Geomorphology and Sedimentology of Contemporary Surging Glaciers

This section provides details of the landform-sediment assemblages observed at contemporary surging glacier margins in Iceland, Svalbard, USA and Canada. Additionally, extensive reference is made to the 1982—83 surge of Variegated Glacier, Alaska, because it is the best documented and instrumented surge event. The time development of surging glacier geomorphology is best represented by the Icelandic examples due to the fact that the surges are historically documented by the local population over the last two centuries, and aerial photography since 1945 (Thorarinsson, 1964, 1969; Evans et al., 1999a).

11.2.1 Thrust-Block Moraines and Push Moraines

Present day thrust-block moraines (including composite ridges and hill-hole pairs; Aber et al., 1989) are found in two ice-marginal settings: at the margins of surging glaciers, (e.g. Sharp, 1985b; Croot, 1988a, b; Bennett et al., 1999; Evans and Rea, 1999; Evans et al., 1999b), and associated with sub-polar glacier margins in permafrost terrains (e.g. Kalin, 1971; Evans and England, 1991; Fitzsimons, 1996a). Proglacial thrusting at the margins of surging glaciers is due to rapid ice advance into proglacial sediments, which may be seasonally frozen, unfrozen or contain discontinuous permafrost. A surging glacier advancing into proglacial frozen sediments is most conducive to the failure and stacking of large contorted and faulted blocks, best exemplified by the wide belts of arcuate thrust ridges of glacimarine and glacifluvial materials at the margins of the polythermal surging glaciers of Svalbard (Fig. 11.1). However, rapid ice advance into unfrozen sands and gravels will still produce high proglacial and sub-marginal compressive stresses. High pore water pressures may be developed in silt and clay layers within the succession, which will act as décollement leading to shearing and stacking of the sand and gravel units.

Excellent examples of proglacially thrust unfrozen materials occur at the surge margins of the Icelandic glaciers Bruarjokull and Eyjabakkajokull, where pre-surge peat layers have been vertically stacked in a series of thrust overfolds (Fig. 11.2) within which individual beds often display slickensides. Thrust-block moraines may be produced also at the margins of polar and sub-polar glaciers where the proglacial sediments contain permafrost and high proglacial stresses can be produced (Evans and England, 1991; O Cofaigh et al., Chapter 3). Because thrust-block moraines can be constructed by surging and non-surging glaciers, their existence in the ancient landform record cannot be ascribed unequivocally to glacier surging. However, former surging activity by sub-polar glaciers that have produced thrust-block moraines cannot be dismissed, particularly as surging has been reported in such environments (Hattersley-Smith, 1969a; Jeffries, 1984).

Although thrust-block moraines are the most spectacular constructional features produced by surging glacier margins, they can be produced only in areas where sufficient sediment is available for glacitectonic thrusting, folding and stacking. The wide surging margin of Bruarjokull has produced thrust blocks only in areas that lie down flow from braided outwash plains where large accumulations of fluvial and/or lacustrine material collected during the

Glacial Flow Fold
Figure 11.1 Aerial photograph of Rabotsbreen, Svalbard (Norsk Polarinstitutt) showing a thrust-block moraine produced during a recent surge.
Native Americans Igloos
Figure 11.2 Thrust overfolds in peat produced at the margin of Bruarjokull during the AD 1890 surge.

quiescent phase. Elsewhere, the glacier constructs low-amplitude push moraines by bulldozing and/or sub-marginally squeezing the veneer of till or peat that drapes the proglacial land surface.

