Inferences from Laurentide Eskers, Canada

Home , Esker, Glaciofluvial deposits, Laurentide Ice Sheet, Oak Ridges Moraine, Rogen moraine, Surge (glacier)

Geomorphology 32Ž. 2000 263±293

Deglacial meltwater drainage and glaciodynamics: inferences from Laurentide eskers, Canada

Dr. Tracy A. Brennand ) Department of Geography, Simon Fraser UniÕersity, 8888 UniÕersity DriÕe, Burnaby, BC, CanadaV5A 1S6 Received 1 August 1998; received in revised form 1 June 1999; accepted 16 August 1999

Abstract

This paper evaluates current knowledge of Laurentide eskers in Canada in the light of developments in glacier hydrology and glacial sedimentology. Questions regarding the morpho-sedimentary relations of eskers, the synchroneity and operation of R-channel systems, the role of supraglacial meltwater input and proglacial water bodies, the controls on esker pattern, and the glaciodynamic condition of the ice sheet at the time of esker formation are discussed. A morphologic classification of eskers is proposed. Five types of eskers are identified and investigated. Type I eskers likely formed in extensive, synchronous, dendritic R-channel networks under regionally stagnant ice that terminated in standing water. Type II eskers likely formed in short, subaqueously terminating R-channels or reentrants close to an ice front or grounding line that may have actively retreated during esker sedimentation. Type III eskers plausibly formed in short R-channels that drained either to interior lakes in, or tunnel channels under, regionally stagnant ice. Type IV eskers may have formed as time-transgressive segments in short, subaerially terminating R-channelsŽ. or reentrants that developed close to the ice margin as the ice front underwent stagnation-zone retreat or downwasted and backwasted regionallyŽ. stagnant ice ; however, formation in synchronous R-channels cannot be discounted on the basis of reported observations. Type V eskers may have formed in H-channels that terminated subaerially. The spatial distribution of these esker types is discussed. The factors that determined Laurentide R-channel pattern and operation were likely a complex combination ofŽ. i supraglacial meltwater discharge, Ž ii . the number and location of sink holes,Ž. iii the ice surface slope, thickness and velocity, and Ž. iv the permeability, topography and rigidity of the bed. These factors cause and respond to changes in ice dynamics and thermal regime over the glacial cycle. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: glacier hydrology; subglacial drainage systems; ice dynamics; eskers; Laurentide Ice Sheet

1. Introduction WisconsinianŽ. 23±10 ka BP; Fulton, 1989 . The melting of this ice sheet produced large volumes of The Laurentide Ice SheetŽ. Fig. 1a covered most meltwater which likely had important implications of Canada and the northern United States in the Late forŽ. i ice motion by basal sliding Ž regelation and slippage; Weertman, 1957; Iken, 1981. and bed de- formationŽ.Ž. e.g., Boulton and Jones, 1979 , ii ice ) sheet profileŽ Arnold and Sharp, 1992; Shoemaker, Corresponding author. Tel.: q1-604-291-3617; Fax: q1-604- 291-5841. 1992a,b.Ž. , and iii glacial landform genesis Ž Boulton E-mail address: [email protected]Ž. T.A. Brennand . and Hindmarsh, 1987; Shaw, 1996. .

Fig. 1.Ž. a Distribution of Laurentide eskers Ž modified from Clark and Walder, 1994 . and basal substrate. Ž A . Long, dendritic eskers in south-central Ontario;Ž. B Abitibi esker, Quebec; Ž. C short, subparallel eskers in the Ottawa±Kingston area, southern Ontario; Ž. D short, deranged eskers in southern AlbertaŽ.Ž. many too small to appear on this map . b Distribution of Laurentide eskers, moraines and proglacial water bodiesŽ. Prest et al., 1968 . T.A. BrennandrGeomorphology 32() 2000 263±293 265

Fig. 2. Karstic meltwater drainage in temperate ice as inferred from field-based glacier hydrology and glaciological theory. See text for explanation. 266 T.A. BrennandrGeomorphology 32() 2000 263±293

How can an understanding of Laurentide meltwa- Ž.Fig. 2 and temporal variability in glacier plumbing ter drainage be best developed? The way forward Že.g., Sharp and Richards, 1996; Hubbard and lies in a dialog between three bodies of research.Ž. 1 Nienow, 1997. . Field research has included bulk Field studies of the hydrology of contemporary meltwater discharge and chemistry measurements, glaciers provide insight into the spatial and temporal borehole measurements, tracer studies and investiga- variability of glacier plumbingŽ e.g., Hubbard and tion of proglacial bedrock exposuresŽ Hubbard and Nienow, 1997.Ž. . 2 Theoretical glaciology describes Nienow, 1997. . It has been mostly conducted on and investigates physically possible drainage scenar- active, temperate alpine glaciers with steep ice sur- iosŽ.Ž. e.g., RothlisbergerÈ and Lang, 1987 . 3 Glacial face slopes that flow over bedrock or bedrock with a geomorphology seeks to identify, describe and ex- thin Ž.-1 m or patchy sediment cover Ž e.g., Brand plain the genesis of glaciofluvial landforms such as et al., 1987; Hubbard et al., 1995. and that terminate eskersŽ. e.g., Banerjee and McDonald, 1975 . subaerially. Comparable research on non-temperate Eskers are stratified ridges of sand and gravel that glaciers is currently lackingŽ. Hodgkins, 1997 , and are inferred to be the casts of subglacial channels soft-bed drainage scenarios are mainly theoretical as Ž.R-channels, Rothlisberger,È 1972 or reentrants into yetŽ. Alley, 1989, 1992; Walder and Fowler, 1994 . the ice frontŽ. Banerjee and McDonald, 1975; Fig. 2 . Field research on the subglacial hydrology of mod- It has been 45 years since Banerjee and McDonald's ern ice sheets is limited due to inaccessibility. Ž.1975 seminal paper on esker genesis. Since that Theory and observation suggest that temperate, time there have been significant developments in alpine glacier plumbing is often karstic in form glacier hydrologyŽ. below and in glacial sedimentol- Ž.Shreve, 1972 , and functions to route supraglacially, ogyŽ. e.g., Rust and Romanelli, 1975 . What can englacially and subglacially generated meltwater to eskers usefully tell us about the R-channel drainage the ice frontŽ. Fig. 2 . Supraglacial lakes may store of the Laurentide Ice Sheet? Do eskers deposited in water where surface ice is coldŽ. Hodgkins, 1997 . R-channels that terminated subaerially exhibit differ- Non-temperate ice is mainly drained by supraglacial ent morpho-sedimentary relations from those de- streams, but subglacial delivery of water via moulins posited in R-channels that terminated in standing Ž.Iken, 1972 and the passage of channels laterally water? Did such R-channels function differently? Do through non-temperate iceŽ Pfirman and Solheim, eskers record synchronous R-channel systems or are 1989. have also been inferred. The persistence of they composed of segments deposited time-trans- channels through cold ice may suggest eitherŽ. i that gressively in R-channels that developed close to the such pathways were inherited, perhaps from earlier ice front as the ice front retreated? What were the warm-ice conditionsŽ.Ž. Hodgkins, 1997 or ii that hydraulic processes responsible for esker formation? supraglacial meltwater input exceeds the threshold Does esker formation require supraglacial meltwater required to keep such channels open. The discussion input? What determined Laurentide esker pattern and below focuses on the more extensively researched distribution? Was the ice sheet regionally active or drainage of alpine, temperate iceŽ. Fig. 2 as a context stagnant at the time of esker formation? This contri- for understanding Laurentide glaciofluvial land- bution evaluates current knowledge of Laurentide forms. However, development of such landforms eskers in CanadaŽ. Fig. 1 in the light of develop- under non-temperate ice cannot be discounted and ments in glacier hydrology and glacial sedimentol- alpine glacier hydrology need not be directly analo- ogy. Developments in glacier hydrology are dis- gous to Laurentide Ice Sheet hydrology. cussed first. Water delivered by rainstorms or generated by surface melting of temperate alpine glaciers may percolate through snow and firn or flow as a sheet, 2. Spatial and temporal variability in glacier but has an incipient tendency to channelize, forming plumbing meandering supraglacial streamsŽ Parker, 1975; Fig. 2. . Surface water enters the glacier either through a A productive dialog between field hydrologists three-dimensional network of veinsŽ Nye and Frank, and glacial theorists has produced a picture of spatial 1973.Ž and tubes connecting to conduits Raymond T.A. BrennandrGeomorphology 32() 2000 263±293 267 and Harrison, 1975.Ž , or through crevasses Robin, velops spatially and temporally variable channelized 1974; RothlisbergerÈ and Lang, 1987. or moulins and distributed systemsŽ e.g., Hubbard and Nienow, ŽHolmlund, 1988; Holmlund and Hooke, 1983; Fig. 1997; Fig. 2. depending on such factors as water 2. . When such sink holes become water-filled the discharge, ice surface slope and thickness, ice veloc- density difference between ice and waterŽ Robin, ity, and the permeability, topography and rigidity of 1974; Weertman, 1974. , and the heat released by the the bedŽ. e.g., Arnold and Sharp, 1992 . Channels waterŽ. Hooke, 1989 can result in the propagation of form efficient drainage pathways, stable at relatively sink holes down through the glacier to the glacier low steady-state water pressures. In contrast, dis- bed. Englacial channels are thus formedŽ. Fig. 2 . tributed systems form inefficient drainage pathways, Water in such channels may initially drain to a stable at relatively high steady-state water pressures. distributed drainage system, but once a threshold discharge is exceeded it will drain to subglacial 2.1. Subglacial drainage on rigid beds channels when the glacier bed is rigidŽ Willis et al., 1990; Fig. 2. . The configuration of the subglacial drainage sys- ShreveŽ. 1972 has argued Ž. i that englacial chan- tem of temperate alpine glaciers flowing over a rigid nels form an upward-branching, arborescent system bed often takes the form of channels and films normal to upglacier-dipping equipotential surfaces andror linked cavitiesŽ e.g., Hubbard and Nienow, andŽ. ii that subglacial channels follow paths normal 1997.Ž . Channels may exhibit one of three forms Fig. to equipotential contoursŽ. Fig. 2 . He assumed that 2.Ž and may only drain a fraction of the bed Hubbard water pressure in such channels is similar to the and Nienow, 1997.Ž. . i R-channels Ž Rothlisberger,È pressure in confining ice and that the bed slope is 1972. are semi-circular channels cut upward into the less than ;11 times the ice surface slope. Equipo- ice. Their size and water pressure reflects a balance tential surfaces are defined as surfaces of equal between channel enlargement by viscous heating and water-pressure potential and equipotential contours closure by ice deformation when the channels are are defined as the intersection of the equipotential water-filled. Rothlisberger'sÈ theory predicts that surfaces with the bedŽ. Shreve, 1972; Fig. 2 . As steady-stateŽ. instantaneous heat transfer water pres- equipotential contours are mainly controlled by ice sures should be lower when discharges are higher surface slope and secondarily by bed topography, and consequently R-channels should form dendritic subglacial channels may also follow topographic networks. WalderŽ. 1982 has argued that supraglacial lows, be routed mid-way up valley sidesŽ Hooke, meltwater input is required for R-channels to be 1998, fig. 8.23.Ž. and cross bumps Shreve, 1972 . persistent at relatively high discharges.Ž. ii H-chan- However, during conditions of variable flow dis- nelsŽ. Hooke, 1984 are broad, flat channels cut up- charge, Shreve'sŽ. 1972 assumption of balanced wa- ward into the ice that tend to follow local bed slope. ter and ice pressure is unlikely to be valid. Under Such channels form where water flows at atmo- such conditions, the slope of the potentiometric sur- spheric pressure beneath thin ice and on steep faceŽ. as defined for each conduit; Fig. 2 or hy- downglacier bedslopes.Ž. iii N-channels Ž Nye, 1973 . draulic headŽ i.e., the elevation difference between are incised into bedrock, perhaps suggesting long- the zone of recharge at the surface and discharge at term channel stability under some alpine glaciers the portal. may be more important in controlling Ž.Walder and Hallet, 1979 . subglacial water flow. Rapidly increasing The presence of linked cavities and filmsŽ. Fig. 2 supraglacial meltwater discharge may cause the have been inferred from proglacial bedrock mapping potentiometric surface to become steeper than the ice Ž.e.g., Walder and Hallet, 1979 , slow dye returns surface slope because small channels throttle flow Ž.e.g., Willis et al., 1990 and high subglacial water ŽRothlisberger,È 1972, Fig. 5f; Menzies and Shilts, pressuresŽ. e.g., Kamb, 1987 . This distributed system 1996. . Consequently, water may flow to the ice is mainly responsible for transferring interchannel, margin even if the ice surface slope is flat. subglacially generated meltwaterŽ basal melt from Channels are only one of many subglacial drainage geothermal and viscous heating ;10y2 may1 ; configurations. The subglacial drainage system de- Paterson, 1994. , or temporarily trapped meltwater 268 T.A. BrennandrGeomorphology 32() 2000 263±293 from other sources, to channelsŽ e.g., Walder and ter is available throughout the year, but may increase Hallet, 1979; Willis et al., 1990. . Water films have in summer as ice motion increasesŽ Willis et al., been attributed to glacier sliding by regelation 1990.Ž . JokulhlaupsÈ e.g., Nye, 1976 . and cavity Ž.Weertman, 1957, 1964 . Water-filled cavities linked drainage eventsŽ. e.g., Kamb et al., 1985 can also by small channels form in the lee of bed bumps affect episodic variability in meltwater discharge. when glacier sliding velocity exceeds a critical value Where temperate, alpine glaciers terminate sub- Ž.Lliboutry, 1968 . aerially, drainage evolves annually as the melt zone expands and contractsŽ e.g., Willis et al., 1990; Richards et al., 1996. . This evolution involves com- 2.2. Subglacial drainage on ``soft'' beds petition and replacement of drainage modes. In the winter, distributed systemsŽ. Fig. 2 dominate sub- Where glaciers flow over permeable substrates, glacial drainage of water supplied by basal melt or either distributed drainageŽ. Walder and Fowler, 1994 storage releaseŽ. Willis et al., 1993 . During the melt or coexisting distributed and channelized drainage season, channels dominate drainage as supraglacial Ž.Alley, 1992 has been proposed. Water may drain meltwater input causes discharge to increase beyond through the voids between substrate clastsŽ Darcian a critical valueŽ. Willis et al., 1990 . Channels de- flow; e.g., Boulton and Jones, 1979. or through velop by coalescence of linked cavitiesŽ. Kamb, 1987 macroporesŽ Clarke et al., 1984; Murray and Clarke, or by downglacier extensionŽ. melt of individual 1995.Ž. . Such porous flow Fig. 2 has been inferred channel segmentsŽ Gordon et al., 1998; Nienow et from borehole measurements of water pressure and al., 1998. . The path of such water-filled channels is turbidity across subglacial channelsŽ e.g., Hubbard et mainly determined by ice surface slopeŽ Shreve, al., 1995; Gordon et al., 1998. . It is rarely sufficient 1972. . This path may persist despite flow at atmo- to evacuate all water flowing at the bedŽ Alley, spheric pressureŽ. in H-channels later in the melt 1989. . Where subglacial pore water pressure exceeds season because channel migration to bed lows may a critical value, dependent on substrate rheology and be limited by local back-pressure effects, the position water flux, the bed is considered to be ``soft'' and of the supraglacial sink point and the time available drainage may occur by advection in mobilized sedi- Ž.Sharp et al., 1993 . The upglacier expansion of the mentŽ.Ž. Alley et al., 1986 , a thin film Alley, 1989 or melt zone during the melt season gives the impres- canalsŽ broad, shallow, ``braided'' drainage path- sion that channel systems grow headwardŽ Gordon et ways; Walder and Fowler, 1994.Ž Fig. 2 . . Canals al., 1998; Nienow et al., 1998. . Diurnal pressure differ from N-channels in that they are proposed to fluctuations may drive porous flow into and out of be stable at high water pressures, and form by a channelsŽ. e.g., Hubbard et al., 1995 . In the fall, combination of bed deformation and fluvial erosion; channels are replaced by distributed systems; re- they have yet to be observedŽ Walder and Fowler, duced meltwater discharge allows ice deformation to 1994. . In contrast, coexisting low pressure channels constrict channels, eventually reversing the sub- and deforming beds have been reportedŽ e.g., Engel- glacial water pressure gradientŽ. e.g., Hooke, 1989 . hardt et al., 1978. and theoretically explained on the Channels may completely collapse in the winter grounds that substrate creep toward such channels is unless they remain water-filledŽ. Haefeli, 1970 . Such limited by substrate rheologyŽ. Alley, 1992 . collapse is most likely to start where ice is thickest, channel size smallest and meltwater discharge least Ž.Hubbard and Nienow, 1997 . Channels may remain 2.3. Temporal Õariability and eÕolution of glacier water-filled in winter ifŽ. i channel collapse Ž Hub- drainage bard and Nienow, 1997.Ž or sediment plugging Boul- ton and Hindmarsh, 1987. is initiated in a downflow The availability of supraglacially and englacially location,Ž. ii a threshold discharge persists through generated meltwater varies according to diurnal, sea- winter, perhaps supplied from stored sourcesŽ Willis sonal and annual melt cycles; the supply is unsteady et al., 1993.Ž. , or iii channels end in a standing water Ž.e.g., éstrem, 1975 . Subglacially generated meltwa- body, in which case they are hydraulically dammed T.A. BrennandrGeomorphology 32() 2000 263±293 269 and will remain water-filled to the level of the winter such a distinction cannot be easily made from mor- potentiometric surfaceŽ. e.g., Powell, 1990 . pho-sedimentary observations.

