Journal of the Geological Society Genesis of 'hummocky moraines' by thrusting in glacier ice: evidence from Svalbard and Britain MICHAEL J. HAMBREY, DAVID HUDDART, MATTHEW R. BENNETT, et al. Journal of the Geological Society 1997; v. 154; p. 623-632 doi: 10.1144/gsjgs.154.4.0623
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© Geological Society of London 1997
Journal of the Geological Society, London, Vol. 154, 1997, pp. 623–632, 10 figs. Printed in Great Britain
Genesis of ‘hummocky moraines’ by thrusting in glacier ice: evidence from Svalbard and Britain MICHAEL J. HAMBREY 1 , DAVID HUDDART 2 , MATTHEW R. BENNETT 3 & NEIL F. GLASSER 1 1 School of Biological and Earth Sciences, Liverpool John Moores University, Byrom Street, Liverpool L3 3AF, UK (e-mail:
[email protected]) 2 School of Education and Community Studies, Liverpool John Moores University, I. M. Marsh Campus, Liverpool L37 6EN, UK 3 School of Earth Sciences, University of Greenwich, Medway Towns Campus, Pembroke, Chatham Maritime, Kent ME4 4AW, UK Abstract: Irregular mounds of glacial debris, commonly referred to as ‘hummocky moraine’, until recently were linked to ice stagnation during rapid climatic amelioration. However, recent work in Scotland has demonstrated that some hummocky moraine, dating from the Younger Dryas event (c. 10 000 years ), was the product of deposition at active ice margins. Observations at modern high-arctic glacier margins in Svalbard (76–80)N) indicate that some moraines of this type form under a dynamic glaciological regime, mainly by thrusting in active polythermal ice undergoing strong longitudinal compression. This situation is particularly common where there is a transition from warm-based ice in the interior of a glacier to cold-based ice at the margins or snout. From their morphological similarity (rectilinear slopes facing up-glacier and irregular downglacier faces), and apparent stacking of slabs of sediment of subglacial derivation, it is concluded that some assemblages of British moraines are also the product of thrusting. This process appears to be most dominant in polythermal glaciers, rather than in temperate glaciers. On this basis, it is tentatively suggested that the Younger Dryas in Britain was characterized by a climatic regime similar to that in Svalbard today (mean annual temperature "5)C; 400–1000 mm precipitation), and that the associated glaciers may have been of polythermal character. Keywords: Svalbard, Great Britain, Younger Dryas, hummocks, moraine, thrust faults.
At the margins of some modern glaciers, and in certain areas formerly occupied by ice, moraine-mound complexes are commonly developed. Generally referred to as ‘hummocky moraine’, these complexes have traditionally been viewed as the product of down-wasting of stagnant ice (Sharp 1949; Hoppe 1952, 1959; Stalker 1960; Winters 1961; Sissons 1967, 1974a, b, 1976, 1980), although a wide variety of other mechanisms have been proposed, as summarized by Johnson et al. (1995). As such, these complexes have often been used to indicate rapid cessation of glacier activity in response to a sudden amelioration of climate. However, the term ‘hummocky moraine’ hides considerable morphological variability, and it is becoming clear from recent work in Scotland that a range of processes are responsible for broadly the same product. Work on modern glaciers in Svalbard suggests that ‘hummocky moraine’ formation is intimately linked to thrusting in glacier ice and in the proglacial sediments. By comparison, we propose that some of the British examples are produced by the same mechanism. In Svalbard, moraine-mound complexes occur in the zone between the Neoglacial maximum and the present-day ice margin, usually a distance of several hundred metres. Detailed observations of the structures in the snouts of several Svalbard glaciers (Comfortlessbreen, Uvêrsbreen, Midre Lovénbreen, Austre Lovénbreen, Pedersenbreen, Kongsvegen and Finsterwalderbreen; Fig. 1), and of the sedimentary facies, structure and morphology of the associated moraines, indicate that this type of moraine complex is the product of thrusting of
