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Since the concept of drumlin formation by glacifluvial processes was first proposed by Shaw (1983), a large number of papers have been published interpreting a wide range of subglacial landforms in North America as products of subglacial megafloods (e.g. Shaw & Kvill, 1984; Shaw et al., 1989, 2000; Shaw & Gilbert, 1990; Fisher & Shaw, 1992; Rains et al., 1993; Brennand et al., 1995; Sjogren & Rains, 1995; Shaw, 1996, 2002; Munro & Shaw, 1997; Beaney & Hicks, 2000; Beaney & Shaw, 2000; Munro-Stasiuk & Shaw, 2002). The sheer volume of peer-reviewed publications promoting the 'megaflood interpretation', and the fact that it has featured prominently in at least two recent compendia of earth science (Young, 2000; Brennand, 2004), may lend it an aura of respectability in the eyes of those unfamiliar with the evidence. However, most Quaternary scientists give little or no credence to the megaflood interpretation, and it conflicts with an overwhelming body of modern research on past and present ice-sheet beds. Despite this, there have been few attempts to systematically scrutinize the megaflood interpretation in print. In large part, this is because most working scientists are busy pursuing their own research programmes and are unwilling to invest time refuting ideas which are clearly incompatible with a huge body of mainstream research.

It is to the credit of journal editors that outrageous hypotheses (Davis, 1926) have been published. Science, after all, should proceed by the testing of ideas in the public domain, not by the censorship of papers simply because they are unorthodox. To become accepted, however, research should conform to the requirements of good science. Non-specialists can rarely be effective judges of whether this is or is not the case, and there is a risk that flawed science may appear to be mainstream in the eyes of a wider public. This is a common problem in the public perception of science, due to the unfortunate tendency for contentious theories to attract disproportionate attention. For this reason alone, the reluctance of many Quaternary scientists to publicly engage with the megaflood interpretation is regretable. This, combined with the fact that the megaflood interpretation has been seized by internet Creationist sites as 'proof' of the Noachian Flood, prompts us to undertake here the task of explaining, for the benefit of those unfamiliar with modern sedimentological literature, why the flood interpretation is unscientific, unnecessary and inconsistent with the evidence.

The megaflood interpretation is not a hypothesis in the Pop-perian sense of a provisional set of ideas that make clear, unambiguous predictions which can be objectively tested against new observations. The clearest example of this is the claim that megafloods can create drumlins by two different mechanisms, (i) by the infilling of subglacial cavities (cavilty fills) and (ii) by eroding away interdrumlin areas (erosional remnants). The cavity-fill interpretation was proposed by Shaw (1983) and Shaw & Kvill (1984), and visualizes drumlins as the infills of scours cut upward into the ice by turbulent waters below. This idea, which was based on the similarity of form between certain drumlins and sole marks below turbidites, and the presence of sorted sediments within some drumlins, makes the clear prediction that other drumlins should also be composed of sorted sediment (e.g. Sharpe, 1987). If this prediction is put to a Popperian test and a drumlin is found that does not contain sorted sediment, then the hypothesis should be rejected. However, this was not the option taken by Shaw et al. (1989,2000) and Shaw (1993), who proposed that till-cored drumlins and flutings are the remnants of preexisting tills left behind when megafloods eroded the material between them. In other words, no matter what the internal composition, drumlins are interpreted as 'evidence' for megafloods. This being the case, the internal composition becomes irrelevant to the megaflood case, which is shown to rely exclusively on the perceived morphological similarity between drumlins and streamlined forms eroded by turbulent flows. Moreover, the megaflood interpretation apparently does not predict any systematic differences in the forms produced by these two mechanisms. If no such differences are expected, how can a single process (subglacial sheet floods) create two sets of erosional forms which are exactly equal and opposite in morphology? That is, why should moulds of erosional scours cut up into overlying ice look exactly like the remnants left behind by erosion of the substratum in other parts of the same flood? This difficulty is not experienced by Boulton's (1987) deformation model of drumlin formation or Tulaczyk et al.'s (2001) ploughing mechanism of substrate fluting (see also Clark et al., 2003), for example, which potentially can explain all drumlins and flutings in terms of a single process, namely the streamlining of pre-existing bed materials be they composed of till, rock, or stratified sands and gravels. The deformation and ploughing models, moreover, make the clear prediction that the drumlins and flutings formed by such processes should be mantled by glacitectonite or till, a prediction that is borne out in our experience. These models do not, as Shaw and his co-workers repeatedly try to assert, champion the cause of pervasive deformation to the exclusion of all other processes.

