In defence of the meltwater (megaflood) hypothesis for the formation of subglacial bedform fields

JOURNAL OF QUATERNARY SCIENCE (2010) 25(3) 249–260Copyright � 2009 John Wiley & Sons, Ltd.Published online 20 April 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/jqs.1264
In defence of the meltwater (megaflood)hypothesis for the formation of subglacialbedform fieldsJOHN SHAW*

Faculty of Science, University of Alberta, Edmonton, Alberta, Canada

Shaw, J. 2010. In defence of the meltwater (megaflood) hypothesis for the formation of subglacial bedform fields. J. Quaternary Sci., Vol. 25 pp. 249–260. ISSN 0267-8179.

Received 30 May 2008; Revised 13 December 2008; Accepted 19 December 2008

ABSTRACT: Characteristics of large-scale fluting and hummocky terrain on the Canadian Prairies testglacial and meltwater hypotheses for landform genesis. These tests defend the meltwater model.Neither sedimentary nor glaciotectonic processes can fully explain such erosional landforms.Province-scale flow paths, which mark palaeo-ice streams and subglacial flood routes, containlarge-scale fluting with flanking hummock terrain. Antecedent relief causes these paths to differ fromother flood landscapes such as the Scablands. Proponents of the glacial hypothesis use an invalidanalogy between Icelandic and Prairie landsystems. They suggest that groove-ploughing formed large-
scale fluting, and that ice pushing created hummocky terrain. However, landform location, form, andextent, surface lags, truncated architecture, and landform associations favour the meltwater hypoth-esis. A simple thought experiment and clear understanding of the principle of least number ofassumptions answer the criticisms that meltwater forms cannot cross-cut and that the meltwaterhypothesis disregards proper hypothesis testing. An example of cross-cutting erosional marks supportsthis theory. No narrow tract of smoothed terrain with fluting terminates at the glacially thrust NeutralHills, negating an important point in the glacial hypothesis. While neither the glacial hypothesis norpostglacial winnowing explain boulder and cobble lags with percussion marks, meltwater processesexplain them well. Copyright # 2009 John Wiley & Sons, Ltd.
KEYWORDS: flow path; large-scale fluting; hummocky terrain; clast lags; ice-pushed moraines.

Introduction

The two main approaches to understanding subglacial bedformfields are the glacial and themeltwater (megaflood) hypotheses:

� G

* CtonE-m

lacial hypothesis – direct erosion and/or deposition by iceor by a subglacial deforming bed create glacial bedforms.

� M
eltwater (megaflood) hypothesis – erosion and/or depo-sition by subglacial sheet floods create meltwater bedforms.
These are not, however, all-or-nothing models. For instance,glacial processes explain most moraines, roches moutonneesand striations; meltwater processes explain eskers, some largearcuate moraines and s-forms. Otherwise, rival glacial andmeltwater hypotheses attempt to explain such landforms asdrumlins, large-scale fluting, ribbed or Rogen moraine, andhummocky terrain.

orrespondence to: J. Shaw, Faculty of Science, University of Alberta, Edmon-, Alberta, Canada T6G 2E9.ail: [email protected]

The meltwater (megaflood) hypothesis for subglacial bed-forms (e.g. Shaw, 2002) is repeatedly criticised, with much ofthis criticism pitting the glacial model against the meltwatermodel for the Albertan Prairie landscape (Benn and Evans,1998, 2006; Evans et al., 2006, 2008). Field studies of, andremote sensing on Prairie landforms – flow paths, large-scalefluting, hummocky terrain and ice-pushed hills – provideobservations to test these twomodels and to counter criticism ofthe meltwater hypothesis.Erosion of bedrock and surficial deposits probably formed

flow paths, large-scale fluting and hummocky terrain on thePrairies (Figs 1 and 2; Rains et al., 1993; Munro and Shaw,1997; Shaw et al., 2000). Consequently, deposition andglaciotectonic thrusting, folding and faulting were merelybackcloths to much landscape evolution; they give informationon events that preceded present-day geomorphology (e.g. Shawet al., 2000; Evans et al., 2008). Nevertheless, Evans et al.(2006) devoted much of their assessment of the meltwaterhypothesis to observation and interpretation of sediments andglaciotectonic structures. They presumed that testing thehypothesis requires the following: determining the environ-ment of deposition of glaciolacustrine sediments; assessing thegenesis of stratified tills; interpreting glaciotectonic structures

250 JOURNAL OF QUATERNARY SCIENCE

as syndepositional or postdepositional, subglacial or ice-marginal; and verifying the fluvial origin of fluting, hummocks,truncation surfaces and clast lags. Only the last of these –verifying the fluvial origin of fluting, hummocks, truncationsurfaces and clast lags – tests the hypothesis. By emphasisingsedimentology and glaciotectonics, Evans et al. (2006) miss thepoint of the debate on Prairie bedforms.

Flow paths

Explanations of fluting and hummocks on theWestern Plains ofCanada must involve the great tracts of mainly flat landextending southwards across the plains (e.g. Shaw et al., 1996;Fig. 2). These anastomosing, flat to undulating zones ofsmoothed terrain are striking on digital elevation models(DEM). Their lengths are measured in hundreds of kilometres,their widths in tens of kilometres, and their slopes are adverse,rising southward. They are scoured to bedrock over much oftheir extent. Large-scale fluting in these tracts (Shetsen, 1987,1990) commonly lies downflow from forward-facing (facinginto flow) valley walls or ridge flanks. Recessional moraines arerare but distinctive. Hummocky terrain and ice thrust hillsalongside tracts have relatively high relief.