4. Classification of Laurentide eskers 3. Laurentide meltwater drainage systems Eskers have been mapped throughout the area covered by the Laurentide Ice SheetŽ Prest et al., It seems reasonable to expect that subglacial 1968; Fig. 1. . The longest Laurentide eskers are over drainage configurations of the Laurentide Ice Sheet 800 km longŽ. Craig, 1964 including gaps, and may included both distributed and channelized systems be nearly unbroken for up to 300 kmŽ Shilts et al., Ž.e.g., Alley, 1996; Fig. 2 , and that such systems 1987. . They attain heights of a few meters to over 50 were likely spatially and temporally variableŽ e.g., m at the land surface, but deposits in excess of 200 Arnold and Sharp, 1992. . Glacial geomorphology m may occur below the surfaceŽ Munro esker, On- provides one approach to elucidating this variability, tario; Banerjee and McDonald, 1975. . Eskers are because morpho-sedimentary observations must be generally less than 150 m in width, but may exceed explained by models of ice sheet hydrology. How- several kilometersŽ e.g., Banerjee and McDonald, ever, this approach is not without limitations; for 1975. . In planform, eskers can be single, sinuous, exampleŽ. i observations are mostly biased toward undulatory ridges with broad or sharp crestsŽ e.g., subglacial and proglacial drainage systems and Shreve, 1985. . Alternatively, they may exhibit peri- deglacial snap-shots of Laurentide Ice Sheet hydrol- odic enlargements or ``beads''Ž e.g., Banerjee and ogy,Ž. ii absolute chronological control is generally McDonald, 1975; Brennand, 1994. , or subdivide and lacking for subglacial landforms and sediments, and recombine downflowŽ anabranched planform, e.g., Ž.iii subglacial sedimentary evidence is often biased Shaw et al., 1989. . Some eskers are relatively iso- toward high magnitude flow events, but sediments in lated, others form integrated dendritic networks with proglacial water bodies contain a more complete up to fourth-order tributariesŽ. e.g., Shilts et al., 1987 . record of ice sheet drainage. Eskers may occur in association with both subaerial To date, most geologic and geomorphic corrobo- and subaqueous ice-marginal landform assemblages ration of subglacial distributed drainage systems has such as pitted subaerial outwash, subaerial or sub- been viewed as equivocalŽ e.g., Benn and Evans, aqueous fansŽ although not all subaqueous fans need 1998. . Roches moutonnees,Â with their abraded stoss be ice marginal, Gorrell and Shaw, 1991. , deltas and sides and plucked lee slopes, suggest drainage by end moraines, and glaciolacustrine and glaciomarine linked cavitiesŽ e.g., Lliboutry, 1968; Iken and Bind- sedimentsŽ. e.g., Shilts et al., 1987; Fig. 1b . They schadler, 1986. . Morpho-sedimentary observations often occur in association with subglacial landforms from drumlins have been interpreted as evidence of such as drumlins, rogen moraine, hummocky terrain both catastrophic sheet flowŽ. e.g., Shaw, 1996 and and tunnel channelsŽ e.g., Shilts et al., 1987; Shaw et advective flowŽ bed deformation; e.g., Boulton and al., 1989; Munro and Shaw, 1997. . Hindmarsh, 1987. . Linear mineral dispersal trains Banerjee and McDonaldŽ. 1975 chose a genetic may suggest advective flowŽ e.g., Dyke and Morris, classification of eskers which identified esker forma- 1988. . Tunnel channels with upslope thalwegs have tion in tunnelsŽ. R-channels and open channels Ž H- been attributed to canal drainageŽ Walder and Fowler, channels or reentrants into the ice front. . They em- 1994.Ž and subglacial deformation Boulton and phasized that these channels could terminate sub- Hindmarsh, 1987. , but catastrophic channel drainage aqueously or subaerially. Shilts et al.Ž. 1987 describe Ž.e.g., Wright, 1973 and more ``normal'' N-channel zonal assemblages of sediments and landforms, in- drainageŽ. Mooers, 1989 have also been proposed. cluding eskers, on the Canadian Shield. Menzies and Geomorphic corroboration of subglacial channelized ShiltsŽ. 1996 describe esker patterns associated with drainage systems has been less controversialŽ with North American ice sheets. The zones described the exception of tunnel channels; O'Cofaigh, 1996. . below and shown on Fig. 1a are based on these zonal Eskers may form in either R- or H-channels, but descriptions. 270 T.A. BrennandrGeomorphology 32() 2000 263±293

In zone 1 eskers are rare; these are regions designated as ice dividesŽ. Fig. 1; Shilts et al., 1987 . In zone 2Ž. Fig. 1a and generally in areas underlain by crystalline bedrock, esker density is highŽ esker spacing ;10 km. except in drift-poor areas. Most eskers form long, partially discontinuous, dendritic net- worksŽ. Fig. 3a which either Ž. i radiate away from regions designated as ice dividesŽ.Ž. Fig. 1a , or ii terminate at major arcuate morainesŽ. Fig. 1b . Gaps along eskers may be occupied by channels eroded into the basal substrateŽ Klassen, 1986; Thorleifson and Krystjanssen, 1993. . Short eskers occur in some places between major dendritic esker systems. In zone 3, esker density is low downflow from major arcuate moraines and in areas generally underlain by sedimentary bedrockŽ. Fig. 1 . Most eskers are short, forming subparallel or deranged patternsŽ. Fig. 3b,c , or are isolated. Several longer, dendritic esker systems are located in tunnel channelsŽ A in Fig. 1a; Brennand, 1994.Ž. . Zone 4 Fig. 1a encompasses regions of high relief, such as the Appalachians. Here, eskers are generally confined to major valleys but may cross drainage dividesŽ Rampton et al., 1984. . Some authors suggest that this esker pattern was substrate-controlled; eskers are present over rigid bedsŽ. zone 2 and rare over soft beds Ž.Ž zone 3 Clark and Walder, 1994; Walder and Fowler, 1994; Fig. 1a. . Other authors have noted that mapping of the complete esker pattern is likely confounded by later burial or reworking in glaciolacustrine or glaciomarine environmentsŽ e.g., Allard, 1974; Shilts et al., 1987; Fig. 1b.Ž. . Shilts et al., 1987 and Menzies and ShiltsŽ. 1996 have argued that sediment availability and changes in ice dynamics during the deglaciation cycle are causal factors; zone 3 eskers formed at an actively retreating ice margin and zone 2 eskers Fig. 3. A morphological esker classification. Each of these esker formed under regionally stagnant iceŽ. Fig. 1a . morphologies may have formed in R-channels terminating in This paper introduces a morphologic classification standing waterŽ. types I, II and III . Long, dendritic eskers may of Laurentide eskers in an attempt to reconcile the also have formed in R-channels terminating subaeriallyŽ. type IV . importance of both individual esker genesis and re- Deranged eskers lack regional alignment. gional esker pattern to an understanding of Lauren- tide R-channel drainage. Three esker morphologies are identified:Ž. i long, dendritic eskers, Ž ii . short, combinations have been reported in the literature:Ž. i subparallel eskers, andŽ. iii short, deranged eskers long, dendritic eskers which formed in R-channels Ž.Fig. 3 . Each of these esker morphologies may have that terminated in standing waterŽ.Ž. type I ; ii short, formed in channels which terminated subaqueously subparallel eskers which formed in R-channels or or subaeriallyŽ. Banerjee and McDonald, 1975 . Five reentrants that terminated in standing waterŽ. type II ; T.A. BrennandrGeomorphology 32() 2000 263±293 271

Ž.iii short, deranged eskers which formed in R-chan- Banerjee and McDonald, 1975; Dyke and Dredge, nels that terminated in standing waterŽ.Ž. type III ; iv 1989. . Morpho-sedimentary diagnostic criteria for long, dendritic eskers which formed in R-channels eskers formedŽ. i in extensive Ž long . , synchronous that terminated subaeriallyŽ.Ž. type IV ; and v short R-channels andŽ. ii as time-transgressive segments in eskers which formed in R-channels or reentrants that Ž.short R-channels or reentrants close to the ice front terminated subaerially. Different morpho-sedimen- are presented in Fig. 4. tary relationsŽ. below for types I, II and III suggest The basis for inferring extensive, synchronous that distinct drainage systems and glaciodynamic R-channels is elaborated below, based on observa- conditions are associated with each of these esker tions from four eskersŽ. 40±70 km long in south- types. Lack of observations and contradicting and central OntarioŽ. Brennand, 1994; A in Fig. 1a , and equivocal interpretations for types IV and V make one eskerŽ.Ž at least 300 km long in Quebec the the discussion of subaerially terminating R-channels Abitibi esker, or Harricana Moraine, Veillette, 1986; inconclusive. Brennand and Shaw, 1996; B in Fig. 1a.Ž. . 1 Esker ridges are relatively continuous. On surficial geology maps, long ridges are separated by short gaps, but 5. Eskers deposited in R-channels that terminated these ridges are not punctuated by subaqueous fans in standing water or deltas. Subaqueous fans do occur at the downflow ends of these eskers; some are superimposed over Eskers that terminate in subaqueous fansŽ identi- esker ridges near their downflow ends.Ž. 2 Paleoflow fied on morphologic and sedimentologic grounds. or variability measured from both sand structures and that exhibit subaqueous fans along their length most gravel fabrics at points along tens to hundreds of likely formed in R-channels which terminated in kilometers of esker ridge is invariably low; higher standing water. Often such landforms are transitional paleoflow variability is observed in subaqueous fans downflow into glaciolacustrine or glaciomarine sedi- at the downflow ends of eskers. This implies deposi- ments. Most of the research on Laurentide eskers in tion within a constrained environment, most likely an Canada has been concerned with such conditions. extensive, synchronous R-channel, terminating in 5.1. Long, dendritic eskers formed in R-channels that standing water. For low paleoflow variability to be terminated in standing water() type I recorded in a segmental esker, either the reentrant must have remained invariably narrow, or sub- 5.1.1. General depositional enÕironment aqueous fan formation at the ice front must have Type I esker systems are tens to hundreds of been inhibited; both conditions seem unlikely.Ž. 3 kilometers longŽ. Fig. 3a . Were such long eskers Regional trends in clast characteristics are observed. deposited in extensive, synchronous R-channel sys- In south-central Ontario, the increase in clast mean tems or were they deposited as time-transgressive roundness downflow along a 70-km long esker sug- segments in less extensive R-channels or reentrants gests relatively long transport distances, vigorous that developed close to the ice front as the ice front flows and low sedimentation rates. In Quebec, the retreated? This question is difficult to answer with upflow portion of the Abitibi esker is relatively certainty as determination of geomorphic and sedi- narrow; there is poor preservation of metabasalt mentary continuity are generally confounded by gaps clasts; most clasts are well rounded and paleoflow along esker ridges, esker burial or removalŽ for variability is low. High flow power and vigorous aggregates. , limited sedimentary exposures and the transport in a narrow R-channel are consistent with non-uniform nature of fluvial sedimentation. Where these observations. Downflow the esker is wider; research has been designed to answer this question, metabasalt clasts are better preserved; clasts are esker sedimentation in extensive, synchronous R- mainly subrounded and paleoflow variability is channels has been inferredŽ Brennand, 1994; Bren- higher. Assuming constant sediment supply along nand and Shaw, 1996. . This conclusion is contrary to the R-channel, this implies lower flow power, less the widespread assertion that long eskers were formed vigorous transport and higher sedimentation rates in successive segments time-transgressivelyŽ e.g., downflow. Together, these regional trends, over 272 T.A. BrennandrGeomorphology 32() 2000 263±293

Fig. 4. Morpho-sedimentary diagnostic criteria for determining the genesis of long, dendritic eskers which formed in R-channels or reentrants that terminated in standing waterŽ derived from Banerjee and McDonald, 1975; Shreve, 1985; Brennand, 1994; Brennand and Shaw, 1996. .

