervasive subglacial till owing to drainage through the till to basal meltwater conduits (Shoe- deformation is quite likely in regions of relatively continuous, low- maker, 1986; Walder and Fowler, in press); thus pe will be greater, permeability, fine-grained till, which occur predominantly over sed- and the sandy till will tend to be stronger. imentary bedrock (exclusive of sandstone), but much less likely

Where the shear stress exerted by the ice exceeds ts, we expect where the till is discontinuous, coarse-grained, and has relatively high the till to deform in a quasi-viscous manner. Low strain rates are permeability, or predominantly over crystalline bedrock and sand- associated with subglacial till deformation (on the order of 10~6s~1 for stone. Where fine-grained discontinuous tills occur over crystalline Breicfamerkuijokull [Boulton and Hindmarsh, 1987] and Trapridge bedrock, exposed bedrock plays the dominant role in balancing basal Glacier [Blake, 1992]). Deformation processes at such slow strain shear stress. rates are probably like those in soil creep. Mitchell's (1976) creep tests These arguments for the distribution of deforming beds agree showed that for a given stress level, the strain rate in clay/sand mix- with geologic data that indicate sediment deformation in areas of fine- tures increased (thus the apparent viscosity decreased) as the relative grained till, including sedimentological evidence (Johnson and Han- amount of clay increased. We thus expect there to be a rheological sel, 1990'; Alley, 1991; Clark, 1991; Hart and Boulton, 1991), low difference between sandy till and fine-grained till, in accord with ad- ice-surface slopes (Mathews, 1974; Beget, 1987; Nesje and Sejrup, mittedly sketchy glaciological evidence. In the relatively coarse 1988; Clark, 1992), and ice-sheet behavior (Clark, 1994). Breiiamerkurjokull till studied by Boulton and Hindmarsh (1987), sand- and gravel-sized clasts composed about 70% of the total; this till Relation of Eskers to Their Substrate: A Hypothesis showed an apparent viscosity during creep of about 1011 to 1012 Pa s (calculated from Boulton and Hindmarsh's data), about two to three The subglacial drainage system is likely to comprise R channels orders of magnitude greater than for where the glacier bed is either free of sediment finer-grained tills (Blake, 1992; Hum- cover or discontinuously covered by sediment, or phrey and others, 1993). where subglacial sediment did not pervasively de- Continuity of till cover should form under stresses applied by the ice. Such con- also play an important role in affecting ditions typify discontinuous, mostly coarse-grained ice movement. Humphrey and others tills overlying crystalline bedrock. In contrast, the (1993) sampled a till with low strength basal drainage system should consist of canals in and effective viscosity from beneath areas characterized by subglacial sediment defor- the Columbia Glacier, . Be- mation, conditions typical of regions of continuous cause the till strength is unable to sup- fine-grained tills overlying sedimentary bedrock. port the basal shear stress of the gla- We therefore propose that eskers should generally cier, Humphrey and others (1993) be confined to areas of crystalline bedrock and as- argued that bedrock protuberances sociated coarse-grained tills. Other factors, partic- contribute most of the basal resistance ularly sediment supply (Aylsworth and Shilts, to ice motion. We believe that this sce- 1989b), will influence whether or not eskers actually form within R channels. In the following, we assess whether observed esker distributions with respect to the subglacial substrate are generally consistent with predictions of the glacier-hydrology model.

DISTRIBUTION OF ESKERS

Laurentide Ice Sheet

The Laurentide Ice Sheet was the largest ice sheet in the Northern Hemisphere. It covered a wide variety of bedrock types, but as discussed, we have simplified these as (1) crystalline bedrock as- sociated primarily with the Precambrian shield in and the Appalachians of and southeastern Canada and (2) sedimentary bedrock gHee of the midwestern , western Canada, the St. Lawrence Lowland, the James Bay and Ice Hudson Bay areas, and the continental shelf along Figure 4. (Continued). the eastern seaboard (Fig. 4).