Figure 1 Location maps

Copyright � 2009 John Wiley & Sons, Ltd.

These low-relief networks record major flow systems, andprobably contained ice streams in the Laurentide Ice Sheet(Anonymous, 1997; Evans et al., 2008). Consequently, they arenamed flow paths. Interpretations, presented later, of flutingsuggest flow path erosion by megafloods. Scour to bedrock andthe location of the northern, eastern and western flow pathsalong the proposed Livingstone Lake flood route (Shaw et al.,1996) add support to the megaflood interpretation.

Relief in paths is subtle and contrasts with the spectacularrelic waterfalls of theWashington Scabland, the classical flood-scoured terrain (Bretz, 1969). This contrast may be taken asevidence against widespread meltwater erosion of the Prairies,but their antecedent landscape had low relief, makingwaterfalls unlikely. By contrast, the high plateaux of theScabland, with their steep cliffs in jointed basalt, are conduciveto spectacular erosion. Using another approach, evidence offloods from glacial lakes in western Canada and the Americanwest suggests discharges of�106m3 s�1 (e.g. Kehew and Teller,1995; Clarke et al., 2004). Since these glacial lake outbursts,with discharges similar to those estimated for Prairie floods, didnot erode gorges with waterfalls, the absence of waterfalls onthe Prairies cannot be taken as evidence against the megafloodhypothesis, but deep coulees (tunnel channels on the Prairies)and giant ripples, which are found in both Scabland andPrairie flow paths (Bretz, 1969; Sjogren and Rains, 1995;Beaney and Shaw, 2000), support the megaflood hypothesis forthe Prairies.

J. Quaternary Sci., Vol. 25(3) 249–260 (2010)DOI: 10.1002/jqs

Figure 2 Flow paths, Alberta. NASA Shuttle Radar Topographic Mis-sion hillshade at 90m horizontal resolution. Courtesy of NASA/JPL-Caltech

Figure 3 (a) Large-scale fluting, Athabasca, Alberta. Note that thefluting starts at the rim of the Athabasca Valley. Flow from top right.White lines illustrate cross-cutting sets. Landsat TM. Courtesy of USGeological Survey. (b) Spindle-shaped erosional troughs in flutingfurrows, Athabasca, Alberta. View in the direction of flow

IN DEFENCE OF THE MELTWATER HYPOTHESIS 251

Large-scale fluting

The glacial model

The origin of large-scale fluting, megaflutes or mega-scaleglacial lineations (MSGL; Clark, 1993) is central to the debateon subglacial bedforms. In compiling a model for the Prairieglacial landsystem, Evans et al. (2008) used an analogy to tielarge-scale fluting on the Prairies to small-scale flutes at thebeds of modern glaciers. Although there is general agreementthat small-scale flutes form by squeezing saturated sedimentinto cavities behind boulders (e.g. Boulton, 1976), the origin oflarge-scale fluting is problematical. The groove-ploughingmodel of Tulaczyk et al. (2001) and Clark et al. (2003) for large-scale fluting underpins Evans et al.’s (2006, p. 1645, 2008, p.259) approach to fluting formation. In this model, moulding ofglacier beds by bedrock roughness forms keels which ploughgrooves in subglacial deforming beds.The glacial hypothesis for large-scale fluting has two

drawbacks. First, the analogy is not valid because small flutesand large fluting are explained by two very different processes:the lateral and upward squeezing of till into small cavitiesand the ploughing of deep furrows are non-analogous.Secondly, the ploughing mechanism for large-scale fluting isat odds with field evidence. For example, Munro-Stasiuk and


Shaw (2002) and Evans et al. (2008) observed large-scale flutingpreferentially located on transverse ridges; it is also founddownflow from forward-facing steps such as steep valley sides(Fig. 3(a); Pollard et al., 1996; Shaw et al., 2000), but flutingthought to be formed by groove ploughing covers vast areas,and individual bedforms are up to 100 km long (Clark et al.,2003). Fluting in Prairie flowpaths should be just as extensive ifit formed by groove ploughing, but it is restricted to ridges andforward-facing steps, and is short relative to lineationsdescribed by Clark et al. (2003). Large-scale Prairie fluting isevidently not well explained by the glacial model.Evans (1996) and Evans et al. (2008, their Fig. 4) illustrated

straight ridges standing high above the Prairie surface. Theseridges could not have formed by ploughing, which excavatesrelatively subdued grooves and ridges, not high ridges withoutgrooves. It is more likely that they are erosional remnants.The above arguments pose hard questions for those who

accept the groove-ploughingmechanism for large-scale fluting.Nevertheless, Evans et al. (2008, p. 257) offered a differentview:

. . . it is unclear why the megaflood proponents prefer ameltwater erosional interpretation for the streamlinedcorridors when such landscapes have been unequivocallyexplained by glacier ice streaming inmodern glacial systems.