;250 km, are consistent with a synchronous R- 5.1.2. Hydraulic processes channel that increases in size downflow as the ice The building blocks of type I eskers are gravel sheet thins toward its margin. The presence of deep, faciesŽ. e.g., Brennand, 1994; Fig. 5; Table 1 . Sand proglacial lake Barlow-Ojibway likely also favoured faciesŽ massive, plane-bedded, cross-bedded, cross- higher sedimentation rates and reduced R-channel laminated and in-phase wave structures. alternate closure rates downflow in the Abitibi esker. with these, but generally form thin, discontinuous Extensive, synchronous R-channels terminating in bedsŽ. Brennand, 1994; Fig. 5c . Limited experimen- standing water must have been water-filled at least to tation has been reported on the sedimentary charac- the level of the potentiometric surface defined by the teristics expected for coarse sediment in pipe flow water body. This surface must have been steep in Že.g., Acaroglu and Graf, 1968; McDonald and Vin- summer to provide sufficient head to drive water to cent, 1972. . On the basis of sedimentary descrip- the ice front, often against topographic gradient tions, grain-size and imbrication measurements, sev- Ž.Brennand and Shaw, 1996 . Gaps along long eskers eral gravel facies have been identified and inter- formed under these conditions may be attributed to pretedŽ Fig. 5; Table 1; the basis for these interpreta- contemporaneous erosion or non-deposition in non- tions is presented in Brennand, 1994. . They record uniform conduitsŽ Shreve, 1972; Banerjee and Mc- deposition from powerful fluid flows and hypercon- Donald, 1975; Brennand, 1994. , as well as post-de- centrated dispersions. Gravel clasts range in size positional erosion and burialŽ. Allard, 1974 . from granules to boulders, and boulder facies are not T.A. BrennandrGeomorphology 32() 2000 263±293 273

Fig. 5. Gravel facies observed in type I eskers.Ž. a Heterogenous, unstratified gravel with poorly delineated lenses of bimodal, clast-supported, polymodal and openwork gravel, Campbellford eskerŽ. Brennand, 1994 . Grid is 1 m2 .Ž. b Heterogenous, unstratified gravel with imbricate clast clusters, Abitibi eskerŽ.Ž. Brennand and Shaw, 1996 . c Massive, imbricate, clast-supported gravel alternating with sand Ž.Ž.truncated at arrow , Tweed esker Brennand, 1994 . Meter rod for scale. Ž. d Imbricate, polymodal, matrix-rich gravel, Norwood esker Ž.Ž.Brennand, 1994 . e Grading in cross-bedded gravel, Abitibi esker Ž Brennand and Shaw, 1996 .Ž. . f Trough cross-bedded gravel, Tweed eskerŽ. Brennand, 1994 . See Table 1 for description and interpretation. Reprinted with permission from Elsevier Science. uncommon. Mean flow velocities required to move Shaw, 1996. . Non-uniform R-channel geometry is boulders up to 1 m in diameter may exceed 5 m sy1 implied by the undulatory crest-line and variable Ž.after Costa, 1983 . width of eskers. Some esker enlargementsŽ wider Gravel facies are arranged into ridge-scale bed- andror higher reaches. , interpreted as deltas in ear- forms or macroforms: composite, oblique-accretion lier literatureŽ e.g., Allard, 1974; Ringrose and Large, avalanche bed, and pseudoanticlinalŽ. Fig. 6 . Macro- 1977; Rondot, 1982; Lindstrom,È 1993. , may be re- forms are sedimentation zones that record powerful classified as macroforms. flows down geometrically non-uniform R-channels, Composite macroforms are architecturally com- the macroform style likely being controlled by R- plex, generally being composed of stacked gravel channel geometryŽ Brennand, 1994; Brennand and and sand faciesŽ. Fig. 6a; Table 1 . They may be 274 T.A. BrennandrGeomorphology 32() 2000 263±293 Ž . -axis parallel to flow direction and imbricate. a support mechanisms -axis imbricate; a p a i : clast b Ž. Ž. Ž. Ž. Ž b Ž. Ž . Ž. Ž. Ž. Ž. Ž. a . Ž. Ž. Ž. Ž. Ž. Ž. Ž. ungraded; framework supported; clusterimbrication with a tand b a i p apseudoanticlinal bed i geometry and high dips; tabular or with the development of cluster deposition during the waning bedforms stages of floods a p aor i pseudoanticlinal bed and geometry high dips; tabular massive in coarser units; polymodal,graded or longitudinally sorted beds; a t bbed i geometry dominant; tabular graded or weak normal orto-normal inverse- grading; a pb a i i ; tabular and or a lenticular bed t geometry clast-supported gravel occasionally with buoyancy; hinderedconvoluted laminae in sand; transport middle B Ð genous openwork gravel; settlingupper C Ð openwork gravelgrain size smaller progradation; longitudinal sediment composite macroform, associated a density interface at points sorting of during flow transport of deposition heterogeneous of gravel, suspended and load lee-side in return with bedform migration or macroform progradation expansion flow beneath a separation eddy -axis transverse to flow direction, a Ž. Ž. Ž . Ž. Modified from Brennand and Shaw 1996 . a t b i : clast a b Table 1 Gravel facies observed in type IFacies eskers Heterogeneous,unstratified gravel vaguely lenticular organization textural in Structural characteristics heterogeneous gravel; fluid turbulence; bed Main sediment longitudinal sediment sorting during facies Origin within of a facies composite tractional structure or transport; sorting associated pseudoanticlinal macroform; Interpretation Massive, imbricate,clast-supported massive; relatively bimodal and clast-supported; generally ungraded; fluid turbulence; bed deposition primarily from traction facies within a composite or transport with minor suspension and pseudoanticlinal macroform gravel pervasive imbrication with a t b i and saltation transport Plane-bedded gravel plane bedded, becoming moreImbricate, polymodal texturally polymodal although bed; fluid turbulence deposition primarily from tractionCross-bedded gravel graded foreset fluid beds: turbulence; facies within A a composite base macroform Ð bimodal, deposition fluid from turbulence; hyper-concentrated bed bedform migration or macroform may form in-phase wave structures large gravel dunes; facies within gravel some may be more matrix-rich; un- dispersive pressure; dispersion homogenous or hetero- associated with the establishment of T.A. BrennandrGeomorphology 32() 2000 263±293 275

Fig. 6. Models of macroform development in type I eskersŽ.Ž.Ž. after Brennand and Shaw, 1996 . a Incremental formation stages 1±3 of a composite macroform in an R-channel enlargement. Over time and with sedimentation the ice roof melts up and back.Ž. b Formation of oblique macroforms resulting in oblique-accretion, avalanche beds. These forms are related to the combined effects of separation vortices generated to the lee of the bedform and convergence scours. Alternating, oblique-accretion avalanche beds are expected where oblique macroforms migrate into R-channel enlargements.Ž. c Formation of pseudoanticlinal macroforms related to paired vortices of similar power in R-channel reaches of uniform geometry. Reprinted with permission from Elsevier Science. more than 10 m in height and over 1 km long. They and Shaw, 1996. . The location and sedimentary de- have gently sloping stoss-sidesŽ. 58±108 , and may tails of composite macroforms were most likely ini- have higher-angled lee slopes. Some composite tially controlled by local R-channel geometry and macroforms exhibit downflow and lateral fining. sediment supply, and later controlled by the interac- Morphologically, such macroforms are associated tion between macroform and R-channel geometries with esker enlargements. These characteristics have Ž.Fig. 6a . Oblique-accretion avalanche bed Ž. OAAB been used to infer that composite macroforms were macroforms exhibit large-scale cross-beds dipping formed at positions of R-channel enlargement and obliquely to the general flow directionŽ. Fig. 6b concomitant flow expansion within a synchronous, indicated by the esker axis and other bedforms. They non-uniform R-channelŽ Brennand, 1994; Brennand occur at esker enlargements. These macroforms 276 T.A. BrennandrGeomorphology 32() 2000 263±293 record alternating bars that likely developed down- nand, 1994; Brennand and Shaw, 1996. . Whereas flow from flow convergence scours under conditions some may be attributed to spatial changes in trans- of high flow powerŽ Brennand and Shaw, 1996; Fig. port and deposition along a non-uniform R-channel, 6b.Ž. . Pseudoanticlinal macroforms q.v. Stone, 1899 where laterally extensive they are more likely to be a exhibit broad, low-angled, arched or anticlinal beds product of abrupt temporal changes in flow compe- with crest-convergent, downflow gravel fabricsŽ Fig. tenceŽ. Brennand, 1994; Brennand and Shaw, 1996 . 6c. . These macroforms occur along geometrically Such unsteady flow may have occurred on daily, uniform, narrow reaches of esker ridges. These ob- weather-related, seasonal, annual or episodicŽ e.g., servations are most simply explained by the opera- jokulhlaupsÈ or other forms of storage release; Nye, tion of secondary currents or vortices of similar 1976; Willis et al., 1993. time scales. As varves are power within narrow reaches of a synchronous R- often transitional to subaqueous fans at the down- channelŽ. Fig. 6c; Shreve, 1972 . flow ends of type I eskers, it is most likely that Minor, subaqueous fan sediments may occur gravel±sand couplets record supraglacial input to alongside bends in type I eskersŽ. Brennand, 1994 . subglacial R-channelsŽ q.v. Banerjee and McDonald, Anabranched reaches and extended, hummocky zones 1975. . Supraglacial meltwater input is likely also are associated with some eskers of this typeŽ Shaw et necessary to maintain powerful flows down discrete al., 1989; Brennand, 1994. . These features contain R-channelsŽ. Walder, 1982 ; intrachannel meltwater, intensely folded and faulted sediment. They are best generated by geothermal and frictional heating or explained as depositional products of episodic flood including supraglacial water that has been stored, eventsŽ. Shaw et al., 1989; Brennand, 1994 . Such likely passes though an inefficient distributed system flood events were most likely the result of meltwater before entering R-channelsŽ Hubbard and Nienow, storage release, perhaps from supraglacial lakes or 1997. . subglacial or englacial water-bodiesŽ. Shaw, 1994 , The implications of supraglacial meltwater input but may have been triggered by seasonal melting and are important. The downflow transition of multiple rainstorms. During such high discharge events, water gravel±sand couplets in eskers to varves raises the pressure within the R-channel was sufficient to breach probability that esker sedimentation occurred over the high pressure seal at the R-channel margin or many years. Type I esker sediments are remarkably cause it to migrate outwardsŽ. Gordon et al., 1998 . undeformed, suggesting that R-channels did not close Three effects are proposed:Ž. i R-channel Ð cavity during esker deposition, despite the normal winter connection for subaqueous fan sediments;Ž. ii the tendencyŽ. q.v. Hooke, 1989 . R-channels may re- establishment of a broad zone of minor R-channels main water-filled in winter ifŽ. i channel collapse for anabranched reaches; orŽ. iii localized hydraulic Ž.Hubbard and Nienow, 1997 or sediment plugging lifting over a broad zone on either side of the Ž.Boulton and Hindmarsh, 1987 was initiated in a R-channel for extended, hummocky zonesŽ Bren- downflow location,Ž. ii a threshold discharge per- nand, 1994. . Decreased flow competence due to flow sisted through the winterŽ.Ž. Willis et al., 1993 , or iii expansion resulted in rapid deposition. Sedimenta- the channel terminated in a standing water body tion ceased when discharge declined and the high Ž.Powell, 1990; Fig. 7a . pressure seal to the R-channel margin was reestab- lished. Sediments were deformed during R-channel 5.1.4. Deglacial drainage and glaciodynamics asso- closure and ice-bed recoupling. Alternatively, ciated with type I esker systems anabranched reaches may be the product of sediment Although the above discussion focuses on particu- pluggingŽ. Shilts et al., 1987 or hydraulic damming lar eskers within extensive dendritic systems, it seems Ž.M. Sharp, 1998, personal communication of R- reasonable to propose that many type I eskers likely channels and their subsequent relocation. formed in synchronous, non-uniform R-channel networks. Such R-channels were likely maintained for 5.1.3. Hydrologic regime many years by powerful, unsteady flows delivered Vertically stacked, gravel±sand couplets are ob- mainly from supraglacial sources during the melt served in many type I esker exposuresŽ e.g., Bren- season, and by the hydraulic damming effects of T.A. BrennandrGeomorphology 32() 2000 263±293 277