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/ x

Figure 5. Distribution of eskers deposited by the Laurentide Ice Sheet (margin shown by heavy line). Not all eskers from New England area of United States are shown. Intermediate heavy line separates crystalline bedrock from sedimentary bedrock (refer to Fig. 2). Data fromCadwel l and Pair (1991), Farrand and others (1984), Fullerton and others (1991), Goebel and others (1983), Lineback and others (1983), Muller and Cadwell (1986), Prest and others (1968), Stewart and MacClintock (1970), and Thompson and Borns (1985).

Eskers deposited by the Laurentide Ice Sheet are found pri- others, 1986) indicate that eskers may be largely absent from these marily in association with crystalline bedrock areas of the Pre- areas as well. Where found, the eskers are short in comparison to cambrian shield in Canada and the Appalachian region of the most of those over crystalline bedrock. Eskers in and eastern United States and southeastern Canada, whereas eskers are associated with sandy till derived from sandstone (Mick- are largely absent from areas underlain by sedimentary bedrock elson and others, 1983). (Fig. 5). Note that there are several areas on the shield where The transition from areas of abundant eskers over crystalline eskers are noticeably absent. In Keewatin and central Ungava- bedrock to areas where they are rare or absent over sedimentary Quebec, these areas correspond to the location of former ice di- bedrock is abrupt, particularly along the boundary between crystal- vides (Shilts and others, 1987). There are no eskers over the high- line and sedimentary bedrock in the western Canadian prairies. Per- lands of eastern . West of the Hudson Bay-James haps the most striking example of this contrast in esker distribution is Bay lowlands, several prominent (Hudwih, Pasquiam, found in the Hudson Bay-James Bay area (Fig. 5). Sioux Lookout, High Rock Lake; see Dyke and Prest, 1986) sep- These observations are generally consistent with our predictions. arate areas of abundant eskers from areas with few eskers (Shilts The absence of eskers near former ice divides is particularly striking and others, 1987; Sharpe and Cowan, 1990). and readily understood within the context of glaciologic restrictions With few exceptions, eskers are conspicuously absent in regions discussed above, namely, the absence of a system of subglacial drain- where fine-grained tills overlie sedimentary bedrock (Fig. 5). Marine age channels upglacier of the zone in which surface meltwater would geophysical surveys in Hudson Bay (Josenhans and Zevenhuizen, have reached the bed. Similarly, the absence of eskers over the east- 1990) and on the continental shelf of (Josenhans and ern highlands of Baffin Island may be explained by thin, cold-based

Geological Society of America Bulletin, February 1994 309

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Cover Figure 7. Three out- crops of fine-grained till with lenticular channel fillings. line of sections lies r transverse to former ice- 1 flow direction. Black rep- resents till; stippled pat- tern represents channel "Cover ' fills, which include , gravels, and laminated clays; covered intervals shown by lined pattern. After Eyles and others r (1982). omfteu"«! rag^ .iflfll^v i Llover 5 t-H-rO 200 0 m

