deformation of glacial materials: introduction and overview

Deformation of glacial materials: introduction and overview

A L E X J. M A L T M A N , B R Y N H U B B A R D & M I C H A E L J. H A M B R E Y

Institute o f Geography and Earth Sciences, University o f Wales,

Aberystwyth, Ceredigion SY23 3DB, UK

(e-mail. [email protected]; [email protected]; [email protected])

The flow of glacier ice can produce structures that are striking and beautiful. Associated sediments, too, can develop spectacular deformation structures and examples are remarkably well preserved in Quaternary deposits. Although such features have long been recognized, they are now the subject of new attention from glaciologists and glacial geologists. However, these workers are not always fully aware of the methods for unravelling deformation structures evolved in recent years by structural geologists, who themselves may not be fully aware of the opportu- nities offered by glacial materials. This book, and the conference from which it stemmed, were conceived of as a step towards bridging this apparent gap between groups of workers with potentially overlapping interests.

Glaciologists have long been aware of the remarkable structures developed in flowing ice. Nineteenth century Alpine mountaineers and natural scientists, such as the Swiss naturalist, Agassiz ('The Father of Ice Ages'), and Forbes and Tyndall from Britain, described a range of structures in glaciers, and were clearly impressed by the similarity with deformation structures in rocks. The first half of the twentieth century saw few advances in glaciological thinking, but renewed interest in structures followed the formulation of a flow law for ice (e.g. Nye 1953; Glen 1955) and its application to glaciers (e.g. Nye 1957). Since then, as outlined by Hambrey & Lawson (this volume), there have been numer- ous studies of deformation in glaciers. Many of these studies link glacier structures to measured deformation rates, notably in the classic case studies of Allen et al. (1960) and Meier (1960) in North America. However, few glaciologists have applied the structural geological concepts of progressive and cumulative deformation to glaciers. Where such an approach has been adopted (primarily within valley glaciers and ice caps), new insights have emerged concerning ice deformation in relation to the development of foliation, folds, and crevasses and other faults (e.g. Hudleston 1976; Hambrey 1977; Hambrey &

Milnes 1977; Hooke & Hudleston 1978; Lawson et al. 1994). However, these concepts remain to be applied to deformation at the scale of ice sheets, where analogous structures are at least an order of magnitude larger. Significantly, these structures may well contain information with the potential for assessing the long-term dynamic behaviour and stability of their host ice masses.

Striking deformation structures are also produced at a wide variety of scales in the sediments associated with ice. Fine examples appear, for example, in the works of Brodzikowski (e.g. in Jones & Preston 1989) and in the volumes by Ehlers et al. (1995a, b). In fact, of all the various kinds of geological deformation structures, among the very first to be described were features ascribed to the action of ice. Two great geological pioneers were involved: both Sir Charles Lyell (1840) and the visionary Henry Sorby (1859) interpreted the disturbance of deposits on the coast of eastern England as being due to the movement of icebergs. Lyell (1840) then expanded his ideas on the deformation of the glacial deposits of Norfolk, and by the end of the century a variety of structures in Europe and North America had been interpreted as the result of glacial processes. A history of these early studies of deformation of sediments by glaciers is given in Aber et al. (1989).

Despite this long pedigree of research, there is commonly disagreement on the actual mechanisms involved in generating glacigenic structures. The dominant concept this century, until a decade or so ago, involved the notion of ice bulldozing into sediments and generating various 'ice-thrust' structures as a result. Thus, most deformation of glacial sediment was envisaged as being 'made at or near glacial termini' (Flint 1971, p. 121). It was the discovery that some glaciers and parts of major ice-sheets rest not on bedrock, but on a layer of sediment (e.g. Engelhardt et al. 1978; Boulton 1979) that launched the 'deformable bed' hypothesis and a new significance to deformation structures in glacial sediments.

From: MALTMAN, A. J., HUBBARD, B. & HAMBREY, M. J. (eds) Deformation of Glacial Materials. Geological Society, London, Special Publications, 176, 1-9. 0305-8719/00/$15.00 �9 The Geological Society of London 2000.

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2 A . J . MALTMAN E T AL.