They presented their analogy between modern glacialsystems and flow paths (streamlined corridors) as unequivocaland claimed that it is superior to the meltwater hypothesis.However, as outlined above, it is inconsistent and does notstand up to scrutiny regarding fluting. What is more, extensive,


Figure 4 (a) Map of glacial features in the forefront of Breidamerkurjokull, Iceland. Note the dominance of flutes and the prominent moraines. FromBenn and Evans (2006), reproducedwith permission. (b) Map of glacial features in theMcGregor Reservoir area, southern Alberta. Note the dominanceof transverse ridges. From Benn and Evans (2006), reproduced with permission


low-relief flow paths with large-scale fluting and flankinghummocky terrain are not reported from modern glacialsystems; therefore, this analogy is not supported by fact.Detailed observations used to support the glacial hypothesis

are also unconvincing. Benn and Evans (2006) presented twolandform maps: one of the glacier forefront at Breidamerkurjo-kull, Iceland, and the other in the vicinity of McGregorReservoir, southern Alberta (Figs 1 and 4(a) and (b)). Althoughthe maps are intended to support the glacial hypothesis byportraying similarities between the Icelandic and Prairie glacial


landsystems, they depict landform assemblages and patternsthat bear little resemblance between the two areas.

The Icelandic glacier forefront is dominated by closelyspaced, small-scale flutes parallel to ice flow (Fig. 4(a)), withsets of flutes beginning and ending at moraines. Continuousridges and aligned segments of ridges form a few prominentend-moraines, which correspond to stillstands or minorreadvance positions of the glacier front. Small eskers lie moreor less parallel to the flutes. Just as longitudinal forms dominatethe Icelandic map, so-called ‘transverse ridges’ dominate the



McGregor Reservoir area map (Fig. 4(b)). These discontinuousridges lack alignment one with another and are clearly notsegments of prominent end-moraines. Figure 4(b) shows justone set of large-scale fluting, which is of low spatial density.Thus the large size and low density of the McGregor fluting aredistinctly different from the small-scale, densely spaced flutes atBreidamerkurjokull. Unlike the Icelandic flutes, the set atMcGregor Lake does not terminate at a well-defined end-moraine (Fig. 4(b); Evans et al., 2008, their Fig. 12).For theoretical and observational reasons, large-scale fluting

at McGregor Lake and small-scale flutes at Breidamerkurjoku-land are non-analogous. It is equally clear that the prominentend-moraines at Breidamerkurjokull and the landformsmapped as discontinuous ridges in the McGregor Reservoirhummocky terrain are also non-analogous. The Evans et al.(2008) form analogy is in question on both counts.The spatial relationship between fluting and minor moraines is

critical to the glacial analogy – Evans et al. (2008, p. 252) wrote:

. . . streamlined landforms (fluting and fluted ridges) . . . fanout in southern Alberta to terminate at a series of minortransverse ridges.

However, Shetsen (1987) and Beaney and Shaw (2000)mapped fluting in southern Alberta, extending over the MilkRiver Ridge (Fig. 1) and well beyond the minor transverseridges. Most minor fluting fields do not terminate at end-moraines (Fig. 5; Evans et al., 2006, their Fig. 3), and many end-moraines are not associated with fluting. Contrary to the glacialhypothesis, minor moraines and large-scale fluting are notclosely related.Evans et al. (2008, p. 259) also proposed a relationship

between ice streams in flow paths, fluting and spectacular ice

Figure 5 Flow patterns and former ice-terminal position at McGregor Reseterrain comprises mainly transverse push-moraines. Adapted from Evans e1958331109078)


thrust hills such as the Neutral Hills (Fig. 6). The hills are said tolie at

. . . the termini of narrow corridors of densely spaced flutings.

It is implicit in Evans et al. (2008) that ice streamsoccupied thenarrow corridors (flow paths) of densely spaced flutings, andexplicit that the thrust hills were constructed by the lobatemargins of these streams. Hence there should be abundantlyobvious patterns of densely spaced fluting in low-relief flowpaths terminating at spectacular ice thrust hills. This would beone of the most striking landform associations on the Prairies –each of its components is distinctive. If they were real, thesepatterns would convincingly support the glacial model.However, the Neutral Hills were thrust from the north-east,where a narrow corridor of densely spaced fluting is expected inthe glacial model, but Fig. 6 shows irregular terrain immediatelynorth-east of the hills, and there is no sign of a narrow corridor offluting in theirvicinity.Criticalobservationsnecessary tosupportthe glacial model for ice thrust hills are missing.Large-scaleflutingnear the townofAthabasca, centralAlberta

(Fig. 1), originated above the southeastern side of the AthabascaValley (Fig. 3(a)), where the valley side faced into the flow andacted as a forward-facing step. Fluting on transverse ridge crests(Munro-Stasiuk and Shaw, 2002; Evans et al., 2008) also liesdownstream from forward-facing steps – the ridge flanks. Thelocation of large-scale fluting downstream from steps is verycommon, and there can be little doubt that it represents afundamental property of the formative flow. The meltwatermodel predicted longitudinal vortices at steps. This predictionwas verified by recent advances on the fluid dynamics ofturbulent flow over steps. Pollard et al. (1996) used experimentandcomputationandWilhelmetal. (2003)appliedcomputation

rvoir southern Alberta, constructed on the assumption that hummockyt al. (2006), reproduced with permission (Elsevier Ltd. License No.