11.2.2 Overridden Thrust-Block Moraines

A number of conspicuous ice-moulded hills occur in the proglacial forelands of Bruarjokull and Eyjabakkajokull. They occur down-ice of topographic depressions from which the hills were originally displaced by thrusting. The surfaces of these features appear extensively fluted and/or drumlinized (Fig. 11.3). Internal structures comprise glacitectonized outwash or lake sediments, the tops of which are usually modified into glacitectonite (Benn and Evans, 1996, 1998) and truncated by subglacial till (Fig. 11.4). These ice-moulded hills are interpreted as overridden thrust-block moraines. The sediments displaced by thrust-block construction are deposited during the surge quiescent phase when proglacial lakes and meltwater streams extensively modify the foreland. Each thrust block demarcates the former glacier margin during a surge, overriding taking place either during the same surge or during a later, more extensive surge. After prolonged periods of modification by overriding ice the former thrust blocks resemble the cupola hills of Aber et al. (1989).

11.2.3 Concertina Eskers

On aerial photographs of the margins of Bruarjokull and Eyjabakkajokull (Knudsen, 1995; Fig. 11.5), sinuous eskers and 'concertina' plan-form eskers are found. Extensive exposures of glacier ice at the base of the eskers (Fig. 11.5c) indicate they were produced englacially or supraglacially (Evans and Rea, 1999; Evans et al., 1999b). Referring to a bulge observed in the snout of Bruarjokull during the 1964 surge, Knudsen (1995) suggests that the concertina

Figure 11.3 Part of an aerial photograph (Landmaelingar Islands, 1993) of an overridden thrust block on the foreland of east Bruarjokull (outlined by broken line). Note the occurrence of flutings and crevasse-squeeze ridges on the surface.
Figure 11.4 Exposure through an overridden thrust block near the AD 1890 surge limit of west Bruarjokull. Contorted interbeds of peat and stratified sediments are overlain by a till carapace that possesses a fluted surface expression.

Figure 11.5 A) Part of an aerial photograph (Landmaelingar Islands, 1993) of a concertina esker on the foreland of west Bruarjôkull, Iceland, produced during the 1963 surge. B) Ground photograph of a concertina esker on the foreland of Bruarjôkull, showing stratification and steep cliffs produced by the melt-out of buried glacier ice. C) Glacier ice exposed in a concertina esker on the Eyjabakkajôkull foreland.

eskers were formed during surging similar in style to the Variegated Glacier surge of 1982—83. Knudsen proposes that the concertina eskers are produced by the shortening of pre-surge sinuous eskers by compression in the glacier snout during surging, the Bruarjokull examples having been compressed during the 1964 surge. It seems unlikely that a situation involving extreme tectonic activity and vertical thickening (>30 m during the surge) could deform an initially sinuous esker to produce the concertina plan-form. Moreover, substantial transverse extension of the snout is required in order for a concertina esker to form in the manner suggested by Knudsen (1995). Initially as the surge wave passed down through the snout region (and the sinuous eskers) there was no room for lateral extension and indeed the snout may have undergone some slight lateral shortening as it passed between topographic highs to the west (Kverkarnes) and east (high point between east Bruarjokull and Eyjabakkajokull). The principal extension at this time would then have been vertical, associated with extensive folding and thrusting. Once beyond these high points, the snout began to splay laterally, resulting in the formation of numerous conjugate shears with the principal axis of extension aligned approximately flow-transverse. The sinuous eskers have undergone flow-parallel compression, with initial vertical extension, followed by flow-transverse extension. The 'limbs' of the concertina eskers are sub-linear, the contact between the sediments and the ice is planar, and the sediments have undergone very little post deposition deformation, the most significant tectonics being normal faulting resulting from meltout of the underlying ice. Thus, it would seem unlikely that concertina eskers are formed by the shortening of pre-surge sinuous eskers in the manner proposed by Knudsen (1995).

It seems more reasonable to assume that the surge destroyed the pre-surge sinuous eskers, and the concertina eskers formed during the surge and/or the immediate post-surge period. The ice-cored nature of the concertina eskers (Fig. 11.5c) demonstrates that they were deposited either in englacial conduits or in supraglacial channels. During the 1982—83 Variegated Glacier surge, substantial quantities of water were stored behind the surge front and these were discharged periodically during, and, at the termination of the surge (Humphrey and Raymond, 1994). Basal water pressures are elevated and so water may drain via an englacial and/or supraglacial drainage system, which would rapidly exploit the extensive network of crevasses created during the surge. Such an event would be short lived, and indeed the high-angled apexes of the concertina eskers suggest the period of water flow along them was not of great longevity. We conclude that the concertina eskers formed during a short-lived, high-discharge event, occurring prior to or very shortly after surge termination. Drainage occurred either englacially or supraglacially and the channel was then rapidly abandoned.