Even so, a common approach today among Quaternary specialists is to see the structures as an aid to deducing glacial environments and directions of ice movement (e.g. Evans et al. 1999), risky though this is without a knowledge of the geometries, kinematics, and physical conditions of the deformation. On the other hand, understanding such aspects of deformation has been a growing theme of structural geology over the last 50 years or so, as the subject has become less descriptive (e.g. Twiss & Moores 1992). These efforts, however, have almost exclusively been restricted to rocks. As noted above, few structural geologists have taken an interest in structures in glaciers. The same is largely true for glacial sediments, and concerted efforts at com-bining ideas developed for rocks with glacial sediments, such as those of Banham (1975), Aber et al. (1989), Warren & Croot (1994) and Harris et al. (1997), are sparse. Yet analysing structures in Quaternary sediments can have an important practical advantage over studying those in lithified rock: the sediment can readily be scraped away to reveal the full three-dimensional arrangement of the structures. Indeed, some of the most spectacular structures appear in working sand and gravel pits, where large-scale sections are con- stantly changing.

Because of the importance of sub-glacial deforming sediments to the motion of many ice masses, it is appropriate to examine the deformation of ice and glacial sediments together. In some cases, sediments shearing beneath an ice sheet are best regarded as being in continuum with overlying debris-rich ice (e.g. Hart 1998). Thus the division of the papers included in this volume into separate sections on ice and sediments is largely for convenience. We have attempted to head each section with a paper that provides some insight and overview. Some papers deal with the linkage between ice and sediments, and there are other papers that could have been placed in more than one group. While some of the papers are explicitly contemporary reviews, others combine review with new find- ings or point strongly to future work.

In many ways, the papers included here con- firm the continuing existence of a gap between different groups of workers - especially in terms of approaches, methods and terminology. Some papers that were refereed by, say, a Quaternary specialist and a structural geologist received conflicting reviews, with contrasting opinions on the practices used and the clarity of the terms. Hence, a number of articles herein represent a working compromise. We have attempted to produce a balanced volume, but are keenly

aware that something of a schism continues. Our aim is that by collecting together papers on similar subjects from workers with a range of backgrounds and approaches, new possibilities and collaborations may open. We hope the mixture presented here will provide a basis for more integrated approaches in the future.

Ice deformation

The volume opens with four papers concerned with processes of ice deformation (relationships between ice deformation and structural development being considered in the second section). Despite the small number of papers in this section, a broad range of approaches to investigating ice deformation is presented. These include results from large- and small-scale ice coring projects, laboratory analysis of ice character (including isotopic, gas and chemical composition and ice crystallography), and laboratory analysis of ice rheology, using both triaxial deformation apparatus and a novel centrifuge apparatus.

The first paper in the section, by Souchez et aL, addresses a complex picture of ice formation by freeze-on and deformation near the base of the Greenland Ice Sheet as revealed in a number of basal-ice core sections. Souchez and his team at Brussels are ideally placed to write such a review (based on both published and new information), since they have been working for some years on the physical properties of the basal sections of many of the world's most important ice cores. The data summarised in this contribution relate mainly to the basal sections of 3 cores located in central Greenland (Dye 3 GRIP and GISP2). Interpretation of the gas, stable isotope and chemical composition of these core sections indicates that their silt-laden basal ice layers are largely composed of ice that was formed prior to ice-sheet advance. Souchez et al. argue that such silt-rich ice is incorporated without a phase change into the advancing ice sheet, and that it is subsequently distributed and mixed tectonically with clean (firn-derived) glacier ice over some metres to tens of metres at the base of the ice sheet.

The second paper in the section, by Tison & Hubbard, presents new information based on a series of eight short ice cores recovered from along a flow-line at Glacier de Tsanfleuron, Switzerland. This paper supplements an earlier classification of the ice facies present in these cores (Hubbard et al. 2000) with a wealth of ice crystallographic data. Significantly, the locations of the ice cores allow a glacier-wide flow-line model of crystallographic evolution to



INTRODUCTION AND OVERVIEW 3

be reconstructed. This progression involves a sequence of four crystallographic units, from initial ice development within 20m of the surface in the accumulation area of the glacier, to strongly metamorphosed ice located within c. 10 m of the glacier bed. The crystallography of this basal ice reflects a steady-state balance between processes of grain growth and strain- related processes of grain-size reduction.

Baker et aL report a set of constant strain-rate compression tests on individual ice crystals at a variety of orientations, comparing the strain response of undoped ice with that of ice doped with H2SO4 at 6.8 ppm. While the test corrobo- rates earlier results indicating that the presence of very low concentrations of H2SO4 reduces both peak stress and subsequent flow stress (Trickett et al. 2000), the present contribution goes on to demonstrate that doping does not significantly affect the stress exponent in the flow law for that ice.