Fig. 7. Plausible conditions for the formation of eskers in R-channels that terminated in standing water.Ž. a Type I eskers Ž dendritic . formed in extensive, synchronous R-channels draining regionally stagnant ice, time 1 and 2.Ž. b Type II eskers Ž subparallel . formed in short R-channels draining actively retreating ice.Ž. c Type III eskers with fans formed in short R-channels draining to interior lakes in regionally stagnant ice.Ž. d Type III eskers without fans formed in short R-channels draining to tunnel channels in regionally stagnant ice. Note,Ž. a is illustrated at a much smaller scale thanŽ.Ž. b , c and Ž. d . See text for further explanation. 278 T.A. BrennandrGeomorphology 32() 2000 263±293 large proglacial water bodies in the winterŽ Figs. 1b local ice flow toward R-channels and explains both and 7a. . esker-convergent striaeŽ. Veillette, 1986 and obser- Was the Laurentide Ice Sheet regionally active or vations of clasts derived from areas lateral to type I regionally stagnant during the operation of such ex- eskersŽ. Brennand and Shaw, 1996 ; these observa- tensive, synchronous R-channel systems? By ``re- tions do not necessitate regionally active ice. Lateral gionally active'' I mean that ice flow was driven by transport of sediment into an R-channel is likely to a regional ice surface slope. By ``regionally stag- occur so long as ice exceeds a thickness of ;50 m nant'' I mean that the ice sheet had a flat ice surface Ž.q.v. Nienow et al., 1998 . and flow occurred only locally toward channels or Is there independent evidence of regionally stag- was induced at the ice front by calving. Shreve nant ice? Many type I eskers are located upflow Ž.1972, 1985 argues for the synchronous operation of from major arcuate moraines and converge on so- extensive, dendritic R-channel systems under slug- called interlobate morainesŽ. Fig. 1b . Ice-frontal gish ice. He defines ``sluggish'' as a condition where landforms such as end moraines are generally lack- ice flowed forward everywhere driven by a gentle ing upflow from arcuate morainesŽ. Prest et al., 1968 . ice surface slope, yet flowed more slowly than pre- This observation is consistent with regional ice stag- sent-day ice sheetsŽ. Shreve, 1985, p. 644 . He sug- nationŽ. Shaw, 1996 , but has also been attributed to gests that R-channel path was primarily determined rapid, active ice retreatŽ. Veillette, 1986 . Active ice by ice surface slope and secondarily by bed topogra- retreat is difficult to rationalize with the preservation phy. However, the underlying assumption of his of extensive and complex esker systemsŽ Shilts et al., analysis, that water pressure approximates ice confin- 1987. . ing pressure, is unlikely to be valid for conditions of How could regionally stagnant ice have devel- unsteady discharge as inferred from type I eskers. In oped? What is the significance of major arcuate contrast, RothlisbergerÈ Ž. 1972, p. 200 states that moraines? Why do eskers appear to radiate out to- R-channels are likely to continuously change their ward them and converge on `ìnterlobate'' positions? position under active ice, a condition not conducive Arcuate moraines have been inferred to record the to preserving intricate dendritic esker systems that terminal positions of ice streams or surging lobes required R-channel stability for many years. Such Ž.e.g., Dyke et al., 1989 . This conclusion is mainly preservation seems more likely if the R-channel sys- based on the radiation of streamlined forms and tems developed under regionally stagnant iceŽ q.v. eskers out toward arcuate moraines and their conver- Mannerfelt, 1981. . Under these conditions the den- gence on `ìnterlobate'' positions. Some arcuate and dritic pattern may have resulted from some combina- interlobate moraines also enclose thick sequences of tion of the pattern of sink points, water pressure exotic tillŽ. e.g., Shilts et al., 1987 . An assumption is gradients between large and small R-channels that streamlined forms were formed during surging Ž.Rothlisberger,È 1972 , and bed topography Ž Bren- by a subglacial deformation processŽ Boulton and nand, 1994. . Hindmarsh, 1987.Ž. . However, 1 the alignment of The confining pressure of the ice and the input of streamlined forms is often consistent upflow and water from supraglacial sources together determine downflow of arcuate morainesŽ. Prest et al., 1968 . the hydraulic gradient and the potentiometric surface Ž.2 Streamlined forms converge on but do not cross within an ice sheetŽ. Rothlisberger,È 1972 . Where `ìnterlobate'' positionsŽ.Ž. Brennand et al., 1996 . 3 supraglacial meltwater input is high and increasing, Some interlobate moraines are eskersŽ e.g., Ringrose the potentiometric surface may become steeper than and Large, 1977; Kaszycki and Dilabio, 1986; Bren- the ice surface slope, as the channel system lacks the nand and Shaw, 1996.Ž. . 4 Eskers are mainly a capacity to efficiently drain surface waterŽ. Fig. 7a . passive record of ice dynamics and morphology. It Consequently, water could be routed through the seems more likely that esker convergence on `ìnter- R-channel system to the ice front even though the ice lobate'' positions and arcuate moraine geometry are surface was flatŽ. Rothlisberger,È 1972, Fig. 5f . The a consequence of events that formed the regional tendency for wall melt to be balanced by creep streamlined fields, rather than coeval with them closure in R-channelsŽ. Rothlisberger,È 1972 causes Ž.Brennand and Shaw, 1996 . Synchronous formation T.A. BrennandrGeomorphology 32() 2000 263±293 279 of streamlined forms by catastrophic meltwater floods Ž.zone 1, Fig. 1a and resulted in the formation of is consistent with current observations for these areas numerous supraglacial lakes so long as near-surface Ž.Shaw, 1996 . Vigorous basal melt and ice sheet ice remained coldŽ. Hodgkins, 1997 . Heat from extension during these ice-bed decoupling events supraglacial meltwater and the density differences would likely have resulted in an intensely crevassed, between water and ice caused moulins and crevasses thin, flat ice sheetŽ. Shoemaker, 1992a,b with local to propagate downward, forming englacial channels saddles along axes of flood path convergenceŽ Bren- and finally connecting those channels to the bed nand and Shaw, 1996. . Esker convergence may Ž.Weertman, 1973, 1974 . Extensive crevassing may record such saddles. Thinner ice at these saddles and have been inherited from earlier meltwater flood the thermomechanical erosion of later meltwater fun- eventsŽ. Shoemaker, 1992a,b . At the bed, meltwater neling below them may have resulted in reentrants in may have connected to a distributed system early in the ice front and an overall lobate appearance to it the melt season. As discharge increased over the Ž.Shaw, 1996 . Large eskers may have formed be- melt season, an integrated R-channel system likely neath these saddlesŽ. Brennand and Shaw, 1996 ; developed by cavity coalescenceŽ. Kamb, 1987 and such eskers have been interpreted as interlobate downglacier channel extensionŽ Gordon et al., 1998; moraineŽ. e.g., Shilts et al., 1987 . Nienow et al., 1998. . These R-channels likely grew Laurentide arcuate moraines are mainly composed headward as the melt zone expanded up-ice over the of subaqueous outwash sediments, occasionally with melt seasonŽ. Gordon et al., 1998 . Summer flow in proximal till depositsŽ. Dubois and Dionne, 1985 ; this integrated drainage system was driven by a steep they mainly record grounding-line positionsŽ Hil- hydraulic gradientŽ. potentiometric surface, Fig. 7a . laire-Marcel et al., 1981; Sharpe and Cowan, 1990. . Episodic drainage of supraglacial lakes through these These grounding-line deposits may record a re-equi- systems may have been responsible for some libration of the potentiometric surface within the ice gravel±sand couplets in type I eskersŽ. Shaw, 1994 sheet, precipitated by a drop in proglacial water level and for anabranched reaches, subaqueous fans at ŽHillaire-Marcel et al., 1981; Sharpe and Cowan, bends and hummocky, extended zonesŽ Brennand, 1990. . However, the similarity of the configuration 1994. . The proglacial water body kept R-channels and occurrence of Laurentide arcuate moraines with open in the winter at least to the level of the poten- those of the Eurasian ice sheetŽ. Punkari, 1997 is tiometric surface defined by the water bodyŽ Fig. striking. The SalpausselkaÈ moraines mainly record 7a. , and permitted multi-year operation of the R- grounding linesŽ. Fyfe, 1990 of Younger Dryas age channels. If the proglacial water body was deep, Ž.Rainio et al., 1995 . Younger Dryas cooling was back-pressure effectsŽ. Shoemaker, 1992b on the ice likely a global eventŽ. Peteet, 1995 . The Hartman mass may have caused ice to float and a grounding Moraine is of late Younger Dryas age Ž;10,400 line to developŽ. Brennand and Shaw, 1996 . Such B.P.; Sharpe et al., 1992; Fig. 1b. . Should many of conditions favour both the formation of subaqueous the Laurentide arcuate morainesŽ excluding the fans superimposed on esker ridges as the grounding Sakami moraine; Hillaire-Marcel et al., 1981. be of line vacillatedŽ. Brennand and Shaw, 1996 , and calv- similar age, they may record Younger Dryas grounding at the ice frontŽ. Fig. 1a; Veillette, 1986 . Calving ing-line re-equilibration positions. Following the may have caused some reactivation of ice close to Younger Dryas, continued climatic warming coupled the ice margin. with thin, flat iceŽ. Shaw, 1996 may have effected the regional ice stagnation which seems most likely 5.2. Short eskers formed in R-channels or reentrants for type 1 esker systemsŽ. zone 2 in Fig. 1a . that terminated in standing water() types II and III Plausible conditions for type 1 esker formation are presented in Fig. 7a. A climatically forced re- 5.2.1. General depositional enÕironment gional melt zone caused ice downwasting and pro- These eskers are tens of meters to tens of kilome- duced high volumes of supraglacial meltwater. On ters long, and exhibit subparallelŽ. type II eskers and flat, stagnant ice this melt zone may have extended derangedŽ. type III eskers regional patterns Ž Fig. well toward the regions designated as ice divides 3b,c.Ž.Ž . They are composed of i a short ridge south- 280 T.A. BrennandrGeomorphology 32() 2000 263±293 ern Alberta, J. Shaw, 1998, personal communication. , McDonald, 1975; Sharpe, 1988; Gorrell and Shaw, Ž.ii a short ridge with a subaqueous fan or a fan 1991; Figs. 8 and 9.Ž. , or iii alternating ridges and complex extending from, or superimposed over, the fansŽ. Banerjee and McDonald, 1975 . The discussion ridgeŽ southern Ontario and Quebec; Banerjee and below draws mostly on research from subparallel or

Fig. 8. Formation of a type II esker-fan sequence.Ž. a Formation of an esker ridge in an R-channel. Ž. b Ice retreat, ridge collapse and superimposition of subaqueous fan sediments.Ž. c Glaciomarine sedimentation and littoral reworking. Diagrams modified from Sharpe Ž.1988; original drawings by R. Gilbert for the Twin Elm esker, deposited in an R-channel that terminated in the Champlain Sea. T.A. BrennandrGeomorphology 32() 2000 263±293 281