ice (hence no basal drainage conduits) in that area (Andrews and source maps (UNESCO, 1967-1980), eskers are also present in north- others, 1985). eastern Scotland (Clapperton and Sugden, 1977). Shilts and others (1987, p. 138) also discussed the relation be- In contrast, eskers are relatively uncommon where the ice sheet tween esker distribution and substrate type for the northwestern sec- overrode sedimentary bedrock, including in the regions underlying tor of the Laurentide Ice Sheet, and they suggested several explana- the North and Baltic Seas. A relatively dense network of shallow tions for the paucity of eskers beyond the edge of the Canadian Shield, seismic surveys of the surficial of the area covered by the including the idea that this phenomenon was related to "the deform- Barents Ice Sheet has not identified any eskers (Solheim and Krist- able nature of the bedrock." In their view, bed deformation would offersen, 1984; Solheim and others, 1990; L. Polyak, 1993, personal have "promoted rapid flow compared to the rigid Shield terrane, commun.). Where present, eskers are commonly significantly shorter causing tunnel integrity to be difficult to maintain." We agree that than those found over crystalline bedrock. Nevertheless, several con- tunnels (that is, R channels) would not be maintained, but the reason centrations of eskers are found over sedimentary bedrock (Fig. 6), is not the ice-flow rate; rather, in line with the Walder-Fowler theory, including in , where the term "esker" originated, and partic- the drainage system over the deformable bed would have consisted of ularly south of the Gulf of in northwestern Russia, eastern nonarborescent canals not conducive to esker formation. Latvia, and northern Estonia. Bedrock underlying the last-named area is predominantly sandstone (area bounded by dashed line, Eurasian Ice Sheets Fig. 6), and till matrix in at least the western part of this region is relatively coarse (A. Dreimanis, 1991, personal commun.). Many of The British, Scandinavian, and Barents Sea ice sheets also over- the small eskers found south of the Baltic Sea in Poland, Germany, rode variable types of substrate, and we make a similar distinction and Denmark (Fig. 6) occur on the floor of tunnel valleys (Michalska, between areas underlain by crystalline bedrock and sedimentary bed- 1969; Kruger, 1983). rock (Fig. 4). In our reconstructions of the limits of these ice sheets As with eskers from North America, the distribution of eskers (Figs. 4,6), we depict the Scandinavian Ice Sheet coalescent with the deposited by the Eurasian ice sheets generally agrees with our pre- British Ice Sheet over the North Sea (Andersen, 1981). The ice sheets, dictions. The largest and best-developed eskers are widespread over however, may have advanced only part way into the North Sea and crystalline bedrock where sediments are discontinuous and coarse. remained separated by an ice-free, open embayment (Sejrup and oth- Their absence over the Norwegian highlands may be associated with ers, 1987). cold and thin ice. An abrupt decrease in esker abundance occurs over Eskers deposited by these ice sheets are well developed and areas where the substrate is fine-grained till overlying sedimentary abundant over crystalline bedrock except over the Kola Peninsula bedrock. The primary exception coincides with the area south of the and the highlands of Norway (Fig. 6). Although not shown on the Gulf of Finland where bedrock is sandstone, and till matrix is corre-