Finally, Irving et al. report a series of ice deformation experiments conducted using the Cardiff Geotechnical Centrifuge in which ice blocks are rotated at a centripetal accelera- tion of 80 g in a beam centrifuge to recreate the true self-weight stress field that drives real ice masses. Even though the laboratory apparatus is still being perfected (particularly in terms of maintaining a constant, controlled temperature through the sample), the preliminary results presented in this paper are encouraging. These indicate that the technique is viable and that it may have significant potential in terms of testing viscosity differences between different ice types. In particular, ice containing 10% by volume of sand strained an order of magnitude slower (10 .7 s -1) than did clean ice (10 -6 s -1) in these experiments.

Glacier flow and structures

Glacier flow is manifested in a variety of structures, commonly reflecting deformation on decadal time-scales in temperate valley glaciers to millenial time-scales in polar ice sheets (e.g. Hambrey 1994, chapter 2; Paterson 1994, chapter 9). The 'primary structure', sedimentary stratification (derived from the accumulation of snow and ice) is subject to considerable mod- ification or even obliteration during glacier flow. It is commonly overprinted by the 'secondary' or deformational structure called foliation, either through folding and transposition of the initial layering, or as a completely new structure resulting from shearing. Extensional flow results in the development of crevasses or tensional

veins, ultimately giving rise to crevasse traces or even a new foliation if these layers are subject to severe cumulative deformation, as below an icefall. In compressive flow regimes, thrusts may develop, especially in polythermal glaciers. All these structures are analogous to those in deformed rocks and can be used to generate models of deformation in fold-and-thrust belts. Like rock structures, those in ice reflect long- term deformation (or cumulative strain). Recog- nizing the significance of structures in glaciers has implications for understanding not only ice dynamics on all scales, but also how debris is incorporated and ultimately deposited by the ice.

The papers in this section focus primarily on the structures observed englacially and at the surface of glaciers, and how they relate to the flow of ice. We start with a review of structural styles and deformation fields in glaciers by Hambrey & Lawson. This paper outlines the historical development of this topic, beginning with the nineteenth century pioneers but con- centrating on developments since the 1950s, when the physics of glacier flow was elucidated. Deformation rates and histories of various glaciers are described, together with measurements indicating measurements which indicate that ice experiences a polyphase deformation history. The importance of cumulative strain in discussing structural development is highlighted, and comparisons are made between ice structures and structures in deformed rocks. The significance of structural glaciology to the way in which debris is incorporated, transported and finally deposited by glaciers is also evaluated. Pointers are offered to future work on linking ice structures to deformation, especially through modelling approaches.

In a paper on deformation histories and structural assemblages of glacier ice, Lawson et ai. evaluate a range of velocity and strain-rate data from the world's most intensively studied surge- type glacier, the Variegated Glacier in Alaska. The strain histories of ice at different positions on this non-steady-state glacier, following the well- documented 1982-83 surge, are compared. The histories of accumulation of cumulative strain are complex, and can mask the effects of large, transient strain events. Substantial cumulative strain can be 'undone', and the cumulative strain signal may be unrepresentative of earlier strain- rates. Structural relationships do not always reflect the complexity of deformation histories, and it is clear that brittle structures in particular can be reactivated several times as they pass through the glacier.

From the large scale, we turn to a paper by Wilson on how deformation is localized in



4 A . J . MALTMAN E T AL.

anisotropic ice; this aspect is studied by means of an experimental study involving a series of creep tests. The effects of initial c-axis preferred orientation and the inclination of primary layering, during both plane strain compression and combined simple shear-compression are evaluated. The author found that significant variations in both the strain rate and in the development of microstructures occurred during the plane-strain compression experiments. In the combined compression and shear experiments, the minimum shear strain rates vary according to anisotropy in the ice, and deviate from the normal power flow law for isotropic ice.

Marmo & Wilson provide an analysis of the stress distribution and deformation history associated with one type of structure, a set of boudins, formed in an outlet glacier of the East Antarctic ice sheet. These boudins arise out of stretching of frozen water-filled crevasses. By documenting the geometrical evolution of the boudins and comparing this with surface strain- rates, it can be determined how these structures form in relation to the stress distribution, as derived from two-dimensional finite-difference modelling. The demonstration that stress is re- fracted across ice-rheological boundaries has implications for analysing planar and linear fabric in rocks.