Fig. 9.Ž. a Type II esker±bead±fan complex, Lanark, Ontario. Ž. b At a grounding line close to flotation, small increases in subglacial water pressure result in hydraulic lifting and the formation of lateral subglacial fansŽ. modified from Gorrell and Shaw, 1991 . Reprinted with permission from Elsevier Science. isolated eskers in southern Ontario and Quebec son, 1988; Rust, 1988; Rust and Romanelli, 1975; ŽBanerjee and McDonald, 1975; Saunderson, 1977; Sharpe, 1988; Gorrell and Shaw, 1991; Spooner and Cheel, 1982; Cheel and Rust, 1982, 1986; Hender- Dalrymple, 1993. . Deranged eskers are most clearly 282 T.A. BrennandrGeomorphology 32() 2000 263±293 observed in southern AlbertaŽ. Shetsen, 1987 , but surface and form deltasŽ. Martini, 1990 . The chan- detailed morpho-sedimentary observations are lack- nels supplying meltwater and sediment to the delta ing. thus became subaerial and may have taken the form Short esker ridges differ in sedimentary detail and of H-channelsŽ. Hooke, 1984 close to the margin. thus depositional environment. Some are dominated Some esker sediments were subsequently modified by gravel facies, exhibit low paleoflow variability, by wave action as the level of the proglacial water lateral faulting, proximal folding, and minor draped body fellŽ Fig. 8; Matile, 1984; Veillette, 1986; or intercalated diamicton bedsŽ Fig. 8; McDonald Sharpe, 1988. . and Shilts, 1975; Henderson, 1988; Sharpe, 1988; Occasionally, the ridge-subaqueous fan sequence Gorrell and Shaw, 1991. . Diapirs and upslope pale- appears to repeat upflowŽ Banerjee and McDonald, oflows in eskers draped over hummocky terrain in 1975; Ringrose, 1982; Henderson, 1988; Spooner the Prairies suggest that they were subglacial forms and Dalrymple, 1993. . In this case, either the esker Ž.Stalker, 1959, 1960; Munro and Shaw, 1997 . Short formedŽ. i in time-transgressive segments in R-chan- esker ridges that terminate in subaqueous fans where nels close to the ice front as the ice front retreated deposited in water-filled R-channelsŽ Banerjee and Ž.Ž.Banerjee and McDonald, 1975; Fig. 4 , or ii un- McDonald, 1975; Henderson, 1988; Gorrell and systematically in R-channels at a fluctuating ground- Shaw, 1991. . It has been suggested that water-filled ing line where the ice front was close to flotation R-channels may also be inferred from the presence Ž.Gorrell and Shaw, 1991; Fig. 9 . of a sliding bed gravel faciesŽ Saunderson, 1977; Ringrose, 1982.Ž , whereas open channel flow flow at 5.2.2. Hydraulic processes atmospheric pressure. was assured by the presence Esker ridgesŽ. cores are mainly composed of mas- of antidunesŽ. e.g., Banerjee and McDonald, 1975 . sive to cross-bedded gravelŽ. pebble±boulder with The logic linking sedimentary descriptions of the low paleoflow variabilityŽ Saunderson, 1977; Hen- sliding-bed facies to its hydraulic interpretation is derson, 1988. . Clasts may exhibit b-ora-axis imbri- incompleteŽ. q.v. Saunderson, 1977; Brennand, 1994 . cation, suggestive of powerful traction or transport as Furthermore, antidunes may form at a range of den- a hyperconcentrated dispersion, respectivelyŽ Rust, sity interfacesŽ. e.g., Hand, 1974; Table 1 . Conse- 1988; Rust and Romanelli, 1975; Gorrell and Shaw, quently, sedimentary facies are not good diagnostic 1991. . For the Guelph esker, southern Ontario indicators of depositional environment, yet they must Ž.Saunderson, 1977 and the Windsor esker, Quebec be consistent with it. Some short esker ridges are Ž.Banerjee and McDonald, 1975 powerful flow is composed of faulted, sandy subaqueous fan sedi- further suggested by the likely presence of macro- ments or offlapping subaqueous fansŽ Rust and Ro- formsŽ. q.v. Brennand, 1994 . Lower meltwater dis- manelli, 1975; Cheel, 1982. . These eskers were de- charge in winter may be indicated by the presence of posited in laterally constrained environments, most diamicton beds and lensesŽ. Gorrell and Shaw, 1991 . likely reentrants into the ice frontŽ Saunderson, 1975; Most of the sedimentary facies and facies associa- Cheel, 1982; Cheel and Rust, 1982. . tions described from short eskers formed in R-chan- Subaqueous fans record ice-frontal or grounding- nels that terminated in standing water actually record line sedimentationŽ e.g., Gorrell and Shaw, 1991; grounding-line or subaqueous fan sedimentation Fig. 9. . Flow decelerated as channelized meltwater Ž.Rust and Romanelli, 1975; Powell, 1990 . This may exited the portal and entered a standing water body. be superimposed over, or extend from, the gravelly This is confirmed by downflow fining, increased esker coreŽ. Sharpe, 1988; Gorrell and Shaw, 1991 . paleoflow variability, and changes in sedimentary In general, the sediments in subaqueous fans fine, structures generally consistent with weakening flow. and exhibit lower-flow-regime structures and higher The upward-fining of subaqueous fan sediments in- paleoflow variability, radially outward from the fan dicates decreased flow intensity due to either local apexŽ Cheel, 1982; Gorrell and Shaw, 1991; Spooner ice retreatŽ. e.g., Sharpe, 1988; Sharpe et al., 1992 or and Dalrymple, 1993. . These characteristics are due a change in portal location. Prolonged ice front to flow expansion and deceleration away from the stability allowed some fans to build to the water R-channel portal; traction transport is replaced by T.A. BrennandrGeomorphology 32() 2000 263±293 283 deposition from suspension downflowŽ Brennand and plumes to the sea surface, thence laterally as over- Sharpe, 1993. , and ultimately glaciolacustrine or flow away from the glacier front. It settles back glaciomarine rhythmites replace and onlap sub- through the water column, generally forming mas- aqueous fan depositsŽ Banerjee and McDonald, 1975; sive beds when plume velocities fall below that Henderson, 1988. . High rates of deposition from critical to counteract grain weight. Suspension set- suspension are inferred from thick sand beds and tling of silt and clayŽ. generally flocculated is con- dewatering structuresŽ Cheel and Rust, 1982, 1986; trolled by the dynamic interaction between the Brennand and Sharpe, 1993. . Despite these generali- oceanward plume movement and migrating tidal ties, the sedimentary character of any one fan may be wedge over the tidal cycle; rhythmically laminated determined by:Ž. i the relative density of the effluent cyclosams and cyclopels are depositedŽ Powell, jetŽ. sediment load, temperature and ambient water 1990. . In Laurentide glaciomarine environments bodyŽ.Ž. sediment load, temperature, salinity , and ii around R-channel portals, the effluent jet was likely unsteady meltwater discharge and sediment transport more dense than the ambient seawaterŽ sediment Ž.Ž.discussed below Powell, 1990 . Consequently, concentration greater than 35 g ly1 . and turbid un- subaqueous fan sediments generally show no strong derflows likely carried coarser sediment further from vertical textural or structural arrangementŽ Gorrell the portal than in modern glaciomarine environments and Shaw, 1991. . Ž.Gilbert, 1983 . R-channels terminating in saline wa- In glaciolacustrine or brackish environments the ter have been proposed from the presence of Hi- high sediment load carried by the effluent jet renders atella arctica Ž.Sharpe, 1988 , although this was likely it relatively more dense than the ambient water and a later colonizerŽ R. Gilbert, 1998, personal commu- consequently underflows are commonŽ Figs. 8 and 9; nication. . The flocculating effect of saline water may Sharpe, 1988; Gorrell and Shaw, 1991; Spooner and have been responsible for thick clay beds in fine- Dalrymple, 1993. . Proximal imbricate gravel is tran- grained esker ridges and fans on Victoria Island sitional downflow to trough cross-bedded gravel Ž.Brennand and Sharpe, 1993 . Ž.pebbles then massive to cross-laminated sand often with dropstonesŽ. interchannel sands of Rust, 1988 . 5.2.3. Hydrologic regime Bouma sequences record turbidity flowsŽ Brennand Vertical variations in sedimentary structures and and Sharpe, 1993. . Mid-fan sands may exhibit textures in short esker ridges and subaqueous fans trough-shaped scours filled with diffusely graded imply fluctuations in meltwater discharge and sedi- sandŽ. channel sands of Rust, 1988 . These structures ment load over time. Short esker ridges may contain may record channels filled with sediment flows gen- several gravel±sand coupletsŽ Banerjee and McDon- erated by slump failuresŽ. Rust, 1977, 1988 or local ald, 1975; Gorrell and Shaw, 1991. . Fans are mainly scour and deposition beneath supercritical sub- composed of stacked packages of fining-upward merged jets at hydraulic jumpsŽ Fig. 9; Saunderson, sand±silt±clayŽ Henderson, 1988; Gorrell and Shaw, 1977; Gorrell and Shaw, 1991. . Fan facies may pass 1991; Brennand and Sharpe, 1993. . Seasonal fluctua- laterally into glaciolacustrine rhythmitesŽ Henderson, tions in meltwater discharge and sediment load are 1988. . Many authors provide extensive descriptions inferred from clay caps and gravel±sand couplets, of the subaqueous fan facies and facies associations and diurnal and weather-related events are invoked of short eskers, for example CheelŽ. 1982 , Cheel and for rhythmic textural and structural variations be- RustŽ. 1982, 1986 , Henderson Ž. 1988 and Gorrell tween clay caps in fansŽ Henderson, 1988; Gorrell and ShawŽ. 1991 . and Shaw, 1991; Brennand and Sharpe, 1993; In modern glaciomarine environments the effluent Spooner and Dalrymple, 1993. . Such unsteadiness in jet may be less dense than the ambient sea water, and meltwater discharge and thus sediment transport and buoyant forces create turbid plumesŽ. Powell, 1990 . deposition requires a supraglacial meltwater input to Gravel is generally deposited very close to the portal, the subglacial drainage system. This input is con- may build to form a barchanoid dune and may be firmed where esker gravels are traceable downflow subject to mass movement due to oversteepening into rhythmitesŽ Fig. 8; Banerjee and McDonald, Ž.Powell, 1990 . Sand is carried by rising turbid 1975; Sharpe, 1988. . 284 T.A. BrennandrGeomorphology 32() 2000 263±293