subglacial material into an englacial or occasionally supraglacial position prior to melting out (Hambrey & Huddart 1995). Exposure is the result of up-valley recession of the ice margin, rather than glacier stagnation. Thrusting of basal ice into an englacial and sometimes a supraglacial position has long been regarded as a valid process (e.g. Swinzow 1962; Boulton 1967, 1970; Hambrey & Müller 1978), although some authors have sought other mechanisms (Weertman 1961; Hooke 1973). The thrusting mechanism has also been applied to assemblages of landforms of late Pleistocene age that formed in a proglacial position, where the term ‘push-moraine’ has sometimes been used (e.g. Boulton 1967, 1986). Interest in the role of thrusting in the incorporation and deposition of debris has recently been revived, particularly with reference to surge-type glaciers in Svalbard (e.g. in Bakaninbreen by Hambrey et al. 1996; Murray et al. in press; and in Kongsvegen, Bennett et al. 1996a, b), although elsewhere other interpretations exist for the incorporation of subglacial debris into ice, such as by the filling of basal crevasses (Sharp 1985). The inference that moraines associated with Younger Dryas ice in Britain were the product of ice-stagnation has recently been questioned (Bennett 1990, 1994; Bennett & Glasser 1991; Benn 1992; Benn et al. 1992; Bennett & Boulton 1993a, b). This recent work suggests that Scottish ‘hummocky moraine’ may be an assemblage of individual ice-marginal landforms generated by the decay of active glacier ice, although the detailed genesis of the individual landform components has only been established at a few locations (Bennett 1990, 1994; Bennett & Glasser 623
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(a)
Fig. 1. Location of glaciers investigated in Svalbard: (1) Kongsvegen*; (2) Pedersenbreen**, Austre Lovénbreen, Midre Lovénbreen; (3) Uvêrsbreen, Comfortlessbreen; (4) Bakaninbreen*; (5) Finsterwalderbreen*, Hessbreen*. Glaciers marked * are known to be of the surge-type; that marked ** is inferred to be of the surge-type.
1991; Benn 1992; Bennett & Boulton 1993a), and the role of thrusting in their formation has not previously been fully recognized. In this paper we argue that the moraine-mound complexes in Svalbard may provide suitable analogues for the formation of some of these landforms, rather than those associated with temperate glaciers, as in Iceland (e.g. Eyles 1983). Thrusting in glacier ice and proglacial landforms has an interest beyond the discipline of Quaternary science. The process may be regarded as analogous to deformation associated with accretionary prisms forming on the sea floor at convergent plate margins (A. J. Maltman pers. comm., Maltman et al. 1993), or to thin-skinned tectonic processes in mountain belts (e.g. Croot 1987).
Morphology and composition of moraine-mound complexes Moraine mounds in Svalbard The Svalbard moraine mounds are typically a few metres to 10 m in height, and of the order of 10–100 m wide and long (Fig. 2a, b). Midre Lovénbreen and Austre Lovénbreen (Fig. 3) are typical examples of polythermal glaciers terminating on land. These glaciers have receded 1–2 km since their Neoglacial position at the turn of the century (Hagen et al. 1993), leaving behind complexes of moraine mounds, braided outwash, lakes and linear trains of supraglacial debris. The
(b) Fig. 2. Moraine-mound complexes comprising stacked sheets of diamicton and sandy gravel in Svalbard: (a) Austre Lovénbreen, with rectilinear slopes inclined to the left towards the glacier. (b) Comfortlessbreen, with rectilinear slopes defining thrusts inclined to the right.