Figure 6 Ice-thrust Neutral Hills, southern Alberta. Arrows indicate the direction of thrusting. The configuration of the thrust ridges relative to theEastern Flow Path illustrates that theywere not constructed by an ice stream in this path. Note that there is no narrow corridor indicating ice stream flowfrom the north-east, nor is there densely spaced fluting terminating at the hills. NASA Shuttle Radar Topographic Mission 90m horizontal resolution.Courtesy of NASA/JPL-Caltech


to demonstrate that scale-independent, longitudinal vorticesextend downflow from forward-facing steps in turbulent flow.Such vortices are likely to erode longitudinal grooves withintervening ridges (Pollard et al., 1996) and will occur over awide range of scale. The meltwater hypothesis proposes deep,broad and turbulent, subglacial meltwater flows, and fluting islocated exactly where theory and experiment predict longitudi-nal vortices in such flows. This systematic location of flutingwhere vortices are expected is unlikely to be coincidental.Fluting furrows formed by groove ploughing will be straight

sided with width changing imperceptibly (Clark et al., 2003).By contrast, furrows in the Athabasca fluting field expandappreciably over short distances, widening where they containspindle-shaped erosional marks (Fig. 3(b)). Munro-Stasiuk andShaw (2002) described similar erosional marks on the Black-spring Ridge (Fig. 1), southern Alberta. Such spindle-shapedmarks are characteristic of meltwater erosion (e.g. Kor et al.,1991) and are most unlikely to result from groove ploughing.Two very different histories of sedimentation and defor-

mation apply to the Century Estates and Schiller sites in theAthabasca fluting field (Fig. 1; see Shaw et al., 2000, forlocations). Fluting ridge sediment at Century Estates ischaracterised by lodgement and melt-out till, intratill conduitsand and gravel, and cavity-fill sand and flow till. Elongateclasts in melt-out and lodgement till are strongly orientatedparallel to the trend of the fluting. Exposures extending to theground surface show no wholesale deformation structures.Sediment at the Schiller site is more complex (Fig. 7). In situdiamicton and cross- and parallel-laminated sand weredeposited first in subglacial cavities. Diamicton dikes tens ofcentimetres thick and numerous millimetre-thick diamictonveins cross-cut the older sediment and record extrusion of low-viscosity sediment into large cavities. To the east, soft-sedimentclasts up to 2m across were rafted upwards in low-viscosity,melange diamicton, which filled a cavity at least as high as thesection (�8m). Clast fabrics measured in the dikes andmelangeshow no collective preferred, orientation – the principal Eigenvector azimuths are highly dispersed. The ridge is erosional,with gravel and a cobble lag at the surface (Fig. 7). Fluting atBlackspring Ridge is also erosional; undisturbed bedrock andgravel are truncated at the ground surface.


A number of conclusions arise from these observations.Undisturbed bedrock and gravel at Blackspring Ridge andintact bedding at Century Estates testify that ploughing andsubglacial deformation did not form fluting at these sites.Meltwater erosion is a more likely explanation. However,principal Eigen vectors for till fabrics at Century Estates areparallel to fluting orientations, which suggests a possible glacialorigin. Yet, the fluvial explanation may be retained if thegradients that drove ice and water flows both depended largelyon ice-surface slope. Veins, dikes andmelange diamicton at theSchiller site point to sediment flow under pressure. Flows with astrong vertical component and dispersed clast fabrics are not,however, consistent with sediment emplacement underglacially induced, unidirectional shear (Shaw et al., 2000).On the other hand, a field of erosional ridges and troughs,together with boulder and cobble lags, and surface gravels,suggests meltwater scour by broad, sheet flow. An inferredmeltwater origin for spindle-shaped erosional marks in theAthabasca fluting field and on Blackspring Ridge adds weight tothis conclusion. Fluting locations where longitudinal vorticesare expected in turbulent sheet flow also support the meltwaterhypothesis. Evans et al. (2006) disputed these findings byspeculating that the gravel at Blackspring Ridge may have atectonised carapace. However, there is no tectonite where thisgravel is exposed beneath thin soil at the crest of a fluting ridge(Munro-Stasiuk and Shaw, 2002).

Evans and others argued that the meltwater model is not aproper hypothesis because it cannot be falsified (Benn andEvans, 1998, 2006; Evans et al., 2006). However, Evans et al.(2006, p. 1634) seem to have forgotten this argument when theyclaimed that cross-cutting fluting negates the hypothesis;therefore, it is falsified with respect to cross-cutting. Themistaken perception that bedforms scoured by meltwater sheetflows cannot cross-cut ought to be put to rest. A simple thoughtexperiment does just that:

It takes time for a subglacial flood to erode fluting or drumlinsor s-forms. If the flow direction changes rapidly because theoutlet shifts or the conduit geometry changes, additional timeis needed to scour a fresh set of bedforms and to erase the firstset. Therefore, two sets of cross-cutting erosional marks


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would exist at the same time. Further shifts in flow directionbring about new erosional sets, which, depending on the rateof erosion, could all survive.

Multiple cross-cutting bedforms are likely to result from thisevolution, and similar events gave rise to fluting cross-cuttingspindle flutes at Wilton Ball Park, Ontario (Fig. 8).