Next, a paper by Hubbard & Hubbard offers a new approach to the understanding of glacier structures using a high-resolution, three-dimensional finite difference model. The model is used to reconstruct the velocity field of Haut Glacier d'Arolla in the Swiss Alps. From this field, it is possible to predict the generation, passage and surface expression of a variety of structures. By means of flow vectors and strain ellipses, the evolution of crevasses, crevasse traces and stratification (mapped from aerial photographs) can be examined. The authors also use the model to track ice that is formed in the accumulation area and then subsequently modified by burial and englacial transport, before being re-exposed in the ablation area. Although this application of the modelling technique is still at an early stage of development, it offers considerable scope to understand better the evolution of a wide variety of structures, including foliation, folds, boudins and a variety of fractures, as highlighted in the first paper in this section.

Ximenis et al. then report an excellent field study of the kind of glacier-wide structural development that fully complements Hubbards' modelling approach. Ximenis et al. deal with folding in Johnsons Glacier, a small cirque- shaped tidewater glacier in the South Shetland Islands of Antarctica. This glacier is unique in

having excellent marker horizons in the form of tephra layers, and a cliff section allowing the three-dimensional structure to be observed. The glacier is characterized by converging flow, and several large folds develop axes sub-parallel to the flow-lines. These folds thus result from transverse shortening in response to reducing channel cross-sectional width, and from an increase in differential flow-rates between the centre of the glacier and the margins. Similar structures have been reported from valley glaciers in Svalbard (Norwegian High-Arctic) by Hambrey et al. (1999).

The final paper in this section is by Herbst & Neubauer, Who report on research at the well- known Pasterze Glacier (Pasterzenkees) in the Austrian Alps. This paper explains how a wide range of structures in the glacier, including foliation, shear zones, folds, thrusts and other faults develop, and how they compare with similar structures in rocks. These structures, combined with an understanding of the kinematics of the glacier, are used to develop the analogy with a model of an extensional alloch- thon, formed on top of an orogenic wedge. The East Carpathian orogen is used as the example.

Subglacial deformation

Subglacial sediment deformation probably represents the single most actively studied glaciological process over the past 15 years or so. However, many aspects of the actual mechanisms that contribute to this sediment deformation are still poorly unders tood- principally because of the extreme difficulties involved in both observing the process first-hand and recreating the boundary conditions of those processes in the laboratory. Partly as a result of these difficulties, the very nature of the deformation itself remains unclear. The debate is principally argued out in terms of whether the deformation can be regarded as essentially viscous (where the rate of sediment deformation varies with applied stress) or essentially plastic (where the sediment is unable to sustain any stress above that at which the material fails). Recent research by workers such as Iverson (e.g. Iverson et al. 1997, 1998, 1999; Iverson 1999) and Tulaczyk (Tulac- zyk et al. 1998, 2000a, b; Tulaczyk 1999) on the details of sediment deformation, and Fischer et al. (1998) and Alley (e.g. 1996) on sediment hydrology reflect a shift in the balance of opinion away from a simple (commonly viscous-based) approach towards a more complex (and probably realistic) analysis involving multiple plastic failure, non-steady pore-water pressures, and the




role of ploughing of large clasts protruding from the base of the overriding ice (or the roughness of the ice itself). The inclusion of these (perhaps non-steady) processes into dynamic models of ice-mass motion provides a major stimulus to current research on subglacial deformation.

The papers presented in this section fully reflect these contemporary issues: a review of the role of continuity in the presence and character of a deformable subglacial sediment layer; an analysis of the basal sediment-basal ice continuum from an outlet glacier of the East Antarctic ice sheet; a theoretical treatment (based on a viscous deformation model) of the relationships between sediment loading and flow-induced landforms; a presentation of field evidence from Iceland indicating that basal motion during a surge is concentrated close to the ice-sediment interface; radar evidence from the Antarctic interior that indicates the presence of extensive areas of water-saturated subglacial sediments, and a report of a suite of laboratory tests investigating the relationships between pore-water pressure, permeability and strength.

The first of these articles is by Alley, who presents a broad review of our current understanding of the physical character of subglacial sediments. Alley concludes that many of the glaciologically important aspects of deforming subglacial sediments are ultimately controlled by till production, continuity and, to some extent, pre-existing surface geology, rather than the finer details of their rheological character.