mounds are concentrated just inside the Neoglacial ice limits, but groupings also occur between this position and the contemporary ice margin (Fig. 3). Additional mounds occur on the snouts of the glaciers themselves, where they are usually associated with gently curving debris-bearing layers which are approximately parallel to the ice margin and dip up-glacier at 15–40) (Fig. 4). Most fresh mounds tend to have a short, straight or gently curved crest aligned parallel to the general moraine-ridge trend. However, these represent median forms, and in practice shapes range from pointed mounds to ridges in excess of 100 m long. Slopes facing towards the glacier have a rectilinear form, with dips of 20–35), unless modified by post-glacial solifluction and mass-flow processes. In contrast, slopes facing away from the glacier tend to be steeper and of much less regular form. Facies associated with moraine mounds are varied, but each mound is composed of a single facies or facies association. Facies are interpreted below with reference to texture and shape characteristics of sediments that can be linked to particular depositional settings (e.g. Bennett et al. 1997). Clast-shape analysis follows the methodology of Benn & Ballantyne (1994), in which a plot of the RA index (percent of very angular and angular clasts) versus the C40 index (percent of clasts with a c-axis/a-axis ratio of ¦0.4) effectively discriminates between subglacially derived from supraglacially derived
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Fig. 3. Glacial geomorphological map of Austre Lovénbreen and Midre Lovénbreen and their proglacial areas, drawn from an aerial photograph (uncorrected for edge-distortion), illustrating the end-moraine complex with associated moraine mounds, other depositional features, and ice structures (including thrusts) in the snouts of the two glaciers.
sediment (Fig. 5a). The most common facies identified in the Svalbard complexes are as follows. (i) Diamicton. This facies is generally massive with a wide range of clasts; grain size ranges from mud to boulder size. The gravel content is typically 20–40%; clasts have a wide range of shapes, but with a predominance in the subangular and subrounded classes, while fine-grained lithologies are commonly striated. Diamicton is interpreted as either basal glacial debris, carried in the zone of traction at the bed of the sliding glacier, or reworked subglacial till. (ii) Sandy gravel and gravelly sand. These facies represent a broad spectrum of textures within the fine sand to cobble range. Thicker units sometimes retain their primary bedding and cross-bedding, although faulting and folding may be in evidence. Clasts shapes are marginally more rounded than those in diamicton, but striations are rare. In terms of texture and clast shape, these facies are identical to proglacial glaciofluvial sediment, which would have been overridden by the glacier as it advanced. (iii) Muddy gravel. This facies is characterised by moderately well sorted pebble/cobble gravels, with clasts being coated with mud. Clasts have a similar range of shapes as diamicton, but are not striated. This facies is only associated with tidewater glaciers, and probably represents proximal, subaquatically discharged glaciofluvial sediment, before being overridden and emplaced on land. (iv) Sand. Moderately well-sorted sand with well-preserved sedimentary structures, but with fold structures verging in the direction of inferred thrusting, have been observed in a number of excavations in moraine mounds. In terrestrial deposits, sand probably represents proglacial lake infills, whereas in glaciomarine settings it originates from subaquatic discharge on the sea floor, before being incorporated in an advancing ice-mass and deposited on land. (v) Mud. Sandy mud is associated mainly with tidewater glaciers and contains occasional pebbles and fragments of shell, calcareous algae and foraminifera, indicating a marine source. Mud represents more distal glaciomarine facies, derived from suspension, with a small amount of ice-rafted material, before being reworked as the ice advanced onto land.
(vi) Sand–mud laminites. Rhythmically laminated mud and sand, occasionally with isolated gravel clasts, form a minor facies associated with tidewater glaciers. The original sediment was probably a tidal rhythmite, to which was added ice-rafted dropstones. The variety of facies incorporated into the moraine-mound complex reflects the ground or sea floor over which the ice flowed. For land-based glaciers such as Midre Lovénbreen, the dominant facies are diamicton and sandy gravel, interpreted as basal till and glaciofluvial sediment respectively. Complexes which involve reworked glaciomarine sediment, such as those at Comfortlessbreen and Kongsvegen, are more variable and reflect the interaction between glacio-terrestrial and glaciomarine processes. In some cases, the sediment making up the moraine mounds is supplemented by angular cobble/boulder gravels, but these only form a drape, and are clearly derived from rockfall material carried through a high-level glacial transport path, when traced back to the source glacier. The overall structure of the moraine-mound complexes can be derived from the three-dimensional form and distribution of facies, which are quite clearly visible in forms that have not undergone slumping. Adjacent mounds are commonly composed of inclined sheets, stacked upon one another progressively in a glacierward direction (Fig. 6a & b). At locations where the internal structure of these moraine mounds can be observed, discrete planar surfaces (parallel to the inner rectilinear slope of the mound) are common in sand/mud facies, as well as the other evidence of deformation referred to above.