Unsteadiness in meltwater discharge is coupled to front or a grounding line. Sedimentation was by variability in subglacial water pressure and results in unsteady flows delivered mainly from supraglacial sedimentary complexity at grounding lines where the sources. Why are these eskers short? Short eskers ice is close to flotation. This situation is well docu- may form whenŽ. i there is an insufficient supply of mented by Gorrell and ShawŽ. 1991 for an esker± supraglacial meltwater to maintain long R-channels bead±fan complex in southern OntarioŽ. Fig. 9 . High Ž.Ž.Walder, 1982 , ii the supraglacial catchment area discharge and water pressure events locally breached of sink holes is constrained by the ice surface topog- the high pressure seals at the margin of an R-chan- raphy,Ž. iii sink holes Ž crevasses and moulins . , which nel; this occurred preferentially at bends where water allow supraglacial water access to the glacier bed, pressure was highestŽ. Streeter and Wylie, 1979 . are limited to positions close to the ice frontŽ q.v., Lateral cavities, perhaps part of a distributed drainage Nienow et al., 1998.Ž. , iv cold, non-crevassed ice system, became temporarily connected to the R- behind a narrow ice marginal zone prevents channel by small distributary and contributory chan- supraglacial meltwater from reaching the bed,Ž. v the nelsŽ. Fig. 9 . Flow escaping from the R-channel ice is active and tends to cause hydrologic switching expanded in the cavities, resulting in subglacial fan Ž.Ž.Rothlisberger,È 1972 , or vi there is limited sedi- sedimentsŽ. beads : pronounced downflow fining and ment supply for esker building. Different combina- structural change are consistent with rapid flow ve- tions of these factors may be responsible for the locity reduction and loss of competenceŽ. Fig. 9 . development of short, subparallelŽ. type II and short, Ice-bed recoupling when discharge and water pres- derangedŽ. type II esker patterns. sure declined resulted in faulting and overfolding in Type II eskers are often few in number but re- bead sediments, confirming a subglacial depositional gionally alignedŽ. Fig. 3b; Rust, 1988 . Such eskers environment. Distally, a series of overlapping fans generally follow level or downslope paths and val- emanate from anastomosing minor ridges lateral to leysŽ Gorrell and Shaw, 1991; Spooner and Dalrym- the main esker ridgeŽ. Fig. 9 . The fans display ple, 1993. . They are common in the Ottawa±King- complex sediment sequences suggestive of highly ston area of southern OntarioŽ C in Fig. 1a; Rust and variable deposition in time and space at a grounding Romanelli, 1975; Cheel, 1982; Cheel and Rust, 1982; line close to flotation. The fans are considered to be Henderson, 1988; Rust, 1988; Sharpe, 1988; Gorrell subglacial as they show limited development of col- and Shaw, 1991; Spooner and Dalrymple, 1993. . lapse structures, occupy a position lateral to the main The regional alignment of type II eskers and their ridge, and exhibit thrust faulting in their upper beds. preferential location in valleys suggests that the R- A similar hydraulic-lift mechanism has been invoked channel path was controlled by a regional ice surface to explain extended deposits and fans lateral to a slopeŽ. Shreve, 1972 or an upglacier-rising potentio- longer esker ridge in the low ArcticŽ Brennand and metric surfaceŽ. Rothlisberger,È 1972 and bed topog- Sharpe, 1993. . This dynamic valve mechanism at a raphyŽ. Shreve, 1972 . Alternating esker ridge±sub- grounding line may also be applicable to other beaded aqueous fan sequences suggest systematic, active ice eskers thus far explained by the segmental, time- retreatŽ. Banerjee and McDonald, 1975 , driven by a transgressive modelŽ e.g., Banerjee and McDonald, regional ice surface slope and perhaps facilitated by 1975; Henderson, 1988; Fig. 4. ; they exhibit similar calving at the ice front. The sparsity of end moraines sedimentary characteristics such as pronounced associated with type II eskers may suggest that re- downflow fining, folds, faults and high paleoflow treat was very fastŽ. q.v. Veillette, 1986 . Active ice variability. Where R-channels terminated in a marine retreat necessitates a negative mass balance, with the environment, tidal forcing may have had similar dominance of melting over accumulation. Such con- effects. ditions favour a regional melt zone, yet eskers are short. Under such conditions esker length and num- 5.2.4. Deglacial drainage and glaciodynamics asso- ber may have been inhibited byŽ. i the location of ciated with type II and III eskers sink holes only close to the ice front, a condition Type II and III eskers formed in subaqueously perhaps resulting from extensional flow behind a terminating R-channels or reentrants close to the ice calving margin,Ž. ii the tendency for R-channels to T.A. BrennandrGeomorphology 32() 2000 263±293 285 close and relocate under active iceŽ Rothlisberger,È vicinity of interior lakes. In contrast, type III eskers 1972; Shilts et al., 1987.Ž , iii . the presence of cold lacking subaqueous fans may have formed in R- ice upflow from a narrow marginal zone, orŽ. iv channels connected to broader tunnel channels in sediment supply. On balance, type II eskers were regionally stagnant iceŽ. Fig. 7d . Higher subglacial most likely formed in short R-channelsŽ or reen- water pressures in R-channels than in tunnel chan- trants. under active ice exhibiting a regional ice nels would have cause R-channels to become con- surface slope. Some were likely formed in time- nected to tunnel channels. Eskers formed in such transgressive segments as the ice front or grounding R-channels may lack subaqueous fans as sediment line retreated during deglaciation. Plausible condi- debouched from R-channels may have been trans- tions for the formation of type II eskers are presented ported away by meltwater flowing down tunnel in Fig. 7b. channelsŽ. J. Shaw, 1998, personal communication . Type III eskers lack regional alignmentŽ. Fig. 3c , may be few in number and may follow paths irre- 6. Eskers deposited in R-channels that terminated spective of bed topographyŽ Stalker, 1959, 1960; subaerially Munro and Shaw, 1997. . They are common in southern AlbertaŽ. Shetsen, 1987; D in Fig. 1a . Here Eskers that end in subaerial outwash or deltas subaqueous fans may or may not be presentŽ Stalker, ŽShilts, 1984; Aylsworth and Shilts, 1989; Thorleif- 1960; J. Shaw, 1998, personal communication. . son and Krystjanssen, 1993. or those that are located The lack of a regional alignment in type III eskers in regions which exhibit little or no independent argues against R-channel path being determined by a evidence of the presence of standing waterŽ e.g., no regional ice surface slope on active iceŽ Shreve, rhythmic glacial lake sediments or raised beaches. 1985. . It seems more likely that R-channel path was were most likely deposited in R-channels that termi- determined by local potentiometric surfaces or hy- nated subaerially. Associations between eskers and draulic head in stagnant iceŽ. Fig. 7c,d . Stagnant ice subaerial outwash have been reported for modern is also consistent with a lack of associated end eskersŽ Gustavson and Boothroyd, 1987; Syverson et moraines on the PrairiesŽ. Shetsen, 1987, 1990 , and al., 1994. . Subaerially terminating R-channels have the meltwater model of sub-ice landscape evolution been inferred forŽ. i some long, dendritic esker sys- proposed for that regionŽ. Rains et al., 1993 . Two tems in the Mackenzie±Keewatin areaŽ Shilts, 1984; plausible conditions for the formation of type III Shilts et al., 1987; St-Onge, 1984. and the Beard- eskers are presented in Fig. 7; Fig. 7c shows condi- more±Geraldton areaŽ Thorleifson and Krystjanssen, tions that may have formed short eskers with fans, 1993.Ž zone 2 in Fig. 1a . , and Ž. ii several short, and Fig. 7d shows conditions which may have formed isolated eskers in southern OntarioŽ Banerjee and short eskers without fans. Interior lakes may have McDonald, 1975; Saunderson, 1975; zone 3 in Fig. developed within regionally stagnant ice in an exten- 1a. . In general though, subaerially terminating R- sive melt zone during deglaciationŽ. Fig. 7c . The channel systems have rarely been inferred from Lau- catchment area for these lakes would have been rentide eskers. This situation may reflectŽ. i that such determined by ice surface topography. The paths of systems rarely existed,Ž. ii that eskers were rarely R-channels draining to such lakes would have been produced in such systems,Ž. iii that eskers produced determined by local ice surface slopes and potentio- in such systems are difficult to identifyŽ Banerjee metric surfaces. Such R-channels are likely to form a and McDonald, 1975.Ž. , or iv that these systems deranged pattern when viewed at a regional scale. existed in regions where research is generally lack- Subaqueous fans formed downflow of R-channel ing. portals in interior lakes. Esker length was most likely 6.1. Long, dendritic eskers formed in R-channels that limited by the supraglacial catchment area for inte- terminated subaerially() type IV rior lakes and sediment supply. Debris-poor ice on the Prairies is consistent with the pristine preserva- 6.1.1. General depositional enÕironment tion of subglacial landforms thereŽ Rains et al., Type IV esker systems are tens to hundreds of 1993. . Local ice activity may have occurred in the kilometers long and may be unbroken for up to 75 286 T.A. BrennandrGeomorphology 32() 2000 263±293 kmŽ.Ž. Shilts et al., 1987 Fig. 3a . Were such long to the conditions of esker formation. However, Gus- eskers deposited in extensive, synchronous R-chan- tavson and BoothroydŽ. 1987, Fig. 3 describe a nel systems or were they deposited as time-transgres- situation at the margin of Malaspina Glacier where sive segments in less extensive R-channels or reen- subaerial outwash deposits are superimposed on an trants that developed close to the ice margin as the esker, yet were supplied by sediment from the same ice front retreated? Some type IV eskers are flanked R-channel that formed the esker.Ž. 5 On the grounds by a series of small, sequential end morainesŽ Gadd, that thick ice near ice divides would tend to close 1973. ; others exhibit repeated downflow transitions R-channels and thin ice would have resulted in greater to subaerial outwashŽ Thorleifson and Krystjanssen, topographic control on esker pathŽ. Shreve, 1985 , 1993. . Such transitions have been defined on mor- ShiltsŽ. 1984 argues against formation in extensive, phologic groundsŽ. aerial photographs but have yet synchronous R-channel systems for type IV eskers in to be confirmed by sedimentologic research. How- the Mackenzie±Keewatin area. However, R-channels ever, these observations make it likely that some are only likely to have closed completely if they type IV eskers were deposited as time-transgressive drained of water, and the relation between eskers and segments in short R-channels or reentrants that de- bed topography has yet to be fully documented in veloped close to the ice margin as the ice front this area. These uncertainties leave open the possibil- retreated or downwasted and backwasted. Such R- ity that some eskers in the Mackenzie±Keewatin channels may have been as short as 750 m based on area may have developed in extensive, synchronous esker-subaerial outwash spacingŽ Thorleifson and R-channel systems prior to ice retreat or backwast- Krystjanssen, 1993. . ing. However, if such R-channels were to remain Other observations and arguments have also been water-filled for many years, then they mustŽ. i termi- presented in support of this depositional environ- nate in standing waterŽ.Ž. Powell, 1990 , ii become ment, yet are perhaps more equivocal.Ž. 1 Esker plugged with sediment or collapse downflowŽ Boul- ridges alternate with channels eroded into the sub- ton and Hindmarsh, 1987; Hubbard and Nienow, strate and anastomosing reaches inferred to record 1997.Ž. thus trapping water over the winter, or iii ice-frontal positionsŽ. Shilts, 1984 . However, chan- experience a discharge sufficient to maintain them nels may be expected to alternate with eskers that over the winterŽ. Willis et al., 1993 . were deposited in long R-channelsŽ. Shreve, 1985 and anabranched reaches need not record ice-frontal 6.1.2. Hydraulic processes positionsŽ.Ž. Brennand, 1994 . 2 Eskers exhibit down- The presence of rounded boulders on the surface flow repetition of boulders and coarse sand along of some eskers indicates powerful flowsŽ St-Onge, their crestsŽ. St-Onge, 1984 . However, crest-line tex- 1984. . Subaerial outwash exhibits the traces of tures may not reflect grain sizes throughout an esker braided channels at its surfaceŽ. Shilts et al., 1987 . ridge and, even if they did, textural patterns in a As additional sedimentological observations are lack- continuous ridge may record flow dynamics along ing, investigation of the hydraulic processes associ- synchronous, non-uniform R-channelsŽ Brennand, ated with type IV eskers awaits further research. 1994.Ž. . 3 Provenance studies suggest relatively short transport distances for esker clastsŽ Bolduc et al., 6.1.3. Hydrologic regime 1987. . Although such observations may favour short Supraglacial meltwater input to R-channels has R-channels, they may also result from sedimentation been proposed to account for the dendritic pattern of zones along long R-channelsŽ. q.v. Brennand, 1994 . type IV eskers in the Mackenzie±Keewatin area Ž.4 Eskers are flanked by time-transgressive terraces Ž.Shilts, 1984 . This pattern is attributed to the im- formed in supraglacial, subaerial outwash that was print of a supraglacial drainage system; the headward initially superimposed on eskersŽ Shilts, 1984; Fig. growth of short R-channels followed the traces of a 10. . The fact that subaerial outwash was superim- connected dendritic supraglacial channel system posed on eskers suggests that it was deposited and Ž.Shilts, 1984 . The development of this supraglacial terraced after esker formation as the ice front re- system required a regional melt zone, cold near- treated or backwastedŽ. Fig. 10 ; it does not inform as surface iceŽ. q.v. Gustavson and Boothroyd, 1987 T.A. BrennandrGeomorphology 32() 2000 263±293 287

Fig. 10. Formation of type IV eskers as proposed by Shilts et al.Ž.Ž 1987 adapted from Menzies and Shilts, 1996; original drawings by J.A. Aylsworth. . The nature of R-channel sedimentation is unknown. Outwash sediments were laid down over stagnant ice containing the esker as the ice backwasted and downwasted. Meltout of buried ice and outwash incision formed pitted and terraced outwash; a partially eroded esker emerged. Reprinted with permission from Butterworth Heinemann Publishers, a division of Reed Educational and Professional Publishing Ltd. 288 T.A. BrennandrGeomorphology 32() 2000 263±293 and sink points preferentially located at the ice front. 6.2. Short eskers formed in R-channels or reentrants Invoking the Coriolis effect, Aylsworth and Shilts that terminated subaerially() type V Ž.1989 have argued that the fact that many esker tributaries join trunk eskers from the left, confirms Type V eskers are tens of meters to tens of the operation of an extensive, dendritic supraglacial kilometers long. The termination of short eskers in channel system on nearly flat ice. St-OngeŽ. 1984 deltasŽ Banerjee and McDonald, 1975; Saunderson, proposes supraglacial meltwater input on the basis of 1975. indicates that at least the latter stage of chan- sediment texture along an esker crest. He suggests nel flow was at atmospheric pressure and may sug- that surface boulders record points of supraglacial to gest that some esker sedimentation occurred in H- subglacial connection, sediments fining toward the channelsŽ. Hooke, 1984 . The facies associations and proposed ice margin; the origin of the boulders is not arguments presented by Banerjee and McDonald addressed. Thorleifson and KrystjanssenŽ. 1993 de- Ž.1975 and Saunderson Ž. 1975 in support of deltaic scribe long glaciofluvial corridors containing discon- sedimentation at the end of a subaerially terminating tinuous eskers in the Beardmore±Geraldton area, H-channel includeŽ. i downflow textural and struc- Ontario. They infer both subaerially and sub- tural trends consistent with weakening flow,Ž. ii aqueously terminating R-channels controlled by bed thick, gravel cross-beds interpreted as topset braid topography. As esker ridges are transitional to sub- barsŽ. Saunderson, 1975 , or as delta or bar front aqueous fans and varved lacustrine sediment depositsŽ.Ž. Banerjee and McDonald, 1975 , iii lateral Ž.Thorleifson and Krystjanssen, 1993 , supraglacial faults consistent with removal of lateral ice support, meltwater input was likely for eskers throughout the andŽ. iv backset beds or antidunes interpreted as region. evidence of subaerial sedimentationŽ Banerjee and McDonald, 1975. . This evidence is equivocal as 6.1.4. Deglacial drainage and glaciodynamics of similar observations are made in subaqueous fans type IV eskers deposited in R-channels or reentrants close to the ice Although the above discussion highlights many frontŽ Rust and Romanelli, 1975; Gorrell and Shaw, uncertainties, some type IV eskers likely formed as 1991. . Yet short eskers terminating in deltas are segments, time-transgressively in R-channels close to observedŽ. Martini, 1990 . It is also plausible that the ice margin as the ice front retreated or down- short eskers terminating in subaerial outwash may be wasted and backwasted. Such R-channels were likely expected from subaerially terminating R-channels, supplied by water from supraglacial sources. Was the but such eskers have not been reported, perhaps ice sheet regionally active or stagnant during the because they are difficult to recognizeŽ Banerjee and operation of these R-channels? St-OngeŽ. 1984 states McDonald, 1975. . Due to lack of evidence on which that eskers show no sign of overriding. Shilts et al., to ground it, a model of possible conditions for the Ž.1987 highlight the excellent preservation of exten- formation of type V eskers is not presented. sive, complex esker systems. Both argue that these observations suggest that the ice was largely stagnant. The segmental nature of the eskers was a 7. Controls on the spatial distribution of Lauren- product of downwasting and backwasting of this ice tide esker types mass. Thorleifson and KrystjanssenŽ. 1993 argue, based on the spacing of esker ridges and outwash Five types of eskers have been identified and deposits and on the presence of kettle holes adjacent discussed in terms of their likely implications for to eskers, that the ice underwent stagnation-zone deglacial drainage and glaciodynamics. How are retreatŽ actively retreating ice with a stagnant fringe these esker types spatially distributed? The answer to within which eskers developed, e.g., Hebrand and this question is by no means certain. Referring to the Amark,Ê 1989. . A model of plausible conditions for zones in Fig. 1a: zone 2 is likely dominated by esker the formation of type IV eskers is not presented due types I and IV, and zone 3 mostly includes esker to lack of consensus and evidence on which to types II and III, although esker type I occurs in ground it. tunnel channels. The distribution of esker type V is T.A. BrennandrGeomorphology 32() 2000 263±293 289 unknown, and few observations are reported for zone basis of type I eskers, but is equivocal on the basis of 4 in CanadaŽ. Rampton et al., 1984 . type IV eskers. It seems probable that the extensive What determined this spatial distribution? Clark esker systems in zone 2 were a response to the late and WalderŽ. 1994 have argued that esker distribu- deglacial coupling of thin, mostly stagnant ice with tion is substrate-controlled: eskers being present on rapid climate warming, possibly following the rigid bedsŽ. zone 2 in Fig. 1a and largely absent on Younger Dryas. Thin ice was likely inherited from soft bedsŽ. zone 3 in Fig. 1a . This argument is based earlier catastrophic events such as surges and sub- on Walder and Fowler'sŽ. 1994 theory which pre- glacial sheet floodsŽ. Shoemaker, 1992a; Shaw, 1994 . dicts that R-channels are replaced by canals on per- Zone 3Ž. Fig. 1a mainly contains short eskers which vasively deforming beds. While elegant, this theory were likely formed under actively retreating ice in may be mute for explaining Laurentide esker distri- parts of southern OntarioŽ. C in Fig. 1a , but under bution. Eskers are present in regions designated as stagnant ice in the PrairiesŽ. D in Fig. 1a . In other soft bed and assumed to be pervasively deforming parts of southern OntarioŽ. A in Fig. 1a type I eskers during ice retreat by Clark and WalderŽ. 1994 . This may suggest stagnation-zone retreatŽ q.v. Shilts et contradiction to theoretical prediction may suggest al., 1987. in parts of zone 3. thatŽ. i R-channels were present over deforming beds This investigation of Laurentide eskers suggests Ž.Ž.q.v. Alley, 1992 , or ii pervasive bed deformation that the factors that determined Laurentide R-channel never occurred, or had ceased, prior to esker forma- pattern and operation were likely a complex combi- tion. In south-central Ontario, the presence of drum- nation ofŽ. i supraglacial meltwater discharge, Ž ii . the lins on a regional till sheet has been interpreted as number and location of sink holes,Ž. iii the ice evidence of pervasive bed deformationŽ Boyce and surface slope, thickness and velocity, andŽ. iv the Eyles, 1991. . Eskers were deposited in tunnel chan- permeability, topography and rigidity of the bed. nels incised into this landscape. However, the re- These factors cause and respond to changes in ice gional till sheetŽ Newmarket Till, Sharpe et al., dynamics and thermal regime over the glacial cycle. 1996. contains horizontal sorted beds which argue Clearly, there is much still to be learned about eskers against pervasive bed deformation, and the eskers and their implications for R-channel drainage of the where formed after till deposition and drumlin for- Laurentide Ice Sheet. mation. Consequently, these eskers likely formed in R-channels on a deformable bed that was not pervasively deforming during esker formation, if indeed it Acknowledgements ever had. Consequently, although R-channels may only be stable on hard beds, deformable beds need This paper grew out of ideas developed for a not deform. Perhaps large volumes of supraglacial lecture presented at the Canadian Geomorphology meltwater delivered at point sources into the sub- Research Group annual meeting in Ottawa in 1998 glacial environment forced the development of an Ž.J. Ross Mackay Award lecture . I thank John Shaw efficient R-channel system on the deformable bed; for his encouragement. The clarity of this manuscript the effects of point-source water delivery to the bed was improved by reviews from Bob Gilbert, Martin is not considered by Walder and FowlerŽ. 1994 . Sharp, Dave Sharpe and Bill Shilts. Figures were Basal substrate certainly has a role in esker distribu- drafted by Paul DeGrace. This research was sup- tion, at least as a sediment sourceŽ Shilts et al., ported by an NSERC Research Grant to T.A. Bren- 1987. , but it is unlikely to be the sole determinant. nand. Shilts et al.Ž. 1987 argue that sediment availability and stagnant ice upflow from major arcuate moraine in zone 2Ž. Fig. 1a was responsible for References extensive, dendritic esker systems, whereas active Acaroglu, E.R., Graf, W.H., 1968. Sediment transport in con- ice retreat was largely responsible for the sparsity of veyance systems: Part 2. The modes of sediment transport and eskers in zone 3Ž. Fig. 1a . The discussion presented their related bed forms in conveyance systems. Bull. Int. here tends to confirm ice stagnation in zone 2 on the Assoc. Sci. Hydrol. 13, 123±135. 290 T.A. BrennandrGeomorphology 32() 2000 263±293