Like some sinuous eskers (e.g. Price, 1969), the gradual melt-out of ice cores in concertina eskers either destroys or heavily disturbs their internal sedimentary structures. Additionally, because the majority of the initial relief of concertina eskers is due to their ice core, they may appear in ancient glacial landform assemblages as discontinuous chains of gravel and sand mounds and therefore be mis-identified as 'kame' forms.

11.2.4 Crevasse-Squeeze Ridges

Crevasse-squeeze ridges are best known from Bruarjokull and Eyjabakkajokull, Iceland and have been used as diagnostic criteria of surging (Sharp, 1985a,b); although Sharp referred to them as crevasse-fill ridges). Prominent crevasse-squeeze ridge networks can be seen on the forelands of both Bruarjokull and Eyjabakkajokull (Fig. 11.6), comprising cross-cutting diamicton ridges that can be traced from the foreland and into crevasse systems in the snouts (Sharp, 1985a, b; Evans and Rea, 1999; Evans et al., 1999b). Crevasse-squeeze ridges have been reported also from Trapridge Glacier (Clarke, et al., 1984) and Donjek Glacier (Johnson, 1972, 1975) in Yukon Territory and from Svalbard (Clapperton, 1975; Boulton et al., 1996; Evans and Rea, 1999) where they are associated with surging glaciers (Fig. 11.7). The extreme tectonics experienced during a surge leave the glacier highly fractured (e.g. Kamb et al., 1985; Raymond et al., 1987; Herzfeld and Mayer, 1997), and many crevasses may extend to the glacier bed. At surge termination as the basal water pressures are reduced and effective pressure at the bed increases, water-saturated sediment rises into open basal crevasses. Currently these are seen as prominent cross-cutting diamict ridges melting out on the glacier surface (Fig. 11.6). Due to the inactivity of the quiescent phase the glacier ice melts away preserving the cross-cutting ridge network. If the glacier returns to slow-flow dynamics then the crevasse-squeeze ridges will be deformed into a typical glacier strain profile.

In the terminus region of non-surging glaciers, crevasses aligned perpendicular to the glacier margin are often found (e.g. Evans and Twigg, 2002). With a bed of deformable sediment such crevasses could fill with sediment and form crevasse-squeeze ridges. In this situation however, active retreat of the glacier margin would destroy or substantially modify the squeeze ridges. Only if the glacier stagnated and wasted away in situ, would preservation be favoured, and even then the radial crevasse pattern typical of such glacier snouts would indicate a non-surging origin. From the evidence presented above crevasse-squeeze ridges are a landform highly suggestive of surging glacier activity, but they cannot be regarded independently as diagnostic

Figure 11.6a Part of an aerial photograph (Landmaelingar Islands 1993) of crevasse-squeeze ridges at the margin of west Bruarjokull, Iceland, produced during the 1963 surge.

Figure 11.6b Ground photograph of the crevasse-squeeze ridges and flutings at the margin of Eyjabakkajokull, Iceland.

Base Surge DepositCrevasse Squeeze Ridges
Figure 11.7 A crevasse-squeeze ridge at the base of the surging glacier Osbornebreen, St Jonsfjorden, Svalbard.

features of palaeo-glacier surging even though widespread development of crevasse-squeeze networks clearly requires extensive fracturing of the glacier, normally associated with surging.