Moraine mounds in Britain In Britain, Younger Dryas ‘hummocky moraine’ has recently been subdivided into three morphological distinct assemblages of landforms (Bennett 1990; Bennett & Glasser 1991; Benn 1992; Bennett & Boulton 1993a): (1) down-valley ridges, interpreted as glacial flutes and drumlins (Peacock 1967; Hodgson 1986; Benn 1992; Bennett 1995); (2) crossvalley ridges, comprising either discrete single-crested ridges or
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(a)
(b) Fig. 4. Thrust ridges forming in the snout of Midre Lovénbreen: (a) diagonal ridge comprising sandy gravel, inferred to be of glaciofluvial origin; (b) close-up view of washed surface of glacier, exposing thin gravel debris layer at the centre and a thicker layer to the left; ice flow is from left to right; the thick drape of debris has melted out from the debris layer.
arcuate belts of low moraine mounds; and (3) larger areas of moraine mounds. The genesis of many of these moraines remains unclear, but the ice-marginal nature of the single-crested, cross-valley ridges has been established (Bennett 1990; Bennett & Glasser 1991; Benn 1992; Bennett & Boulton
1993a). Of these assemblages, it is the arcuate belts of moraine mounds and the larger moraine-mound complexes that closely resemble the landforms found in front of Svalbard glaciers. These moraine mounds have crests which formed parallel to the inferred ice margin, have rectilinear upglacier slopes of between 25) and 35), and are composed of a wide range of different sedimentary facies, interpreted as flow tills, valley-side debris, basal diamicton and glaciofluvial gravels (Benn 1992; Benn & Evans 1993; Bennett & Boulton 1993a). One of the most comprehensive sedimentological investigations of Scottish ‘hummocky moraine’ was undertaken by Benn (1992) on the Isle of Skye. In some moraine mounds he found evidence of flow till and valley-slope debris deposits, all of which indicated extensive sediment-reworking during deposition. Within other moraine mounds, he also found evidence of glaciotectonic deformation in the form of fractured bedrock rafts and sheared diamictons. This work illustrates the range of different processes responsible for the formation of moraine-mound complexes in Scotland. A similar range of sedimentary facies was noted by Bennett & Boulton (1993a) in the NW Highlands, who also recognized evidence of glaciotectonic deformation within some moraine mounds. Some of the most extensive areas of moraine mounds within the limits of the main Younger Dryas icefield occur in the Torridon area of the North West Highlands. Of particular note is the well-known moraine-mound complex in Coire a’ Cheud-chnoic (The Valley of a Hundred Hills) in Glen Torridon [NG 950 550]. Here, low conical and straight-crested mounds are the dominant landform on the floor of the corrie and cover an area of c. 2.5 km2 (Figs 7 & 8a, b). Straightcrested ridges are approximately parallel to each other, extending across the valley in a transverse fashion. Crest-lines are typically 5–50 m long, and the mounds show little consistency in height. In cross-section, many moraine mounds have a well-developed rectilinear slope (15–35)) which is inferred to face upglacier (Figs 6c & 8b). From limited exposures observed during path construction, the internal composition of the mounds varies from clast-rich sandy diamicton to sandy gravel. Clast shapes, analysed using the methodology of Benn & Ballantyne (1994), demonstrate a range of characteristics from edge-rounded clasts typical of those found in lodgement tills, to more angular clasts with a greater supraglacial affinity (Fig. 5b). Torridonian Sandstone bedrock is observable in streams near the foot of some mounds, suggesting that nearly the entire subglacial debris zone was involved in thrusting and that the bedrock represents the de´collement surface. The same sort of relationships are observable at the northern moraine complex of Kongsvegen in Svalbard. Another example of moraine mounds with similar form to those in Svalbard is present in Cwm Idwal, a well-known cirque in North Wales. These mounds have rectilinear slopes that face across a lake basin towards a high-level subsidiary cirque, which was the source of the Younger Dryas ice (Figs 6d & 8c). Thrusting may have been facilitated by strong compression as the ice moved up the reverse slope after crossing the floor of the cirque, which is now occupied by a lake. An exposure yielded a sandy diamicton with clasts having the characteristics of basal glacial debris. In addition, a number of bedrock rafts, their flat faces lying parallel to the rectilinear slopes, indicate uplifting from the glacier bed, and emplacement on the backs of mounds of other debris. Indeed, the largest raft, found nearest the lake shore (Fig. 8c), shows well-developed Nye-channels, cut by glacial meltwater under high pressure at the bed.