Allard, M., 1974. GeomorphologieÂ des eskers Abitibiens. Cahi. Glaciofluvial and Glaciolacustrine Systems. GeoBooks, Nor- Geogr. Quebec 18, 271±296. wich, pp. 279±295. Alley, R.B., 1989. Water-pressure coupling of sliding and bed Cheel, R.J., Rust, B.R., 1986. A sequence of soft-sediment defor- deformation: I. Water system. J. Glaciol. 35Ž. 119 , 108±118. mationŽ. dewatering structures in late Quaternary subaqueous Alley, R.B., 1992. How can low pressure channels and deforming outwash near Ottawa, Canada. Sediment. Geol. 47, 77±93. tills coexist subglacially? J. Glaciol. 38, 200±207. Clark, P.U., Walder, J.S., 1994. Subglacial drainage, eskers, and Alley, R.B., 1996. Towards a hydrological model for computer- deforming beds beneath the Laurentide and Eurasian ice sheets. ized ice-sheet simulations. Hydrol. Processes 10, 649±660. Geol. Soc. Am. Bull. 106, 304±314. Alley, R.B., Blakenship, D.D., Bentley, C.R., Rooney, S.T., 1986. Clarke, G.K.C., Collins, S.G., Thompson, D.E., 1984. Flow, Deformation of till beneath Ice Stream B, West Antarctica. thermal structure and subglacial conditions of a surge-type Nature 322, 57±59. glacier in its quiescent state, Trapbridge Glacier, Yukon Terri- Arnold, N.S., Sharp, M., 1992. Influence of glacier hydrology on tory. J. Glaciol. 37, 158±169. the dynamics of a large Quaternary ice sheet. J. Quat. Sci. 7, Costa, J.E., 1983. Paleohydraulic reconstruction of flash-flood 109±124. peaks from boulder deposits in the Colorado Front Range. Aylsworth, J.M., Shilts, W.W., 1989. Glacial features around the Geol. Soc. Am. Bull. 90, 986±1004. Keewatin Divide, Districts of Mackenzie and Keewatin. Geol. Craig, B.G., 1964. Surficial geology of east-central District of Surv. Can., Pap. 88-21. Mackenzie. Geol. Surv. Can., Bull., 99. Banerjee, I., McDonald, B.C., 1975. Nature of esker sedimenta- Dubois, J.-M.M., Dionne, J.-C., 1985. The QuebecÂ North Shore tion. In: Jopling, A.V., McDonald, B.C.Ž. Eds. , Glaciofluvial moraine system: a major feature of Late Wisconsin deglacia- and Glaciolacustrine Sedimentation. Soc. Econ. Paleontol. tion. In: Borns, H.W. Jr., LaSalle, P., Thompson, W.B.Ž. Eds. , Mineral., Spec. Publ. 23, 304±320. Late Pleistocene history of northeastern New England and Benn, D.I., Evans, D.J.A., 1998. Glaciers and Glaciation. Arnold, adjacent Quebec. Geol. Soc. Am. Spec., Pap. 197, 125±133. London. Dyke, A.S., Dredge, L.A., 1989. Quaternary geology of the Bolduc, A.M., Klassen, R.A., Evenson, E.B., 1987. Cobble northwestern Canadian Shield. In: Fulton, R.J.Ž. Ed. , Quater- lithologies in eskers of central Labrador. Geol. Surv. Can., nary geology of Canada and Greenland. Geol. Surv. Can., Pap. 87A, 43±51. Geol. Can. 1, 189±214. Boulton, G.S., Hindmarsh, R.C.A., 1987. Sediment deformation Dyke, A.S., Morris, T.M., 1988. Drumlin fields, dispersal trains, beneath glaciers: rheology and geological consequences. J. and ice streams in Arctic Canada. Can. Geogr. 32, 86±90. Geophys. Res. 92, 9059±9082. Dyke, A.S., Vincent, J.-S., Andrews, J.T., Dredge, L.A., Cowan, Boulton, G.S., Jones, A.S., 1979. Stability of temperate ice caps W.R., 1989. The Laurentide Ice Sheet and an introduction to and ice sheets resting on beds of deformable sediment. J. the Quaternary geology of the Canadian Shield. In: Fulton, Glaciol. 24, 29±43. R.J.Ž. Ed. , Quaternary Geology of Canada and Greenland. Boyce, J.I., Eyles, N., 1991. Drumlins carved by deforming till Geol. Surv. Can., Geol. Can. 1, 178±189. streams below the Laurentide Ice Sheet. Geology 19, 787±790. Engelhardt, H.F., Harrison, W.D., Kamb, W.B., 1978. Basal Brand, G., Pohjola, V., Hooke, R.Le.B., 1987. Evidence for a till sliding and conditions at the glacier bed as revealed by layer beneath Storglaciaren,È Sweden, based on electrical resis- borehole photography. J. Glaciol. 20, 469±508. tivity measurements. J. Glaciol. 33Ž. 115 , 311±314. Fulton, R.J., 1989. Foreword to the Quaternary geology of Canada Brennand, T.A., 1994. Macroforms, large bedforms and rhythmic and Greenland. In: Fulton, R.J.Ž. Ed. , Quaternary geology of sedimentary sequences in subglacial eskers, south-central On- Canada and Greenland. Geol. Surv. Can., Geol. Can. 1, 1±11. tario: implications for esker genesis and meltwater regime. Fyfe, G.J., 1990. The effect of water depth on ice-proximal Sediment. Geol. 91, 9±55. glaciolacustrine sedimentation: SalpausselkaÈ I, southern Fin- Brennand, T.A., Sharpe, D.R., 1993. Ice-sheet dynamics and land. Boreas 19, 147±164. subglacial meltwater regime inferred from form and sedimen- Gadd, N.R., 1973. Quaternary geology of southwest New tology of glaciofluvial systems: Victoria Island, District of Brunswick, with particular reference to Fredericton area. Geol. Franklin, Northwest Territories. Can. J. Earth Sci. 30, 928±944. Surv. Can., Pap. 71-34. Brennand, T.A., Shaw, J., 1996. The Harricana glaciofluvial Gilbert, R., 1983. Sedimentary processes of Canadian fjords. complex, Abitibi region, Quebec: its genesis and implications Sediment. Geol. 36, 147±175. for meltwater regime and ice-sheet dynamics. Sediment. Geol. Gordon, S., Sharp, M., Hubbard, B., Smart, C., Ketterling, B., 102, 221±262. Willis, I., 1998. Seasonal reorganization of subglacial drainage Brennand, T.A., Shaw, J., Sharpe, D.R., 1996. Regional-scale inferred from measurements in boreholes. Hydrol. Processes meltwater erosion and deposition patterns, northern Quebec, 12, 105±133. Canada. Ann. Glaciol. 22, 85±92. Gorrell, G., Shaw, J., 1991. Deposition in an esker, bead and fan Cheel, R.J., 1982. The depositional history of an esker near complex, Lanark, Ontario, Canada. Sediment. Geol. 72, 285± Ottawa, Canada. Can. J. Earth Sci. 19, 1417±1427. 314. Cheel, R.J., Rust, B.R., 1982. Coarse-grained facies of glacioma- Gustavson, T.C., Boothroyd, J.C., 1987. A depositional model for rine deposits near Ottawa, Canada. In: Davidson-Arnott, R., outwash, sediment sources, and hydrologic characteristics, Nickling, W., Fahey, B.D.Ž. Eds. , Research in Glacial, Malaspina Glacier, Alaska: a modern analog for the southeast- T.A. BrennandrGeomorphology 32() 2000 263±293 291

ern margin of the Laurentide Ice Sheet. Geol. Soc. Am. Bull. Lliboutry, L., 1968. General theory of subglacial cavitation and 99, 187±200. sliding of temperate glaciers. J. Glaciol. 7, 21±58. Haefeli, R., 1970. Changes in the behaviour of the Unteraar- Mannerfelt, C., 1981. Stagnation or activity in the last ice rem- gletscher in the last 125 years. J. Glaciol. 9, 195±212. nants? Geogr. Ann. 63AŽ. 3±4 , 139±147. Hand, B.M., 1974. Supercritical flow in density currents. J. Sedi- Martini, I.P., 1990. Pleistocene glacial fan deltas in southern ment. Petrol. 44, 637±648. Ontario, Canada. In: Colella, A., Prior, D.Ž. Eds. , Coarse- Hebrand, M., Amark,Ê M., 1989. Esker formation and glacier grained deltas. Int. Assoc. Sedimentol., Spec. Publ. 10, 281± dynamics in eastern SkaneÊ and adjacent areas, southern Swe- 295. den. Boreas 18, 67±81. Matile, G., 1984. Quaternary geology map of the Birds Hill area. Henderson, P.J., 1988. Sedimentation in an esker system influ- Manitoba Energy Mines Map, AR84-5, scale 1:20,000, 1 enced by bedrock topography near Kingston, Ontario. Can. J. sheet. Earth Sci. 25, 987±999. McDonald, B.C., Shilts, W.W., 1975. Interpretation of faults in Hillaire-Marcel, C., Occhietti, S., Vincent, J.-S., 1981. Sakami glaciofluvial sediments. In: Jopling, A.V., McDonald, B.C. moraine, Quebec: a 500-km-long moraine without climatic Ž.Eds. , Glaciofluvial and glaciolacustrine sedimentation. Soc. control. Geology 9, 210±214. Econ. Paleontol. Mineral., Spec. Publ. 23, 123±131. Hodgkins, R., 1997. Glacier hydrology in Svalbard, Norwegian McDonald, B.C., Vincent, J.S., 1972. Fluvial sedimentary struc- high arctic. Quat. Sci. Rev. 16, 957±973. tures formed experimentally in a pipe, and their implications Holmlund, P., 1988. Internal geometry and evolution of moulins, for interpretation of subglacial sedimentary environments. Storglaciaren,È Sweden. J. Glaciol. 34Ž. 117 , 242±248. Geol. Surv. Can., Pap. 72-27. Holmlund, P., Hooke, R.LeB., 1983. High water pressure events Menzies, J., Shilts, W.W., 1996. Subglacial environments. In: in moulins, Storglaciaren,È Sweden. Geogr. Ann. 65A, 19±25. Menzies, J.Ž. Ed. , Past Glacial Environments: Sediments, Hooke, R.LeB., 1984. On the role of mechanical energy in Forms and Techniques. Butterworth-Heinemann, Oxford. maintaining subglacial water conduits at atmospheric pressure. Mooers, H.D., 1989. On the formation of the tunnel valleys of the J. Glaciol. 30, 180±187. Superior Lobe, central Minnesota. Quat. Res. 32, 24±35. Hooke, R.LeB., 1989. Englacial and subglacial hydrology: a Munro, M., Shaw, J., 1997. Erosional origin of hummocky terrain qualitative review. Arct. Alp. Res. 21, 221±233. in south-central Alberta, Canada. Geology 25Ž. 11 , 1027±1030. Hooke, R.LeB., 1998. Principles of Glacier Mechanics. Prentice Murray, T., Clarke, G.K.C., 1995. Black box modelling of the Hall, NJ. subglacial water system. J. Geophys. Res. 100Ž. B7 , 10231± Hubbard, B., Nienow, P., 1997. Alpine subglacial hydrology. 10245. Quat. Sci. Rev. 16, 939±955. Nienow, P., Sharp, M., Willis, I., 1998. Seasonal changes in the Hubbard, B., Sharp, M., Willis, I.C., Neilsen, M.K., Smart, C., morphology of the subglacial drainage system, Haut Glacier 1995. Borehole water-level variations in the structure of the D'Arolla, Switzerland. Earth Surf. Processes Landforms 23, subglacial hydrological system of Haut Glacier d'Arolla, 825±843. Valais, Switzerland. J. Glaciol. 27, 572±583. Nye, J.F., 1973. Water at the bed of a glacier. Int. Assoc. Sci. Iken, A., 1972. Measurements of water pressure in moulins as part Hydrol., Publ. 95, 189±194. of a movement study of the White Glacier, Axel Heiberg Nye, J.F., 1976. Water flow in glaciers: jokulhlaups,È tunnels and Island, Northwest Territories, Canada. J. Glaciol. 11, 53±58. veins. J. Glaciol. 17, 181±207. Iken, A., 1981. The effect of the subglacial water pressure on the Nye, J.F., Frank, F.C., 1973. Hydrology of the intergrannular sliding velocity of a glacier in an idealized numerical model. J. veins in a temperate glacier. Int. Assoc. Sci. Hydrol., Publ. 95, Glaciol. 27, 407±421. 157±161. Iken, A., Bindschadler, R.A., 1986. Combined measurements of O'Cofaigh, C., 1996. Tunnel valley genesis. Progr. Phys. Geogr. subglacial water pressure and surface velocity of Findelen- 20Ž. 1 , 1±19. gletscher, Switzerland: conclusions about drainage system and éstrem, G., 1975. Sediment transport in glacial meltwater streams. sliding mechanism. J. Glaciol. 32, 101±119. In: Jopling, A.V., McDonald, B.C.Ž. Eds. , Glaciofluvial and Kamb, B., 1987. Glacier surge mechanisms based on linked cavity glaciolacustrine sedimentation. Soc. Econ. Paleontol. Mineral., configuration of the basal water conduit system. J. Geophys. Spec. Publ. 23, 101±122. Res. 92, 9083±9100. Parker, G., 1975. Meandering of supraglacial melt streams. Water Kamb, B., Raymond, C.F., Harrison, W.D., Englehardt, H., Resour. Res. 11Ž. 4 , 551±552. Echelmeyer, K.A., Humphrey, N., Brugman, M.M., Pfeffer, Paterson, W.S.B., 1994. The Physics of Glaciers. 3rd edn. Per- T., 1985. Glacier surge mechanism: 1982±1983 surges of magon, Oxford. Variegated Glacier, Alaska. Science 227, 469±479. Peteet, D., 1995. Global Younger Dryas? Quat. Int. 28, 93±104. Kaszycki, C.A., DiLabio, R.N.W., 1986. Surficial geology and till Pfirman, S.L., Solheim, A., 1989. Subglacial meltwater discharge geochemistry, Llyn Lake-Leaf Rapids region, Manitoba. Geol. in the open-marine tidewater glacier environment: observa- Surv. Can., Pap. 86-1B, 245±256. tions from Nordaustlandet, Svalbard Archipelago. Mar. Geol. Klassen, R.W., 1986. Surficial geology of north-central Manitoba. 86, 265±281. Geol. Surv. Can., Mem., 419. Powell, R.D., 1990. Glaciomarine processes at grounding-line Lindstrom,È E., 1993. Esker enlargements in northern Sweden. fans and their growth to ice-contact deltas. In: Dowdeswell, Geogr. Ann. 75AŽ. 3 , 95±110. J.A., Scourse, J.D.Ž. Eds. , Glaciomarine environments: pro- 292 T.A. BrennandrGeomorphology 32() 2000 263±293