Fitzsimons e t al. then present investigations of ice deformation undertaken beneath Suess Glacier, Antarctica, where a 25m long subglacial tunnel has been painstakingly con- structed by chainsaw to gain direct access to the ice-bed interface. This remarkable facility has allowed basal ice velocity to be measured over an extended period. Results indicate that fine- grained amber ice containing relatively high sol- ute concentrations deforms more readily than adjacent clean ice. Further, sediment-laden ice is characterized by more brittle deformation features than cleaner ice, suggesting different modes of failure in this -17~ ice.

Next, Hindmarsh & Rijsdijk evaluate whether a viscous model of sediment deformation is consistent with field observations of the nature and scale of loading instabilities within glacigenic sediments. The application of Rayleigh- Taylor theory to layered sediments that are assumed to be characterized by a pre-existing density inversion is found by the authors to result in features that are consistent with those observed in the field. Further, Hindmarsh & Rijsdijk's model indicates that variations in

layer thickness are likely to exert a stronger control over the scale of the resulting instability form than are variations in sediment viscosity.

Fuller & Murray report painstaking field evidence from the forefield of recently surged Hagafellsj6kull Vestari, Iceland. Detailed analysis of macro-scale and micro-scale structures in recently exposed flutes and drumlins sug- gests that surge deformation is confined to the upper 16 cm of the glacier's subglacial till layer. Detailed field-based evidence also indicates that the (surge phase) coupling between the glacier base and the subglacial sediment layer was strong and that ploughing by clasts entrained within, but protruding from, the ice was widespread.

In the penultimate paper in this section, Siegert reports evidence from the remarkable SPRI- TUD airborne radar database of the East Antarctic ice sheet relating to the nature of the ice-bed interface. Although the presence and character of now well-documented sub-ice lakes have been inferred from flat basal radar returns in earlier papers (e.g. Siegert et al. 1996; Siegert & Ridley 1998), this contact is here supplemented by a frozen ice-bed interface (weak and scattered radar return) and an ice-saturated subglacial sediment interface. The latter radar return is similar to, but less flat than, that produced at an ice-lake interface. This significant development points to the potential for ice radar not only to map areas of presumably deformable (and deforming) subglacial sediments, but also to identify large-scale structures within those sediments.

Finally, Hubbard & Maltman report on a suite of laboratory investigations of the dynamic permeability of subglacial sediments recovered from Haut Glacier d'Arolla, Switzerland, and a (late Pleistocene) glacierized beach cliff section in South Wales. Results from these experiments indicate that, although large variations in permeability did not occur during deformation (to total strains of <20%), pore-water pressure did exert a major influence over permeability, whether dynamic or static. The nature of this relationship between permeability and effective pressure is best described by an inverse power relationship above a base permeability such that permeability increased dramatically (several orders of magnitude) at effective pressures of less than c. 150 kPa.

Glaciotectonic structures

Before considering the individual papers in this section we contrast terminologies and approaches used. Since the mention of 'glacial tectonics' by Slater (1926), the term has evolved



6 A . J . MALTMAN E T A L .

into 'glaciotectonics' and become well estab- lished. It can, however, cause confusion in interdisciplinary work because of the way that structural geologists use 'tectonics'. In structural geology the term has a dual meaning. Originally it referred to the 'architecture' (its meaning translated from the original Greek) or config- uration of rock masses, as in papers describing the 'tectonics' or 'tectonic style' of a particular region. This would seem to be the usage perpetuated in 'glaciotectonics', but modern usage in structural geology tends to use 'tectonic' to refer to the origin of the forces that caused the deformation. Tectonic forces origi- nate from physico-chemical changes within the Earth, contrasting strongly with gravity-based forces that drive glacial motion (Maltman 1994). Hence the recent habit of some workers to drop the prefix glacio- is dangerous if structural geologists and glaciologists are to communicate clearly. 'Tectonic detachment', for example, has been reported by glaciologists as a process occurring within ice, but the term might puzzle structural geologists since tectonic forces are unlikely to operate within glaciers. Owen (1989) described some structures in sediments in the Himalayas that are partly the product of tectonic forces associated with crustal uplift, and others that are due to ice movement, yet both are referred to as tectonic. To avoid this confusion, all structures directly associated with glacial processes should be referred to as glaciotectonic, a practice we have followed in this volume.