Hummocky terrain

The glacial model

Evans et al. (2006, 2008) presented a new explanation forhummocky terrain on the Prairies. They considered hummocksto be depositional landforms (Evans et al., 2008, p. 256), butsuggested that glaciotectonic push-moraines make up largetracts of hummocky terrain. Thus, ridges in the modernIcelandic glacial environments are considered to be analoguesfor hummocks (ridges?) on the Prairies. For example, hum-mocky terrain flanking the Eastern Flow Path is described as

. . . a prominent ridge complex . . . comprising a 470 km longbelt of hummocks that display numerous discontinuousridges (Evans et al., 2008, p. 255, my italics).

Note that the terms hummock and ridge are used inter-changeably.In their model, Prairie fluting terminates at associated push-

moraines. If this is the case, fluting ridges should divergetowards arcuate moraines; they should not cross the distal limitof the contemporaneous set of moraines; and they should beorthogonal to these moraines. If the model is universal, allminor fluting should terminate at moraines.This model can be tested by assuming that Evans et al. (2006,

2008) are correct and hummocky terrain comprises recessionalmoraine ridges at which fluting terminates. A slightly amendedversion of Evans et al. (2006, their Fig. 5) allows assessment ofthis interpretation (Fig. 5). Limits to minor ice readvances arelocated at the distal limits of ridge domains, which are definedas areas of similar ridge orientation. Each domain is designated

Figure 8 Spindle erosional marks (s) cross-cut by fluting (f), WiltonBall Park, southeastern Ontario. The erosional marks are sculpted inPalaeozoic carbonate. Light, superimposed striations are absent fromthe proximal parts of the fluting furrows, indicating that they wereprobably formed by meltwater



by a Roman numeral and is associated with an ice-marginalposition. Younger domains usually cross-cut older domains. Icetermini are numbered from oldest to youngest (Fig. 5). Theevolution of these terminal positions requires unlikely shifts inglacier flow. For example, flow shifted from north-west at thetime of terminus 1 to north at the time of terminus 2, and back tonorth-west at the time of terminus 3. These shifts are unlikely inan ice sheet, given the short distances of retreat between termini.The relationship between terminal positions, ridge domains

and large-scale fluting raises telling objections to the glacialhypothesis. After mapping transverse moraines and fluting inthe McGregor Lake area, Evans et al. (2006, their Fig. 4(b))referred to fluting in the mapped area (Fig. 5) and concludedthat (p. 1643)

. . . recent flutings terminate at a series of transverse ridgeswith intervening pond-filled depressions.

This conclusion implies that there is a genetic relationshipbetween fluting and transverse moraine, but there is only onefluting field in Fig. 5. This field crosses and is, therefore, older thanterminal positions 7 and 8, and also positions 5 and 6, where theywere overrun by readvance 7. Consequently, terminus 4 is theyoungest possible marginal position for the fluting. Thus, in theice-push interpretation for the transverse ridges, fluting survivedreadvances 5, 6, 7 and 8, but not some recessional ridges indomains IV, V, VI and VII, which were destroyed by overridingice. In thismodel, flutingwould be destroyed too. However, if theridges and fluting formed subglacially, ridges at, say, position8 could format the same time as fluting crossing position 5, 6, and7 and extending to position 4. This arrangement allows fluting tobe superimposed on ridges (hummocks) in a way that is notpermitted with ice-marginal ridge formation (e.g. Hoppe, 1957;Stalker, 1960).Further field evidence contradicts the active ice model for

hummocky terrain. Although fluting in the glacial model issupposed to fan out towards arcuatemoraines, fluting lineationsin theMcGregor Reservoir area are either parallel or showminorconvergence (Fig. 5). Clearly, the configuration of flow linesinferred fromflutingorientation isnotconsistentwithpredictionsof flow in the glacial model. Evans et al. (2006) proposed thatfluting and recessional moraines are closely related becausefluting terminates at series of ridges. This proposal is question-ablebecause, inmanycases, thenumbersofflutingsetsandridgedomains do not match. For example, Fig. 5 depicts only onefluting field, and eight ridge domains and marginal positions.Obviously, the relationship between fluting and marginalpositions is not one-to-one. Thus, ice-frontal position and ridgedomains, fluting alignment, and the number of moraine andfluting sets present strong evidence against the glacial hypoth-esis. These conclusions are hardly surprising, given that, as thename implies, most of hummocky terrain in the McGregorReservoir area is in hummocks, not ice-pushed ridges (Munro-Stasiuk, 1999; Evans et al., 2006, their Fig. 4(b)).Moraines in flow paths present further arguments against the

active-ice hypothesis. Unequivocal examples of recessionalmoraines at Frank Lake, in theWestern Flow Path near the townof Nanton (Fig. 1), are completely unlike the ‘ridges’ inhummocky terrain (compare Figs 9 and 10) and, despite thepredictions of the glacial model, there is no minor fluting in thevicinity. The recessional ridges record the extent of Cordilleranice flowing down the Bow Valley – ice lobe 2 clearly flowedacross, not along, the Western Flow Path (Fig. 10). By contrast,Evans et al. (2008) suggested moraine formation by theLaurentide Ice Sheet flowing along the Western Flow Pathunder stable climatic conditions.Sjogren et al. (2002) described relatively young, incipient

tunnel channels dissecting hummocky terrain upland, and


eskers are superimposed on hummocks. Since the channels andeskers are subglacial, the older ridges and hummocksare probably subglacial too; they could only be ice-marginalif they formed during ice sheet advance. If that was so, theridges and hummocks had to survive at the ice bed over a fullglacial cycle. Such preservation is unlikely in the glacialhypothesis, which proposes glacial transport in a subglacial,deforming bed.