By invoking the 'erosional remnant' mechanism to account for drumlins that cannot be explained by the 'cavity fill' process, Shaw is using a classic ad hoc protection device (Chalmers, 1976), the sole purpose of which is to remove difficulties encountered by the original theory. Any model that is protected from awkward evidence in this way is in effect unfalsifiable. This is not the hallmark of a hypothesis, but of a self-reinforcing belief system in which the interpretation of the evidence depends on pre-existing conclusions. Although many of the papers by Shaw and co-workers invoke the language of hypothesis testing, this does not stand up to scrutiny. For example, Shaw (this volume, Chapter 4) asks us to 'imagine' scenarios 'for the sake of hypothesis testing', then we find later in the paper that the imaginings are facts, which are then used to support further imaginings, and so on. Nowhere is there a serious and objective comparison of prediction with evidence.

As noted above, the megaflood interpretation relies very heavily, if not exclusively, on the morphological similarity between drumlins and certain kinds of small-scale erosional scours on the one hand, and between Rogen moraine and wavy fluvial bedforms on the other. There is indeed a superficial resemblance between drumlins and sole marks, as we illustrated in fig. 11.25 of Glaciers and Glaciation (Benn & Evans, 1998). However, this resemblance does not imply that they were necessarily formed by the same medium. The simple explanation is that obstacles below flowing media exhibit shadow effects, such that their presence influences patterns of erosion far down-flow. This effect applies not only to turbulent flows, but also to non-turbulent flowing media, such as ice. One need only to think of fluted moraines exposed on modern glacier forelands to see that this is so. Indeed, drumlins and megaflutings show a much stronger resemblance to streamlined subglacial landforms exposed by recent glacier retreat (in form and in scale) than they do to scours formed by turbulent media. The similarities are not merely superficial, but extend to numerous characteristics at a wide range of scales. Furthermore, historically produced fluting fields such as those in front of Breidamerkurjokull in Iceland allow us to relate sediment and landform characteristics to genetic processes with a high degree of confidence (Evans & Twigg, 2002). At Brei-damerkurjokull, flutings are aligned parallel to known former ice-flow directions in slightly offset flow sets that terminate at moraines (Fig. 8.1). Tills in the flutings commonly have erosional lower contacts with glacitectonized or undisturbed outwash. Detailed process studies have demonstrated the role of subglacial lodgement and deformation in the origin of the tills and the flutings (e.g. Boulton & Hindmarsh, 1987; Benn, 1995; Benn & Evans, 1996; Boulton et al., 2001). Eskers mark the location of channelized meltwater. This landsystem provides us with a clear process-form model and it is a small logical step to assume that it can be applied to ancient landform-sediment assemblages that have a wide range of closely similar characteristics.

To this end, landform assemblages illustrated through digital elevation models (DEMs) by Munro & Shaw (1997) have been remapped from aerial photograph mosaics, and have been shown to consist of discrete fields of glacially streamlined features (flutings) terminating at a series of inset transverse ridges (moraines) organized in broad arcuate bands (Fig. 8.2). More localized moraine arcs record topographically induced lobation of the ice margin during recession. Minor readvances of these lobes are documented by the localized superimposition of transverse ridges. This is a landform characteristic difficult to explain as a subglacial fluvial erosional ripple mark, the genesis of the transverse ridges suggested by Munro & Shaw (1997). Also evident are misaligned and cross-cutting flow sets, represented either by superimposed flutings or adjacent fluting fields with orientations that are significantly different and which cannot be explained by contemporaneous ice flow deviations. Juxtaposed fluting fields that display different orientations are convincingly interpreted as glacier flow sets (Clark, 1997) and the superimposition of flow sets is often identifiable in cross-cutting flutings (Dyke & Morris, 1988; Boulton & Clark, 1990; Clark, 1993). Flow sets have been interpreted as the subglacial imprint of fast glacier flow in an ice mass with shifting loci of ice dispersal and termination. Cross-cutting can be explained by the ice streamlining hypothesis but not the sheetflood hypothesis of fluting formation. In addition, evidence of subglacial and/or englacial meltwater activity is manifest in fragmented single and anabranched esker networks and occasional elongate water-filled depressions. This assemblage of landforms is similar in every respect to the glacial landsystem reported by Evans & Twigg (2002) from southern Iceland, characterized by inset sequences of integrated subglacial and ice-marginal landforms produced by lobate marginal recession of active temperate glaciers.