'Glaciotectonic' refers to the direct deformation and structures resulting from the movement or loading of glacier-ice, outside the ice itself. The definition and limitations of the term have been discussed by Aber et al. (1989). In line with their suggestions, the term as employed here does not involve structures within glacier ice, and structures resulting from primary deposi- tional processes (such as till fabric) are excluded, as are the effects of freeze/thaw and iceberg grounding. Lithospheric adjustments to changing ice loads can induce deformation, but these effects are also excluded, as the role of ice here is indirect. Glaciotectonic structures include a wide range of features conventionally studied by structural geologists and the larger scale effects can grade into major landforms (e.g. Aber et al. 1989; Warren & Croot 1994; Van der Wateren 1995). The structures themselves include a wide variety of folds and faults, on a range of scales, together with a host of related features.

Almost all glaciotectonic structures, unlike those normally encompassed by structural geology, involve only one deformation mechanism:

frictional grain-boundary sliding, sometimes called independent particulate flow (e.g. Malt- man 1994). In this, the mineral particles themselves undergo negligible deformation but simply slide past each other; in most glacial deposits the mineral grains are stronger than any bond- ing between them. Aggregates of clays may be deformed, and, particularly in frozen sediments, any brittle lithic clasts or bedrock that are involved may undergo cataclasis (grain break- age). However, the deformation mechanisms central to most crustal structures, that is the various modes of crystal plasticity and diffusion mass transfer, are unlikely to be significant in the relatively cold and rapid conditions of glaciotectonic deformation. One significant aspect of this difference between glacial sediments and deforming rocks is that terms for the resulting structures have to be used carefully if confusion is to be avoided. Mylonite, for example, is a structural geological term that by definition applies to materials that deformed dominantly by plastic, intracrystalline, processes such as dislocation creep and dynamic recrystallization (e.g. Hippert & Hongn 1998). Yet the term 'mylonitic' has been applied to glacial deposits that almost cert- ainly deformed chiefly by grain-boundary sliding rather than any form of crystal plasticity.

Differences in approach between structural geology and glacial geology are particularly marked in the microscopic study of deformation features, and these differences are reflected in some of the papers in this section. Even the subject is named differently: 'micromorphology', a term used in this volume, is foreign to structural geologists, who tend to talk about 'micro- structure'. Basic working terms such as fabric, texture, matrix and structure are minefields of different meanings.

These differences in the structural and Qua- ternary approaches seem fundamentally to result from their differing heritages. Since Sorby's pioneering work in the middle of the nineteenth century (Sorby 1859), structural geology has drawn largely on materials science and metal- lurgy (e.g. Kameyama et al. 1999), and there has been strong emphasis on the motion geometry of rock particles in response to deforming stresses, in particular the notion of employing 'shear sense indicators'. In contrast, the field concerned with micromorphology of glacial sediments has existed for little more than about twenty years, and is still in the process of cataloguing the features and evolving the most useful terminology. So far, the subject has drawn more on the concepts and terminology of soil science (e.g. Fitzpatrick 1984) and less on the experi- ence of deformed rocks, or for that matter the




approaches of soil (geotechnical) engineering. This has the advantage of enabling biological and other high-level constituents that are not normally present in lithified rocks to be dealt with. It also provides a detailed terminology, based on the term 'plasma' with various prefixes, for the fine-grained, typically clayey material that structural geology tends to have neglected. However, it does reduce the emphasis on deformation, not normally an important aspect of agricultural soils. The result is that, at the present time, microstructural studies are rather impenetrable to Quaternary geologists, some of whom use a terminology quite alien to structural geologists. Therefore, there seems a particularly pressing need in the field of microscopic studies of glaciotectonism for greater commonality of approach and terminology. However, it seems premature to attempt a unison here; we have not attempted to lay down yet another set of working definitions and impose editorial pre- judices. While any particularly arcane terminology has hopefully been eliminated, authors' preferences have been honoured. Consequently, there is some duality of usage in the following papers and the exact meaning has to be deduced from the context.