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spondingly relatively coarse. Eskers associated with tunnel valleys subglacial flowing in tunnels was so fine that it was simply may be special cases associated with the poorly understood process removed. Even a fine-grained till (that is, a till in which clay and silt of formation (Boulton and Hindmarsh, 1987; Mooers, compose >50% of the matrix), however, still contains considerable 1989). sand in the matrix, and the gravel fraction, while rarely recorded, may be significant. Furthermore, outwash sediments that occur in regions POSSIBLE GEOLOGIC RECORD OF DRAINAGE of fine-grained till are composed of sand and gravel such as found in IN A CANAL SYSTEM eskers (Costello and Walker, 1972; Fraser and Cobb, 1982; Dredge and others, 1986; Thorleifson and others, 1993). Thus, coarse material In this section, we explore the geologic record for sedimento- was available for deposition in the form of eskers if the drainage net- logical evidence that might reflect deposition within a canal system work had been such as to favor their formation—that is, an arbores- having the characteristics (anastomosing, wide and shallow with flat cent network of tunnels. roofs, low flowvelocities ) predicted by Walder and Fowler (in press). Alternatively, the paucity of eskers over fine-grained till may We focus on the geologic record in areas covered by fine-grained till, reflect the fact that those substrates generally coincide with the outer where eskers are rare or absent. parts of the former ice sheets. In simple models of ice-sheet flow (for In northeastern England, an area of fine-grained till with no es- example, Paterson, 1981), the ice-surface slope increases progres- kers, Eyles and others (1982) described "shoestring" deposits, which sively downglacier; thus the rate of meltwater production due to sur- they interpreted as channel fills, that may be the best-described ex- face melting would also increase downglacier. If all surface meltwater ample of a canal system in the sedimentary record (Fig. 7). These reached the bed, the sediment-carrying capacity of the rivers in sub- deposits are elongate subparallel to the former direction of ice flow. glacial tunnels would increase rapidly in the downstream direction, so They have nearly flat upper surfaces and irregular, concave-up lower that all sediment might have been transported out of the tunnels and surfaces. Although they have variable dimensions, these deposits are no eskers deposited. Two lines of evidence call this hypothesis into all much wider than thick. Gravel lags at the deposit base are overlain question. First, reconstructed ice-surface gradients over regions of mostly by sand but also include laminated clay. Lenses of till are fine-grained till are low, corresponding to driving stresses of 1-25 kPa present within some of these deposits. Eyles and others (1982) pro- posed that these deposits represent cut-and-fill structures formed by subglacial drainage, with grain-size variations within the deposit in- dicating variable discharge. Diapiric deformation at the channel bases indicates high pore-water pressures in the till at the time of channel formation. Lenses of till represent sediment masses that dropped into the channels from overlying ice. Brown and others (1987) interpreted characteristics of lenses of water-laid, sorted sediment within the Vashon Till deposited by the Puget lobe of the late (an area of no eskers) as having been deposited "from water flowing in broad sheet- like layers under the ice." They further inferred that water pressure beneath the Puget lobe was very near the ice overburden pressure. Such low effective pressures in channelized flow would be consistent with canals, but not with R channels incised into the ice. The discon- Deforming tinuous character of the water-laid subglacial deposits described by Brown and others (1987) is consistent with a picture of broad, braided Bed canals (similar to alluvial gravel-bedded streams), reflecting the con- stantly shifting geometry of the braids. Similar subglacial channelized sediments in fine-grained till have been described from many areas in North America where eskers are largely absent, including (Shaw, 1987; Catto, 1984), (Dredge and Nielsen, 1985; Dredge and others, 1986; Klassen, 1986), (Sharpe, 1988; Thorleifson and others, 1993), and the mid- continent region of the United States (Clayton and others, 1989; John- son and Hansel, 1990; Clark and Rudloff, 1990).

SUMMARY AND DISCUSSION

The distribution of eskers deposited by the Laurentide and Eur- asian ice sheets with respect to the nature of the underlying substrate generally supports our predictions based on the Walder-Fowler the- ory of subglacial drainage. Two alternative, related hypotheses, how- ever, might also be posed to explain the geographic relationship be- tween esker distributions and substrate type. First, eskers may be Figure 8. A. Schematic illustration of drainage over a deforming rare where the substrate comprised fine-grained till over sedimentary bed via a system of canals. B. Schematic illustration of drainage over a bedrock (other than sandstone) because the sediment delivered to rigid bed via a system of tunnels.