Given the availability of clear modern analogues for ice-sheet beds on modern glacier forelands, the case for the megaflood interpretation is seriously weakened, because turbulent flows are shown to be unneccesary to explain streamlined subglacial landforms. Historical jokulhaups have occurred at Breidamerkurjokull, but these were associated exclusively with small ice-dammed lakes at the western and eastern margins. Almost all of the glacier foreland (which is extensively fluted) has been unaffected by jokulhlaups. If sheetfloods are unnecessary (indeed, impossible) as an explanation for streamlined subglacial landforms at Breidamerkurjokull, why must we invoke them to explain closely similar sediment-landform associations in, say, Alberta? In the fourteenth century, William of Ockham wrote, 'pluralitas non est ponenda sine neccesitate', which translates as 'entities should not be multiplied unnecessarily'. The principle that explanations of phenomena should not invoke any agencies not actually required by the evidence, led the way out of medieval superstition into scientific enlightenment. It still stands as a central tenet of science today. If we already have an ice sheet, and

— Flutings and drumlin crests '' ' " | Moraine ridges .'■'/.';;

0 1000 m ^ -C^----' _______

Figure 8.1 Push moraines and flutings on the foreland of Breidamerkurjokull, extracted from a 1998 map of the area by Evans & Twigg (2000). Each flow set of flutings is located between push moraines and when compared the flow sets record a slightly offset ice flow direction.

Figure 8.1 Push moraines and flutings on the foreland of Breidamerkurjokull, extracted from a 1998 map of the area by Evans & Twigg (2000). Each flow set of flutings is located between push moraines and when compared the flow sets record a slightly offset ice flow direction.

flowing ice can create streamlined landforms, why do we need a flood?

Thus, although the resemblances between streamlined subglacial landforms and features such as sole marks and sastrugi may appear significant, modern glacial landforms provide much stronger analogies. Attempts to boost the argument for turbulent flows by appealing to required Reynolds numbers are spurious. The Reynolds number Re is the ratio between inertial and viscous forces in a fluid. If it is assumed that streamlined subglacial land-forms must have been made by turbulent flows (high Re), then of course it follows that ice was not the medium, since ice flow is not turbulent. But this is mere circular reasoning, in which the conclusion comes first, disguised as an argument.

The case for the megaflood interpretation would be strengthened if there were independent evidence that requires us to believe that large volumes of water were stored at the beds of former ice sheets. Shaw (this volume, Chapter 4) tackles this issue by arguing that the presence of tills implies that meltwater was 'abundant' in the inner parts of the Scandinavian and Laurentide ice sheets. Munro-Stasiuk (2000, 2003) uses the occurrence of stratified diamictons in an area of south-central Alberta to support the contention that englacial debris was melting out into large subglacial lakes beneath the southwest Laurentide Ice Sheet. The use of stratified diamictons as indicators of subglacial melt-out is a contentious one, but even if we accept that melt-out was the primary depositional process (and observations on sedimentation rates in modern subglacial lakes (e.g. Siegert, 2000; Siegert et al., 2001) suggest that this is almost insignificant), the implied volume of water produced does not come anywhere close to the quantities required by successive manifestations of the megaflood interpretation. Consider the quantities involved. We are told that a megaflood draining into the Gulf of Mexico (only one component of the proposed cataclysm) was sufficient to raise global sea level by 3.7m. This is about half the mass of the Greenland Ice Sheet. To claim that the occurrence of stratified tills supports the existence of enough water to supply floods of the required magnitude involves a logic jump that bypasses numerous stages of hypothesis testing. One obvious query relates to the nature of water release during melt-out till production—surely it must be released slowly and passively in order to preserve the delicate structures? Moreover, the melt-out process would have continued in areas of the bed of the former Laurentide Ice Sheet well after it had disappeared; it continues today in northern Canada (Dyke & Savelle, 2000; Dyke & Evans, 2003). The very nature of melt-

Figure 8.2 Sequences of push moraines and flutings mapped from aerial photographs of a part of south-central Alberta previously mapped from DEMs by Munro & Shaw (1997). Note the overprinting of push moraines with slightly offset alignments and the localized lobation that coincides with topographic hollows.