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Fig. 5. Clast shape analysis from Svalbard and Scotland, following the method of Benn & Ballantyne (1994). The RA index (percent of very angular and angular clasts) versus the C40 index (percent of clasts with c-axis/a-axis ratios ¦0.4). Each sample contained 50 clasts and was taken at random from a different moraine mound or sedimentary unit. (a) Midre Lovénbreen; (b) Coire a’ Cheud-chnoic (Valley of a Hundred Hills), Torridon, NW Scotland. In the case of the Torridon sample, scree is used as a surrogate for supraglacial debris and the lodgement till samples are derived from samples of unequivocal origin in northern Torridon.
In each case mentioned above, stacking of slabs of sediment, bounded by thrusts, is inferred by comparison with the Svalbard examples. Although the morphology and structure of the British moraine mounds referred to above suggests thrusting, other hummocky moraines may be related to temperate ice-marginal processes, such as winter freeze-on and advance followed by summer melt-out (e.g. Matthews et al. 1995; Kruger 1996), or by melting out of debris from relict conduits (e.g. Kirkbride & Spedding 1996). However, these forms are smaller than thrust moraines, and lack well-defined rectilinear slopes.
Discussion Incorporation of debris into glacier ice The manner in which the debris is incorporated into ice from the bed is dependent on the thermal regime of the glacier. From deep-ice temperature measurements, many glaciers in Svalbard are known to be polythermal, comprising cold ice frozen to the bed near the snout, and warm ice with a sliding bed in the upper reaches (Hagen & Sætrang 1991). This condition facilitates the freezing onto the base of the glacier of subglacial debris as part of a regelation-ice complex, in the transitional zone between the cold margin and warm interior. In this zone, a decrease in ice sliding velocity takes place, resulting in longitudinal compression. Debris frozen to the base of the glacier then becomes incorporated englacially by folding or along thrusts. However, the nature of regelation ice, as found in temperate glaciers (e.g. Hubbard & Sharp 1995), remains to be determined in polythermal glaciers. In addition, the stress imposed by even slow-moving ice on the zone where the glacier is frozen to the bed induces dislocation in the subglacial, permafrost-affected sediment. Both debris-rich regelation ice and rafts of permafrost-affected materials are then incorporated into the body of the glacier by thrusting in the zone of longitudinal compression. Individual thrusts may extend laterally for 100 m and typically dip upglacier at 30–50). Debris films and layers are associated with thrusts, the debris concentration varying from a few percent to nearly 100%
where slabs of frozen substrate are involved (Fig. 4). Most debris does not reach a supraglacial position by thrusting, but rises only several metres from the base, although in surgetype glaciers greater uplift of at least 200 m has been noted (Hambrey et al. 1996). However, within a few tens of metres of the ice margin, thrusts with debris layers crop out at the surface, and here they are closely spaced and anastomosing.
Debris release and moraine-mound formation Surface ice melting and lowering of thrust ridges onto the substrate leads to the formation of moraine mounds (Fig. 9). The mounds are apparently forming in an isolated manner today, despite rapid recession. When Svalbard glaciers were at their maximum Neoglacial extent, they had steep fronts (Liestøl 1988). Because of their more dynamic nature at that time, it is likely that the flux of debris via thrusts to the surface of the glacier was greater and more continuous around the advancing ice margin than it is today. Consequently, the density of moraine mounds is greater near the outer limit of these glaciers, where there are distinct arcuate moraine belts or complexes, superimposed upon which are moraine mounds. At all the glaciers investigated, deposition is occurring from active, though rapidly receding ice, and there is little evidence of wholesale ice-stagnation, even where there is an extensive cover of supraglacial debris. The ice margins, in fact, are clearly defined (Fig. 3), and the only evidence of dead ice is in lateral moraines and in moraine-mound complexes now disconnected from the glacier.