The meltwater hypothesis

Further observation and interpretation raise more complexissues on the formation of hummocky terrain. Hummockscontain a wide variety of surficial sediment and bedrock (e.g.Stalker, 1960, 1973; Tsui et al., 1989; Munro and Shaw, 1997;Boone and Eyles, 2001; Evans et al., 2006; Munro-Stasiuk andSjogren, 2006). Glaciotectonic thrusting and folding indicatestrong coupling, probably under frozen conditions, betweenthe ice sheet and the subglacial bed. However, glaciotectonicstructures and sedimentary stratification in hummocks andridges are invariably truncated by erosion; the hummockysurface forms a regional landscape unconformity (Figs 9(c)and 11; Munro and Shaw, 1997; Munro-Stasiuk and Sjogren,2006). As discussed earlier, incipient tunnel channels truncateor dissect hummocks or are deflected around them (Sjogrenet al., 2002). Boulders, commonly with percussion marks, forma lag where the underlying sediment is coarse.

Moraine plateaux commonly appear as dissected hills andcluster amongst hummocks with strong relief and relativelythick deposits (Stalker, 1960; Shetsen, 1987, 1990; Figs 7 and 9).Plateaux are irregular in shape, flat topped with underlying lakeor pond deposits, and their elevations are a fewmetres to tens ofmetres higher than adjacent hummocks. Swales are sculptedinto their margins and, occasionally, their top surfaces.Distinctive rim ridges commonly contain deformed lakedeposits, and were pressed by ice surrounding plateaux (e.g.Hoppe, 1952; Stalker, 1960; Boone and Eyles, 2001).

Plateaux obviously form together with hummocks andswales; their erosional margins and dissection suggest thatthey are remnant hills. Ice pressing of lake deposits requiresdeposition prior to ice removal, with accumulation in ice-walled lakes (e.g. Stalker, 1960; Clayton et al., 2008).Underlying surficial sediment and bedrock in plateaux riseabove the surrounding terrain and are elevated residuals ofwidespread deposits. Whatever the conclusion on the processof plateaux formation, the rim ridges show that they formed instagnant ice. Therefore, hummocky terrain cannot compriseactive-ice push-moraines that form at ice margins.

A clear picture emerges of the formation of hummockyterrain. At first, and for a relatively long period of time,sedimentation and glaciotectonic deformation acted overmuch larger areas than the present extent of hummockyterrain. At that time, deposition probably extended over thesites of the flow paths. Then, in a single violent event or inmultiple events, erosion scoured swales and sculpted hum-mocks; denudation and hummock formation left residualmoraine plateaux. Dissection of broad plateaux created smallerones, and some plateaux were eroded away. Hummocks andplateaux remain as residuals. Lags of cobbles and boulderswere concentrated on the ground, and bombarded by boulders,cobbles and pebbles transported by floodwater. Percussionmarks resulted where clast-to-clast collisions pitted andfractured rock surfaces. Only a megaflood or floods couldhave accomplished this geomorphological work. As the floodabated, subglacial streams eroded channels, melted conduits inthe ice and deposited eskers. Lacustrine sediment accumulated


Figure 9 (a) Hummocky terrain near Rumsey, Alberta. The arrow indicates the flow direction in the adjacentWestern Flow Path. Courtesy of NaturalResources Canada, National Air Photo Library. (b) Clustered moraine plateaux (cultivated) with surrounding hummocky terrain. Note how hummocksform symmetrical polygons around swales. This characteristic is accentuated where the swales contain ponds (black). Courtesy of Natural ResourcesCanada, National Air Photo Library. (c) Morphology and structure in hummocky terrain

Figure 10 Recessional moraines at Frank Lake, southern Alberta.Note the absence of minor fluting. The moraines are superimposedon large-scale fluting. Cordilleran ice, Lobe 2, flowed across, not alongthe Western Flow Path. Shuttle Radar Topographic Mission hillshade at90m horizontal resolution. Courtesy of NASA/JPL-Caltech



in ice-walled lakes and was pressed into ridges by surroundingglacier ice. Finally, the ice melted away, leaving a remarkablycomplete record of events. Erosion by broad, subglacial floodsaccounts for swales between hummocks, dissection of highground to form clustered plateaux, eventual removal of someplateaux, and boulder and cobble lags (Munro and Shaw, 1997;Sjogren et al., 2002; Munro-Stasiuk and Sjogren, 2006). Theregular form and scale of hummocks and swales are typical ofaqueous regime bedforms and support a meltwater origin forhummocky terrain. It is difficult to see how the ice-pushhypothesis for hummocky terrain could explain this landscape.Alternative mechanisms for the formation of lags involve