11.2.5 Flutings

Flutings occur on the forelands of many glaciers and are certainly not diagnostic of glacier surging. However, fluting length may provide important evidence for rapid advances over substantial distances. Excellent examples of this exist on the foreland of Bruarjokull where regularly spaced parallel-sided flutings (Benn and Evans, 1996, 1998) are continuous for more than 1 km inside the 1964 surge moraine (Fig. 11.8), and display remarkable uniformity in long- and cross-section. Another prominent feature of the fluting fields are numerous boulders with short sediment prows/flutes on their down-flow sides, which are interpreted as ploughs/incipient flutes produced by boulders embedded in the glacier sole or just lodged into the till at the surge termination. Clast fabrics were measured from a number of the Bruarjokull flutes and ploughs/incipient flutes. Low fabric strengths are recorded in both the elongate flutes

Figure 11.8 Long flutings formed during the 1964 surge of Bruarjokull.

and the short ploughs/incipient flutes (Fig. 11.9). Other clast fabric measurements made on flutes from non-surging glaciers suggest that either herring-bone or flow-parallel fabrics are the norm, with flow-parallel clustering being expected for parallel-sided flutings of the type described here (Rose, 1989; Benn, 1994, 1995; Eklund and Hart, 1996). The very low fabric strengths are taken to indicate that there was little coupling between the glacier and the bed. The elongation of these flutes suggests that they were formed during a single flow event when basal water pressures and thus the degree of ice-bed coupling and sediment strength remained 'constant'. Flutes formed by non-surging Icelandic glacier margins tend to be substantially shorter and much less uniform in long section, due probably to fluctuations in ice-bed interface conditions, driven by seasonal or annual cycles.

Additionally, the association of flutings and crevasse-squeeze ridges is an important aspect of the subglacial geomorphology of surging glaciers. Sharp (1985a, b) notes that fluting crests at Eyjabakkajokull rise to intersect the crevasse-squeeze ridge crests, indicating that subglacially deformed till was squeezed into basal crevasses as the glacier settled onto its bed at the end of the surge. The contemporaneous production of the flutings and crevasse-squeeze ridges at Eyjabakkajokull renders them diagnostic of glacier surging in the landform record. Due to the range of glaciers and glacial conditions under which flutings are produced they cannot be used independently as diagnostic of glacier surging.

11.2.6 Thrusting/Squeezing

Sharp (1985a), in his model of sedimentation for Eyjabakkajokull, suggests a zone of thrusting exists in the snout where sediment is lifted from the bed. Thrusting can occur in surging glaciers where the surge front propagates into thin ice (Raymond et al., 1987). In such settings supraglacial sediment on the thin ice is entrained along the thrust. Similar features, dipping in an up-glacier direction, have been cited as the products of debris thrusting from basal to

Figure 11.9 Typical low strength clast fabric plots measured in an elongate parallel sided flute on the Bruarjokull foreland. Arrows indicate flute long axis/ice flow direction and location of plot represents position of sample on the fluting (i.e. left side, crest and right side).

englacial/supraglacial positions in surging glacier snouts on Svalbard (Bennett et al., 1996b; Hambrey et al., 1996; Murray et al., 1997; Porter et al., 1997; Glasser et al., 1998b). However, close inspection of a number of englacial diamict bands, which look temptingly like thrusts, exposed along the margins of Bruarjokull and Eyjabakkajokull, show no evidence of basal ice thrust over firnification ice, which would be the case if the diamict was emplaced by thrusting . It is believed that many of the features reported from Svalbard may be tilted crevasse-squeeze ridges rather than sediment emplaced by thrusting. Crevasse-squeeze ridges are predominantly formed vertically to sub-vertically. Subsequently, if normal glacier flow resumes or a small amount of forward momentum remains after ridge construction, crevasse fills will be compressed and tilted into a down-glacier direction (Fig. 11.10). Excavation of a crevasse-squeeze ridge melting out of the snout of Bruarjokull exposed slickensided, fine-grained sediment, indicating a small amount of post-emplacement shearing through the sediment in the ridge. If crevasse-squeeze ridge tilting occurs, the preservation potential of the squeeze ridge form is very poor. The melt-out of tilted ridges will produce a landform-sediment signature similar to that envisaged for englacial thrusting (see below). Specifically, this includes low-relief hummocky moraine comprising interbedded sediment gravity flows and crudely bedded stratified sediments with the possible preservation of small ridges where the thrust intersected the bed. Based upon observations of crevasse-squeeze ridges melting out from Bruarjokull, it is