Comparison with temperate glaciers In temperate alpine glaciers, thrusts are developed to a lesser degree than those in the High-Arctic, and debris transport is dominated by supraglacially-derived material. Thick basal debris layers are unusual, and only small quantities of debris are incorporated from the bed into an englacial position (Small 1987). Furthermore, proximal proglacial landforms are normally destroyed by meltwater emerging from a central
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Fig. 6. Cross-sections through short segments of moraine-mound complexes in Svalbard and Britain. The internal structure in the Svalbard examples is derived from detailed morphology, facies relationships and occasional exposed cross-sections. In British examples the geometry of the thrusts is inferred from morphology.
portal. Consequently, alpine glaciers do not produce the same landform assemblages as those in the Arctic in which englacial thrusting is usually well-developed, and where proximal landforms are commonly bypassed by meltwater which usally emerges along the glacier-flanks. The presence of moraine-mound complexes, which appear to have formed from debris-bearing thrust zones, is therefore strongly suggestive of the presence of polythermal glacier ice. The style of thrusting, such as the angle and relationship to bedding in the thrust slice, reflects the differences in the sedimentary sequences affected by the overriding ice. For example, Uvêrsbreen shows a dominance of high-angle thrusting in brittle sandy gravels (Hambrey & Huddart 1995), whereas Comfortlessbreen shows low-angle thrusting and strong
internal deformation in more ductile diamictons and muds (Huddart & Hambrey 1996).
Development of landforms associated with surge-type glaciers It is important to note that englacial thrusts also develop during glacier surges, as noted at Bakaninbreen (Hambrey et al. 1996; Murray et al. in press), Kongsvegen (Bennett et al. 1996a, b) and Hessbreen (Hambrey & Dowdeswell in press) (Fig. 1). The resulting landform assemblage resembles that of non-surge type glaciers. Further comparisons are needed before it will be possible to distinguish between surge and non-surge landform assemblages.
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Fig. 7. Glacial geomorphological map of Coire a’ Cheud-chnoic (Valley of a Hundred Hills), Torridon, NW Scotland.
(a)
(b)
(c) Fig. 8. British examples of moraine mounds (‘hummocky moraines’) inferred to be the product of thrusting: (a) General view of moraine-mound complex in Coire a’ Cheud-chnoic. Arrow indicates location of (b). Ice-flow was from top right to lower left. Note cottage, lower left, for scale. (b) Close-up view of hummocks and their stacked thrust sheets arrowed in (a). The relative height of the complex here is about 10 m. The profile in Fig. 6c passes over the highest ridge crest from right to left. (c) Cwm Idwal, North Wales. The profile in Fig. 6d extends from the lake to the ridge crest on the left, traversing behind the prominent hillock in the middle. Ice-flow was across the cirque from right to left.
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Fig. 9. Conceptual model for the formation of moraine-mound complexes.