clasts sinking in deforming beds (e.g. Clark, 1991) andpostglacial winnowing (Evans et al., 2006). Boulders andcobbles in the glacial explanation form buried pavements, andthey are generally faceted and striated by tools in overriding ice(Clark, 1991). In the absence of surface deformation till, thismechanism for lag formation is unlikely. Also, many clasts inlags originate in coarse substrates, not in overlying till. They aremainly subrounded to subangular with percussion marks,unlike the faceted and striated glacial boulders and cobblesdescribed by Clark (1991). Thus, the characteristics of the clastsare not consistent with glacial emplacement. Evans et al. (2006,p. 1643) proposed that lags formed by wind deflation of till, butaeolian deposits are generally derived from glaciolacustrinedeposits, not hummocks (Shetsen, 1997). They also maintained(p. 1663) that, because they rest on ground with underlyingcoarse material, lags must result from postglacial winnowing byrunning water. It is not clear why location on coarse materialrules out lag formation by meltwater sheet flow. Furthermore,neither deflation nor postglacial winnowing accounts forpercussion marks.


Figure 11 Truncated sequence, McGregor Lake, indicating complex deposition and glaciotectonics prior to the formation of hummocks by erosionthat cuts indiscriminately across sedimentary and glaciotectonic structure. From Munro and Shaw (1997). Courtesy of the Geological Society ofAmerica


Hummocky terrain tracts and flow paths:landform and sediment

Narrow tracts of inter-flow path hummocky terrain are over400 km long (Fig. 12; Shetsen, 1987, 1990; Rains et al., 1993;Shaw et al., 1996; Munro and Shaw, 1997; Munro-Stasiuk andSjogren, 2006; Evans et al., 2008). Surficial sediment thicknessin hummocky terrain is about 10–15m, much thicker than inflow paths where bedrock is exposed over much of their extent(Shetsen, 1987, 1990; Rains et al., 1993). Rains et al. (1993) and

Figure 12 Distribution of hummocky terrain southern and central Alberta.


Shaw et al. (1996) assumed original thicknesses of surficialdeposits were about the same in flow paths and hummockyterrain. They recognised flow paths as zones of pronouncederosion, and suggested that the difference in present surficialsediment thickness between hummocky terrain and flow pathsgives a minimum depth of erosion in the paths. Simplecalculations demonstrate the significance of such erosion tolandscape evolution. Assuming net erosion of 10m for surficialdeposits and bedrock along a flow path about 100 kmwide and600 km long, the volume of eroded sediment is about 600 km3.If this sediment is deposited in moraines with surface slopes of

From Munro-Stasiuk and Sjogren (2006), reproduced with permission



58, it could produce 50 ridges, each about 20m high. However,the landscape is not consistent with this prediction; only minormoraines, which are almost imperceptible from the ground, arefound in flow paths (Fig. 10; Shetsen, 1987; Rains et al., 1993;Beaney and Shaw, 2000).Evans et al. (2006, 2008) concluded that glacial erosion

progressed by incorporating bedrock and surficial deposits intothe base of a deforming bed. This process, however, explainsneither the dearth of sediment nor the absence of high morainesat the Last Glacial Maximum of the Laurentide Ice Sheet. Bycontrast, extraglacial meltwater flow and sediment transportaccount for the sediment budget dilemma and ‘missing’moraines – sediment was carried by floods to the MississippiDelta and Fan, where thick deposits accumulated rapidly(Shaw et al., 1996).In the Evans et al. (2008, p. 255) interpretation, hummocky

terrain immediately adjacent to flow paths comprises ice-marginal push-moraines formed by minor advances during icesheet recession. It is easy to determine whether or not this viewis realistic. Assuming ridge spacing of about 300m (Evans et al.,2008, their Fig. 3), and hummocky tract length of 470 km, about1570 minor readvance moraines are expected. Yet, other thanin the very south of Alberta and in the vicinity of Frank Lake(Fig. 10; Shetsen, 1987), there are no equivalent moraines inadjacent flow paths. It is possible that such moraines have notbeen recognised, but the lobate moraines around Frank Lake(Fig. 10) are so distinctive that it is most unlikely that similarfeatures are undetected elsewhere in flow paths. Rather, exceptin deglacial lake basins where lacustrine and aeolian sedimentsblanket their morphology (Shetsen, 1987, 1990), pristinesubglacial landscapes are preserved in flow paths (Fig. 2;Shaw et al., 1996). It is most unlikely that an ice sheet builthundreds of recessional push-moraines adjacent to the Easternand Western Flow Paths (Fig. 12) without building moraines inthe paths themselves. Consequently, the glacial hypothesisshould be rejected.