cesses and sediments. Geol. Soc. London, Spec. Publ. 53, glaciolacustrine sedimentation. Soc. Econ. Paleontol. Mineral., 53±73. Spec. Publ. 23, 155±176. Prest, V.K., Grant, D.R., Rampton, N., 1968. Glacial map of Saunderson, H.C., 1977. The sliding bed facies in esker sands and Canada. Geol. Surv. Can., Map 1253A, scale 1:5,000,000, 1 gravels: a criterion for full-pipeŽ. tunnel flow? Sedimentology sheet. 24, 623±638. Punkari, M., 1997. Glacial and glaciofluvial deposits in the inter- Sharp, M., Richards, K.S., 1996. Integrated studies of hydrology lobate areas of the Scandinavian Ice Sheet. Quat. Sci. Rev. 16, and water quality in glacierized catchments. Hydrol. Processes 741±753. 10, 475±478. Rainio, H., Saarnisto, M., Ekman, I., 1995. Younger Dryas end Sharp, M.J., Richards, K., Willis, I., Arnold, N., Nienow, P., moraines in Finland and NW Russia. Quat. Int. 28, 179±192. Lawson, W., Tison, J.-L., 1993. Geometry, bed topography Rains, B., Shaw, J., Skoye, R., Sjogren, D., Kvill, D., 1993. Late and drainage system structure of the Haut Glacier d'Arolla, Wisconsin subglacial megaflood paths in Alberta. Geology 21, Switzerland. Earth Surf. Processes Landforms 18, 557±572. 323±326. Sharpe, D.R., 1988. Glaciomarine fan deposition in the Champlain Rampton, V.N., Gauthier, R.C., Thibault, J., Seaman, A.A., 1984. Sea. In: Gadd, N.R.Ž. Ed. , The Late Quaternary development Quaternary geology of New Brunswick. Geol. Surv. Can., of the Champlain Sea Basin. Geol. Assoc. Can. Spec., Pap. 35, Mem., 416. 63±82. Raymond, C.F., Harrison, W.D., 1975. Some observations on the Sharpe, D.R., Cowan, W.R., 1990. Moraine formation in north- behaviour of the liquid and gas phases in temperate glacier ice. western Ontario: product of subglacial fluvial and glaciolacus- J. Glaciol. 14, 213±233. trine sedimentation. Can. J. Earth Sci. 27, 1478±1486. Richards, K.S., Sharp, M., Arnold, N., Gurnell, A., Clark, M., Sharpe, D.R., Pullen, S.E., Warman, T.A., 1992. A basin analysis Tranter, M., Nienow, P., Brown, G., Willis, I., Lawson, W., of the Wabigoon area of Lake Agassiz, a Quaternary clay 1996. An integrated approach to modelling hydrology and basin in northwestern Ontario. Geogr. Phys. Quat. 46Ž. 3 , water quality in glacierised catchments. Hydrol. Processes 10, 295±309. 479±508. Sharpe, D.R., Dyke, L.D., Hinton, M.J., Pullen, S.E., Russell, Ringrose, S., 1982. Depositional processes in the development of H.A.J., Brennand, T.A., Barnett, P.J., Pugin, A., 1996. eskers in Manitoba. In: Davidson-Arnott, R., Nickling, W., Groundwater prospects in the Oak Ridges Moraine area, Fahey, B.D.Ž. Eds. , Research in Glacial, Glaciofluvial and southern Ontario: application of regional geological models. Glaciolacustrine Systems. Geo Books, Norwich, pp. 117±137. Geol. Surv. Can., Curr. Res. 1996-E, 181±190. Ringrose, S., Large, P., 1977. Quaternary geology and gravel Shaw, J., 1994. A qualitative view of sub-ice-sheet landscape resources of the Leaf Rapids local government district. Mani- evolution. Prog. Phys. Geogr. 18, 159±184. toba Dept. Mines, Resources Environmental Management, Shaw, J., 1996. A meltwater model for Laurentide subglacial Mineral Resources Div., Geol. Rep. 77-2. landscapes. In: McCann, S.B., Ford, D.C.Ž. Eds. , Geomorphol- Robin, G.deQ., 1974. Depth of water-filled crevasses that are ogy Sans Frontieres.Á Wiley, Chichester, pp. 181±236. closely spacedŽ. correspondence . J. Glaciol. 13, 543. Shaw, J., Kvill, D., Rains, B., 1989. Drumlins and catastrophic Rondot, J., 1982. L'esker du Lac Berry. MinistereÁ de l'Energie et subglacial floods. Sediment. Geol. 62, 177±202. des Ressources, Direction de la GeologieÂ publication. Shetsen, I., 1987. Quaternary geology of southern Alberta, Alberta Rothlisberger,È H., 1972. Water pressure in intra- and subglacial Research Council map, 1:500,000, 1 sheet. channels. J. Glaciol. 11, 177±203. Shetsen, I., 1990. Quaternary geology of central Alberta, Alberta Rothlisberger,È H., Lang, H., 1987. Glacier hydrology. In: Gurnell, Research Council map, 1:500,000, 1 sheet. A.M., Clark, M.J.Ž. Eds. , Glacio-fluvial Sediment Transfer. Shilts, W.W., 1984. Esker sedimentation models, Deep Rose Lake Wiley, Chichester, pp. 207±287. map area, District of Keewatin. Geol. Surv. Can., Pap. 84-1B, Rust, B.R., 1977. Mass flow deposits in a Quaternary succession 217±222. near Ottawa, Canada: diagnostic criteria for subaqueous out- Shilts, W.W., Aylsworth, J.M., Kaszycki, C.A., Klassen, R.A., wash. Can. J. Earth Sci. 14, 175±184. 1987. Canadian Shield. In: Graf, W.L.Ž. Ed. , Geomorphic Rust, B.R., 1988. Ice-proximal deposits of the Champlain Sea at Systems of North America. Geol. Soc. Am., Centennial Spec. south Gloucester, near Ottawa, Canada. In: Gadd, N.R.Ž. Ed. , 2, 119±161. The Late Quaternary development of the Champlain Sea Basin. Shoemaker, E.M., 1992a. Subglacial floods and the origin of low Geol. Ass. Can. Spec. Pap. 35, 37±46. relief ice lobes. J. Glaciol. 38, 105±112. Rust, B.R., Romanelli, R., 1975. Late Quaternary subaqueous Shoemaker, E.M., 1992b. Water sheet outburst floods from the outwash deposits near Ottawa, Canada. In: Jopling, A.V., Laurentide ice sheet. Can. J. Earth Sci. 29, 1250±1264. McDonald, B.C.Ž. Eds. , Glaciofluvial and Glaciolacustrine Shreve, R.L., 1972. Movement of water in glaciers. J. Glaciol. 11, Sedimentation. Soc. Econ. Paleontol. Mineral., Spec. Publ. 23, 205±214. 177±192. Shreve, R.L., 1985. Esker characteristics in terms of glacial Saunderson, H.C., 1975. Sedimentology of the Brampton esker physics, Katahdin esker system, Maine. Geol. Soc. Am. Bull. and its associated deposits: an empirical test of theory. In: 96, 639±646. Jopling, A.V., McDonald, B.C.Ž. Eds. , Glaciofluvial and Spooner, I.S., Dalrymple, R.W., 1993. Sedimentary facies rela- T.A. BrennandrGeomorphology 32() 2000 263±293 293

tionships in esker-ridgeresker-fan complexes, southeastern Walder, J.S., Fowler, A., 1994. Channelized subglacial drainage Ontario, Canada: application to the exploration for asphalt over a deformable bed. J. Glaciol. 40Ž. 134 , 3±15. blending sand. Quat. Int. 20, 81±92. Walder, J.S., Hallet, B., 1979. Geometry of former subglacial Stalker, A.MacS., 1959. Surficial geology of the Red Deer±Stet- water channels and cavities. J. Glaciol. 23, 335±346. tler map-area, Alberta. Geol. Surv. Can., Mem., 306. Weertman, J., 1957. On the sliding of glaciers. J. Glaciol. 3, Stalker, A.MacS., 1960. Ice-pressed drift forms and associated 33±38. deposits in Alberta. Geol. Surv. Can., Bull. 57. Weertman, J., 1964. The theory of glacier sliding. J. Glaciol. 5 Stone, G.H., 1899. The glacial gravels of Maine. U. S. Geol. Surv. Ž.39 , 287±303. Monogr., 34. Weertman, J., 1973. Can a water-filled crevasse reach the bottom St-Onge, D.A., 1984. Surficial deposits of the Redrock Lake area, surface of a glacier? Int. Assoc. Sci. Hydrol., Publ. 95, District of Mackenzie. Geol. Surv. Can., Pap. 84-1A, 271±278. 139±145. Streeter, V.L., Wylie, E.B., 1979. Fluid Mechanics. 7th edn Weertman, J., 1974. Depth of water-filled crevasses that are McGraw-Hill, New York. closely spaced. J. Glaciol. 13, 544. Syverson, K.M., Gaffield, S.J., Mickelson, C.D.M., 1994. Com- Willis, I.C., Sharp, M.J., Richards, K.S., 1990. Configuration of parison of esker morphology and sedimentology with former the drainage system of Midtdalsbreen, Norway, as indicated ice-surface topography, Burroughs Glacier, Alaska. Geol. Soc. by dye-tracing experiments. J. Glaciol. 36, 89±101. Am. Bull. 106, 1130±1142. Willis, I.C., Sharp, M.J., Richards, K.S., 1993. Studies of the Thorleifson, L.H., Krystjanssen, F.J., 1993. Quaternary geology water balance of Midtalsbreen, Hardangerjokulen,È Norway: II. and drift prospecting, Beardmore±Geraldton area, Ontario. Water storage and runoff prediction. Z. Geltscherkunde Geol. Surv. Can., Mem, 435. Glazialgeologie 27r28, 117±138. Veillette, J.J., 1986. Former ice flows in the Abitibi±Timiskaming Wright, H.E., 1973. Tunnel valleys, glacial surges, and subglacial region: implications for the configuration of the Late Wiscon- hydrology of the Superior Lobe, Minnesota. Geol. Soc. Am., sinian ice sheet. Can. J. Earth Sci. 23, 1724±1741. Mem., 136. Walder, J.S., 1982. Stability of sheet flow of water beneath temperate glaciers and implications for glacier surging. J. Glaciol. 28, 273±293.