Climatic and dynamic implications for British moraine-mound complexes Although there are situations in Britain where basal topographic constraints could have increased the propensity for thrusting (e.g. the reverse slope in Cwm Idwal), the similarities between some types of the Younger Dryas moraine mounds in the British Isles, and those in Svalbard, suggest that environmental conditions may have been similar. In particular, we tentatively suggest that some of the British Younger Dryas glaciers had a polythermal character, and developed moraines under conditions similar to those in Svalbard today. The contemporary western Svalbard climate is characterized by mean annual temperatures of "5)C and precipitation of between 400–1000 mm a "1, while the ablation season usually lasts about 2.5 months (Liestøl 1988). These temperature estimates (Fig. 10) are similar to those inferred from periglacial
evidence for the Younger Dryas in Britain (Ballantyne & Harris 1994). Although the Younger Dryas glaciers emanating from the main highland icefield in Scotland, centred on Rannoch Moor, were considered to have been fast-flowing and temperate (e.g. Thorp 1991; Payne & Sugden 1990), it is likely that the more isolated and thinner glaciers in NW and E Scotland, the Lake District and North Wales, were cold-based in part, and thus more sluggish than temperate ones of similar size. This is consistent with the some glacial reconstructions based on erosional evidence. For example, in Cwm Dyli in North Wales, Sharp et al. (1989) showed that the erosive power of the Younger Dryas glacier was limited. This could be related to the bed having being frozen for part of the shortlived advance. A further implication is that the termination of the Younger Dryas glaciation was not as abrupt in Britain as would be implied by large-scale ice-stagnation in response to sudden climatic warming (Coope & Brophy 1972), thereby
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sediment and till, which were thrust into the ice from a subglacial position before being deposited. (3) Some so-called ‘hummocky moraine’ in Britain, dating from the Younger Dryas event, are morphologically and sedimentologically similar to those in Svalbard, and thrusting is therefore inferred to have been the dominant mechanism in their formation. (4) Temperate alpine glaciers are poor analogues for the British Younger Dryas moraines. (5) Some British Younger Dryas moraines are provisionally inferred to have developed under a climatic regime similar to that of Svalbard today, with a mean annual temperature of the order of "5)C, low precipitation and permafrost conditions.
Fig. 10. Annual temperature curves for Britain in the Younger Dryas (Ballantyne & Harris 1994) and Svalbard today (Liestøl 1988). Curve 1: Isfjord Radio Station (78)4* N, 13)37* E), western Svalbard. Curve 2: Conjectural range of mean monthly temperatures in central England based on an assumed mean annual air temperature of "5) derived from periglacial evidence. Curve 3: Conjectural range of mean monthly temperatures in northern Scotland based on an assumed mean annual air temperature of "8) derived from periglacial evidence.
supporting the views of Benn et al. (1992) and Bennett & Boulton (1993b). Rather, the moraine patterns suggest that the glaciers remained dynamic during climatic amelioration, responding by recession of the snout. This interpretation contrasts with isotopic evidence obtained from cores through the Greenland ice sheet, which indicates that the Younger Dryas event ended abruptly (Dansgaard et al. 1989; Dowdeswell & White 1995), and was responsible for a sharp rise in sea level around 10 000 years ago (20 mm a "1) (Fairbanks 1989). What is not known at this stage is whether individual glaciers switched from a polythermal to a temperate state as climatic warming took place. Long-term monitoring of Svalbard glaciers, which are currently responding to sharp twentieth century warming, would help to resolve this issue. Outside Britain, moraine-mound complexes occur widely within the limits of former mid-latitude ice sheets, where they have also been used as evidence of glacier stagnation. The formation in Svalbard of moraine-mound complexes by thrusting in ice margins suggests that large-scale glacier stagnation may not always explain the development of these landforms satisfactorily, a point already made by Sollid & Sørbel (1988).
Conclusions (1) Moraine-mound complexes in the High-Arctic (Svalbard) are largely the product of thrusting of subglacially derived sediment into polythermal glacier ice and its subsequent release as the ice recedes. (2) Recognition of thrust-related moraine-mound complexes is based on (i) the presence of rectilinear and relatively gentle (c. 30)) proximal slopes, and a relatively irregular and steep distal slope; (ii) evidence of stacking of discrete slabs of sediment, some of which retain their original sedimentary structures, while others show signs of recumbent folding; and (iii) the wide range of facies present, such as glaciofluvial
Work in NW Spitsbergen was supported financially by Liverpool John Moores University (1992–5), The University of Greenwich (1995), the Royal Society (1992) and the UK Natural Environment Research Council (Grant no. GR9/02185) (1996). We thank N. Cox and NERC for provision of logistical support and facilities in the Kongsfjorden area. Work at the southern localities was supported by the European Union Environment Programme Grant EN5V-CT93-0299 to J. A. Dowdeswell (1994, 1995). We thank the journal referees (including D. I. Benn and N. Spedding) for helpful reviews.
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Received 24 June 1996; revised typescript accepted 5 February 1997. Scientific editing by Graham Shimmield.