Figure 8.2 Sequences of push moraines and flutings mapped from aerial photographs of a part of south-central Alberta previously mapped from DEMs by Munro & Shaw (1997). Note the overprinting of push moraines with slightly offset alignments and the localized lobation that coincides with topographic hollows.

out till production precludes any central role it could have played in the production of catastrophic subglacial meltwater floods.

The idea of floods of such 'unimaginable' (Shaw, this volume, Chapter 8) dimensions is the outcome of taking flawed assumptions to their logical conclusion, a form of reductio ad absurdum in which the final absurdity is taken not as evidence of false premises but as fact. The initial flawed assumption, that the form analogy between drumlins and scours made by turbulent flows implies a common mode of formation, led to the conclusion that the Livingston Lake drumlin field in north Saskatchewan records a megaflood event (Shaw, 1983). Once this was accepted, then the close association between drumlins and other landforms (such as Rogen and hummocky moraine) was taken to imply that they too must have a flood origin (Munro & Shaw, 1997). As such land-forms are very widespread inside the limits of the Laurentide Ice Sheet, this assumed origin in turn leads to the conclusion that 'unimaginable' megafloods must have occurred. Each stage of this chain of thought is deeply flawed, but has been woven into such

46 D. I. Benn & D. J. A. Evans/J Shaw & M. Munro-Stasiuk a dense network of assumptions and conclusions that it appears solid to its proponents and to unwary onlookers. The apparently supporting 'evidence', such as widespread streamlined forms visible on DEMs (Shaw et al., 1996; Beaney & Shaw, 2000), and the modelling work of Shoemaker (1992a,b) are no more than assumptions in disguise, which serve to perpetuate the myth in a self-reinforcing cycle in which the distinction between assumption and conclusion is constantly blurred.

This process is apparently so beguiling that the evidence itself becomes distorted to fit the interpretation. As an example, Shaw (this volume, Chapter 4) tells us that the landscape within the limits of the last Laurentide Ice Sheet represents a snapshot of the glacier bed at one moment in time, a pristine, synoptic assemblage preserved by continent-wide ice stagnation following a megaflood. The lack of recessional moraines or of cross-cutting relationships within drumlin fields are presented as facts, whereas several researchers have convincingly shown that the opposite is the case (see Fig. 8.2). Widespread cross-cutting streamlined land-forms have been thoroughly documented for example by Dyke & Morris (1988), Boulton & Clark (1990) and Clark (1993). As was illustrated above with the Alberta case study, the very features used to support a subglacial megaflood (ripple marks of Munro & Shaw, 1997) in fact constitute the 'missing' recessional moraines that only become 'visible' when mapped objectively and systematically from aerial photographs.

It must be emphasized that our refutation of the arguments of Shaw and co-workers does not force us to conclude that no large subglacial outburst floods occurred during the lifetime of the great Pleistocene ice sheets. Indeed, evidence for such floods, in the form of tunnel channels for example, is found in many places (e.g. Brennand & Shaw, 1994; Patterson, 1994; 0 Cofaigh, 1996; Clayton et al., 1999; Cutler et al., 2002). Subglacial reservoirs, mostly of the order of 10km across, are now known to be widespread below the Antarctic Ice Sheet (Siegert, 2000), and some such reservoirs appear to have drained catastrophically in the past (Shaw & Healy, 1977; Denton et al., 1993). Tunnel channels and anastomosing channel networks such as the Labyrinth in the Antarctic Dry Valleys provide compelling evidence for catastrophic drainage events, but acknowledgement of this does not lead logically to the conclusion that landforms such as drumlins, flutings and hummocky moraine record vastly larger floods. The juxtaposition of demonstrably glacifluvial landforms (channels) with flutings or other landforms does not imply that all were formed simultaneously by the same mechanisms, as consideration of the landform associations at Breidamerkurjokull makes clear (Evans & Twigg, 2002). To make the step from local, albeit large floods to events of Biblical proportions falsely polarizes the situation into an 'all or nothing' scenario, which actually distracts attention away from the important business of determining the true importance of catastrophic discharges from former ice sheets.

Reply to Benn and Evans by John Shaw and Mandy Munro-Stasiuk

This response to the comments of Benn and Evans is divided into three parts: fact, omission and philosophy.

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