Discussion and conclusions

While the glacial model for fluting and hummocky terrain onthe Prairies is erroneous, supporting evidence for the meltwaterhypothesis is strong. The two models are working hypotheses,and confusion on hypothesis testing requires correction. Evanset al. (2008, p. 257) wrote that the meltwater interpretation hasbeen widely rejected because it fails to provide the leastnumber of assumptions, but they do not spell out how manyassumptions it requires. Nevertheless, if they appeal to theprinciple of least number of assumptions, they must demon-strate that their glacial model satisfies this requirement. More tothe point, why did they cite this principle in the first place? Itshould be invoked only when all else is equal betweenhypotheses, but all else is not equal between the glacial andmeltwater hypotheses – meltwater processes explain flowpaths, fluting and hummocky terrain better than do glacialprocesses.Testing a hypothesis amounts to assessing how well

observations and interpretations fit its proposals. For example,subrounded lag boulders with percussion marks are explainedby the following interpretation: boulders excavated from thesubstrate are immersed in a fast-flowing, highly turbulent flow(meltwater); this flow is competent to carry cobbles and smallboulders; and impacts between lag and transported clastsproduce percussion marks. The meltwater hypothesis proposesthat powerful subglacial meltwater sheet floods scour land-


scapes. Thus the interpretations that explain observations onlags are an excellent fit to the hypothesis; therefore, it issupported. If other interpretations fit the hypothesis (e.g.longitudinal vortices at steps), it gains support. There may beother, relevant hypotheses, for example the glacial hypothesis,and the same procedure should be applied to them. If anobservation arises for which there is no interpretation that fits ahypothesis, the hypothesis should be amended or rejected.Thus, percussion marks on boulders are interpreted to resultfrom collisions between stationary boulders and cobbles intransport. The aeolianmodel for percussion marks fails becausewinds do not transport pebbles and cobbles, and there is noobvious way to amend this hypothesis to make the interpret-ations fit. By rejecting the aeolian explanation, we canconcentrate on more promising avenues of research.For the most part, the following interpretations and

conclusions emphasise the roles of observation and theory intesting the glacial and meltwater (megaflood) hypotheses. Theybring out inconsistencies in the glacial hypothesis and the waysinwhich they are avoided in themeltwater model. This exerciseis a defence of the meltwater model.Erosional flood paths across the Prairies are probably

footprints of ice streams that drained part of the LaurentideIce Sheet. They also carried megafloods that carved large-scalefluting. Scabland topography is more rugged than Prairie relief,but this does not mean that there were no megfloods on thePrairies; cataclysmic glacial lake discharges on the CanadianPlains did not erode rugged landscapes.The glacial hypothesis for flow paths and large-scale fluting,

which depends on an analogy with small-scale flutes at modernglaciers, is invalid because different glacial processes generatethe two forms. Also, fluting at ridges and forward-facing steps isshorter and less extensive than is predicted by the groove-ploughing mechanism favoured in the glacial model.Maps of an Icelandic glacier forefront and a Prairie

landsystem, designed to portray analogy between the twolandscapes, show little similarity and, despite predictions in theglacial system, fluting on the Prairies is not closely related to‘arcuate push-moraines’.Using the locations of fluting the meltwater hypothesis

predicted high-energy vortices in turbulent flow at forward-facing steps. Experimental and computational fluid dynamicsverified this prediction. Spindle-form erosional marks in flutingfurrows cannot be explained by groove ploughing, and becausethey are identical to s-forms elsewhere these erosional marksare naturally interpreted as meltwater forms.Space shuttle images present unequivocal evidence that ice

thrust hills, flow paths and fluting are not directly related; amajor component of the glacial hypothesis lacks supportingfield observation.Since surficial sediment and bedrock in fluting may be

undeformed or intensely deformed, subglacial deformationcannot be responsible for fluting formation. Sedimentary anddeformation structures and clast fabrics support this interpret-ation. Truncation of structures at the ground surface and surfacelags point to erosion; depositional and glaciotectonic processeshad little bearing on fluting formation – at most, they may havecreated landforms that influenced initial locations of scour.Cross-cutting bedforms are frequently presented as proble-

matical by proponents of the glacial hypothesis, but a thoughtexperiment demonstrates that cross-cutting is easily formed.Cross-cutting meltwater erosional marks are illustrated.The glacial hypothesis on hummockymoraine includes some

unusual interpretations. The term hummock is said to besynonymous with the term transverse ridge, and the twolandforms are interpreted as push-moraines. Fluting is said toterminate at these moraines. A map of landforms at McGregor



Lake, used to test this model, illustrates major inconsistencies:fluting does not terminate at most end-moraines; it does notsplay towards arcuate moraines; and there are many more‘glacial termini’ than sets of fluting.Indisputable recessional moraines at Frank Lake bear no

similarity to ‘push-moraines’ in hummocky terrain. Thisobservation undermines the glacial hypothesis.In the meltwater model, tunnel channels and eskers

superimposed on hummocks are evidence for a subglacialorigin. The alternative view of ice-marginal genesis in theglacial model requires that hummocks survive in a subglacialenvironment over a full glacial cycle. On the other hand,fluvioglacial and residual origins are supported by the followingevidence: rounded hummocks, bedforms of similar form andsize to their neighbours, truncated internal structure and surfacelags. Moraine plateaux are also residual; clusters of residualplateaux record the dissection of high ground. Ice-pressingtoward plateaux created rim ridges, requiring that glacier icesurrounded the plateaux.

Acknowledgements With thanks to the Natural Sciences andEngineering Research Council of Canada (contract grant numberG12110428) for a Discovery Grant. Bruce Rains, Mandy Munro-Stasiuk, Darren Sjogren and Rob Young taught me about the Prairielandscape. I am grateful to Roger Hooke for a critical and helpfulreview. An anonymous reviewer expressed doubts about this paper, butgenerously recommended that it be published. Don Rodbell madecritical editorial suggestions.

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Documents

In defence of the meltwater (megaflood) hypothesis for the formation of subglacial bedform fields