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Beget, J., 1987, Low profile of the northwest Laurentide Ice Sheet: Arctic and Alpine Research, v. 19, (Mathews, 1974; Clark and Josenhans, 1986; Beget, 1987; Nesje and p. 81-88. Sejrup, 1988; Clark, 1992). Second, the region of esker-free, fine- Blake, E. W., 1992, The deforming bed beneath a -type glacier: Measurement of mechanical and electrical properties [Ph.D. thesis]: Vancouver, University of , 179 p. grained till underlying the Hudson Bay and James Bay lowlands lies Blankenship, D. D., Bentley, C. R.. Rooney, S. T., and Alley, R. B., 1986, Seismic measurements reveal a saturated porous layer beneath an active Antarctic ice stream: Nature, v. 322, p. 54-57. near the geographic center of the former ice sheet, for which the Blankenship, D. D., Bentley, C. R„ Rooney, S. T., and Alley, R. B., 1987, Till properties beneath ice hypothesis is not relevant. stream B. 1. Properties derived from seismic travel times: Journal of Geophysical Research, v. 92, p. 8903-8911. There may have been relatively small areas near the ice-sheet Boulton, G. S., 1979, Processes of glacier erosion on different substrata: Journal of , v. 23, p. 15-38. margin where the absence of eskers is attributable primarily to melt- Boulton, G. S., and Hindmarsh, R.C.A., 1987, Sediment deformation beneath glaciers: Rheology and water flux sufficient to flush sediments. For example, in central Min- geological consequences: Journal of Geophysical Research, v. 92, p. 9059-9082. Boulton, G. S-, and Jones, A. S., 1979, Stability of temperate ice caps and ice sheets resting on beds of nesota, eskers formed within tunnel valleys but only after formation deformarle sediment: Journal of Glaciology, v. 24, p. 29-43. Boulton, G. S., Smith, G. D., Jones, A. S., and Newsome, J., 1985, Glacial geology and glaciology of of those tunnel valleys. This change in the subglacial drainage system the last mid-latitude ice sheets: Geological Society of London Journal, v. 142, p. 447-474. from an erosional mode to a depositional one may have been due to Brown, N. E., Hallet, B., and Booth, D. B., 1987, Rapid soft bed sliding of the Puget glacial lobe: Journal of Geophysical Research, v. 92, p. 8985-8998. decrease in the meltwater flux or increase in sediment supply (Moo- Cadwell, D. H., and Pair, D. L., 1991, Surficialgeologicmap of : Adirondack sheet: New York ers, 1989, 1990), although either conclusion remains speculative. State Museum-Geological Survey, Map and Chart Series No. 40, scale 1:250,000. Catto, N. R., 1984, Glacigenic deposits at the Edmonton Convention Centre, Edmonton, Alberta: Ca- Geological evidence and glaciological theory indicate that esker nadian Journal of Earth Sciences, v. 21, p. 1473-1482. Clapperton, C. M., and Sugden, D. E., 1977, The late Devensian glaciation of north-east Scotland, in networks are time-transgressive. Accordingly, we envisage the fol- Gray, J. M., and Lowe, J. J., eds., The Scottish lateglacial environment: New York, Pergamon Press, p. 1-14. lowing scenario for evolution of the subglacial drainage network dur- Clark, P. U., 1991, Striated clast pavements: Products of deforming subglacial sediment?: Geology, ing ice-sheet retreat. In the terminal (ablation) zone of the ice sheet, v. 19, p. 530-533. Clark, P. U., 1992, Surface form of the southern Laurentide Ice Sheet and its implications to ice-sheet surface meltwater enters the basal drainage system via a system of dynamics: Geological Society of America Bulletin, v. 104, p. 595-605. Clark, P. U., 1994, Unstable behavior of the Laurentide Ice Sheet over deforming sediment and its englacial conduits (Shreve, 1972). Where the ice sheet locally rests on implications for change: Research (in press). a deforming bed, the meltwater drains through a canal system Gark, P. U., and Josenhans, H. W., 1986, Late Quaternary land-sea correlations, northern Labrador, Canada: Geological Survey of Canada Paper 86-1B, p. 171-178. (Fig. 8A); where the substrate is rigid, meltwater drains through an Clark, P. U-, and Rudloff, G. A., 1990, Sedimentology and stratigraphy of late Wisconsinan deposits, Lake Michigan bluffs, northern Illinois, in