The section on glaciotectonics begins with two reviews that illustrate the differences in approach to microscopic scale analyses and that may help coordinate them in future. Each uses a simple terminology and approach that should make the reviews fully accessible. Menzies reviews the state-of-the-art in Quaternary studies that draw from the soil science heritage, providing a qualitative classification of deformation structures in glacial sediments and then attempting to link this taxonomy to the different glacial environments. The review minimizes specialist terminology and introduces some terms common to structural geology. Menzies makes it clear that his work is still provisional; deformed glacial sediments have not been surveyed comprehen- sively at the microscopic scale and features may remain unrecognized. The ideal goal of this kind of work is to seek criteria at the microscopic scale for recognizing particular lithofacies. However, Menzies emphasises that the complexities of nature are likely to preclude this in practice. Rather, microstructural criteria will have to be used together with other features to diagnose particular glacial settings.

Van der Wateren et aL review aspects of a structural geological topic that has advanced greatly in recent years: that of diagnosing the kinematics of the deformation. They explain how various microstructural features can lead to a better understanding of deformation move-

ments and magnitudes, but here also, the analyses are best used in conjunction with other kinds of observations. Structural geology has by now evolved methods for dealing with shear sense in flowing materials to a considerable degree of sophistication (e.g. Passchier 1998), with details of the relative roles of different strain symmetries being hotly debated (e.g. Lin et al. 1999). No doubt, particularly complex symmetries exist in deforming glacial materials, through such things as non-planar boundaries and accompa- nying heterogeneous volumetric strains, but at this stage of the subject's development in glacial geology, Van der Wateren et al. felt it more instructive to restrict their review largely to simple shear.

A structural geological topic that has by now matured is that of recognising and deciphering multiple or polyphase deformation, common, for example, in the rocks that form the mountain belts of the world. The contribution by Phillips & Auton illustrates how such concepts can be used to unravel sequences of events in glacial deposits. The paper also shows how a more coordinated terminological approach can be achieved. It deals explicitly with 'micromorphology' yet employs conventional structural geology terminology of S 1, $2, for successive generations of fabrics. Structural geological terms such as Riedel shears, boudinage, and pressure shadows are used as appropriate alongside terms derived from Van der Meer (1993) such as lattisepic and unistrial plasmic fabrics, quite foreign to structural geology. Such capitalizing on the strengths of the different approaches in order to employ the most effective terms and concepts must be the way forward if we are to gain before long a more unified approach to microscopic studies of deformed glacial sediments.

Structural geological advances in recent dec- ades on fold-and-thrust belts have already been employed in glacial geology (e.g. Croot 1987; Hambrey et al. 1997) and glaciology (e.g. Hambrey et al. 1999), but the article by Huuse & Lykke-Andersen extends such adapta- tions further. All the glaciotectonic structures they report are submarine and are interpreted from high-resolution multi-channel seismic data, giving unprecedented quality of resolution for such structures. Moreover they are documented on a scale of kilometres that is normally simply too large to observe on land. The profiles they derive compare closely with on-land fold-and- thrust belts (e.g. Macedo & Marshak 1999), and the rooting of the thrusts into an underlying detachment zone is comparable to the architecture of accretionary prisms forming at conver- gent plate margins (e.g. Morgan & Karig 1995).



8 A . J . M A L T M A N ET AL.

The two final papers in this section illustrate the interface between glaciotectonic structures and landforms. Fowler provides a dynamic- numerical analysis of the flow processes that may give rise to that still controversial feature - drumlins. Graham & Midgley outline a case study of moraines in Nor th Wales, in which the interplay between deformat ion processes and structures, and part icularly the role of thrusting, is explored. Their paper also nicely reflects the pedigree of the studies reported in this volume. It was in the same locality in Nor th Wales that Charles Darwin in 1842 showed that, not only must ice have once extended over southern Britain, but also that the associated processes had much influence on today 's landscape.

This volume represents the product of an interdisciplinary conference, entitled the 'Deformation of Glacial Materials', held at the apartments of the Geological Society of London in Burlington House in September 1999. The editors would first and foremost like to thank the conference participants, who con- tributed so fully to the success of both the conference and this volume. We would also like to thank the staff of Burlington House and the staff of the Geological Society Publishing House respectively for their assis- tance in organizing the meeting and in producing this volume. In addition, we acknowledge the efforts of the many referees who gave up their time to read and comment on the manuscripts contained in this volume. Finally, we gratefully acknowledge the support of the sponsors of the conference- the Tectonics Studies Group of the Geological Society, the International Glaciological Society, the Quaternary Research Asso- ciation, and our commercial sponsors, Lasmo plc and Badley Ashton & Associates Ltd.

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