Schneider, A. F., and Fraser, G. S., eds., Late Qua- arborescent tunnel system (Fig. 8B). Thus as the ice sheet retreats ternary history of the Lake Michigan Basin: Geological Society of America Special Paper 251, from a deforming substrate to a rigid substrate, the character of the p. 29-41. Clayton, L., Teller, J. T., and Attig, J. W., 1985, Surging of the southwestern part of the Laurentide Ice subglacial drainage changes from a distributed system, represented in Sheet: Boreas, v. 14, p. 235-242. Clayton, L., Mickelson, D. M., and Attig, J. W., 1989, Evidence against pervasively deformed bed the geologic record by broad, water-worked deposits in fine-grained material beneath rapidly moving lobes of the southern Laurentide Ice Sheet: Sedimentary Geol- till, to a network of arborescent tunnels, represented by eskers. We ogy, v. 62, p. 203-208. Costello, W. R., and Walker, R. G., 1972, Pleistocene sedimentology, Credit , southern Ontario: believe such drainage-system evolution occurred as the margins of the A new component of the model: Journal of Sedimentary Petrology, v. 42, p. 389-400. Dredge, L. A., and Cowan, W. R., 1989, Quaternary geology of the southwestern Canadian Shield, in Laurentide and Scandinavian ice sheets retreated from deforming Fulton, R. J., ed., Quaternary geology of Canada and Greenland: Geological Survey of Canada, sediments onto Precambrian shield bedrock. We suggest that the con- Geology of Canada, no. 1, p. 214-249. Dredge, L. A., and Nielsen, E., 1985, Glacial and deposits in the Hudson Bay Lowlands: A verse sort of evolution—that is, a change from arborescent tunnels to summary of sites in Manitoba: Geological Survey of Canada Paper 85-1A, p. 247-257. Dredge, L. A., Nixon, F. M., and Richardson, R. J., 1986, Quaternary geology and of distributed canals—occurred as the margin of the Laurentide Ice northwestern Manitoba: Geological Survey of Canada Memoir 418, 38 p. Sheet retreated onto the James Bay and Hudson Bay lowlands Drewry, D., 1986, Glacial geologic processes: London, U.K., Edward Arnold, 276 p. Dyke, A. S., and Dredge, L. A., 1989, Quaternary geology of the northwestern Canadian Shield, in (Fig. 5), which are areas of fine-grained till virtually devoid of eskers, Fulton, R. J., ed., Quaternary geology of Canada and Greenland: Geological Survey of Canada, Geology of Canada, no. 1, p. 189-214. yet surrounded by crystalline bedrock where eskers are abundant. Dyke, A. S., and Prest, V. K-, 1986, Late Wisconsinan and Holocene retreat of the Laurentide Ice Sheet: Geological Survey of Canada Map 1702A, scale 1:5,000,000. Dyke, A. S., and Prest, V. K., 1987, Paleogeography of northern North America, 18,000-5,000 years ago: Geological Survey of Canada, Map 1703A. ACKNOWLEDGMENTS Dyke, A. S., Vincent, J.-S., Andrews, J. T., Dredge, L. A., and Cowan, W. R„ 1989, The Laurentide Ice Sheet and an introduction to the Quaternary geology of the Canadian Shield, in Fulton, R. J., ed., Quaternary geology of Canada and Greenland: Geological Survey of Canada, Geology of K. Bevis assisted with data collection. A Dreimanis and T. Vor- Canada, no. 1, p. 178-189. Embleton, C., and King, C.A.M., 1975, Glacial geomorphology: New York, John Wiley and Sons, 573 p. ren provided helpful information on the glacial and bedrock geology Engelhardt, H., Humphrey, N., Kamb, B., and Fahnestock, M., 1990, Physical conditions at the base of parts of the Scandinavian Ice Sheet. We benefited from discussions of a fast moving Antarctic ice stream: Science, v. 248, p. 57-59. Eyles, N., Sladen, H. A., and Gilroy, S., 1982, A depositional model for stratigraphie complexes and with R. B. Alley. R. LeB. Hooke and R. L. Shreve carefully reviewed facies superimposition in lodgement till: Boreas, v. 11, p. 317-333. Farrand, W. R., Mickelson, D. M., Cowan, W. R., and Goebel, J. E., 1984, Quaternary geologic map an earlier draft of the paper and made many useful comments. We also of the Lake Superior 4°x6° quadrangle, United States and Canada: U.S. Geological Survey Map appreciate critical reviews and comments by W. W. Shilts, D. M. 1-1420 (NL16), scale 1:1,000,000. 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