Crevasse Squeeze Ridges
Figure 11.10 Crevasse-squeeze ridge emerging at the surface of Bruarjokull. Note that the ridge is dipping up-glacier (towards the right) and mass flowage of the down-glacier side of the ridge is producing an extensive spread of bouldery rubble to the left.

clear that mass flowage of ridge sediment down the glacier surface results in the production of boulder lags and thin debris flow diamictons resting on the subglacial till surface (Fig. 11.10).

11.2.7 Hummocky Moraine

Production of prominent belts of hummocky moraine, particularly at the margins of lowland glaciers, is reliant upon widespread and effective transportation of large volumes of material to (eventual) supraglacial positions. In lowland surging glaciers, thrusting has been cited as the dominant process in transporting large volumes of debris into englacial and supraglacial positions. Subsequent ice stagnation leads to the production of hummocky moraine (Sharp, 1985b; Wright, 1980). Substantial amounts of sediment may also be intruded into the glacier by squeezing of sediment up into crevasses, which eventually melts out on the glacier surface during quiescence. Thick debris sequences on the glacier surface may preserve underlying stagnant ice for long periods, creating a feedback loop. Thus, successive surges may involve overriding, overthrusting and incorporation of debris-rich stagnant ice preserved from a previous surge, producing thick sequences of debris-rich and debris-covered ice in surging snouts (Johnson, 1972). The resulting landform assemblage after ice melt-out would comprise aligned hummocky moraine ridges and kame and kettle topography. The widespread distribution of hummocky moraine has been cited by Clayton et al. (1985) and Drozdowski (1986) as evidence of palaeo-surging at the margins of the Laurentide and Scandinavian Ice Sheets based upon observations of contemporary, debris-rich surge snouts by Clapperton (1975) and Wright (1980). However, such a landform assemblage can be produced by non-surging glaciers, particularly where debris provision rates are high, and so cannot be used solely as a diagnostic criterion.

Conspicuous mounds of hummocky topography occur on the down-ice sides of topographic depressions on the forelands of Bruarjokull and Eyjabakkajokull. These can be differentiated from overridden thrust-block moraines by the fact that they are characterized by extensive evidence of on-going melt-out of buried ice. This melt-out has disturbed the faint fluting patterns that occur on the hummock surfaces (Fig. 11.11). Stratigraphic exposures are rare but indicate that the hummocks comprise intensely glacitectonized, fine-grained stratified sediments and diamictons or poorly sorted gravels. Pockets of stratified sediments interbedded with diamictons occur in small depressions on the hummocky topography. These sediments have been contorted into low-amplitude folds by the melt-out of underlying ice. The interpretation of this hummocky topography is that it evolves from surging by the thrusting, squeezing and bulldozing of proglacial lake sediments and outwash over pre-existing stagnant ice dating to a previous surge. The faint flutings on the hummock surfaces indicate that the sediment and stagnant ice were overridden by the surging snout. Supraglacially reworked sediments are locally deposited over the bulldozed sediments as they emerge from beneath the melting ice, leading to the deposition of the small pockets of stratified sediments. The post-surge melt-out of the older buried ice results in the production of chaotic hummocky terrain upon which flutings may still be observed as discontinuous linear ridges, at least during the early stages of melt-out. Mapping

(B)

Figure 11.11 Examples of hummocky moraine at the margins of Icelandic surging glaciers. A) Part of an aerial photograph (Landmaelingar Islands, 1993) of ice-cored hummocky moraine tracts (H) located on the down-ice sides of topographic depressions at west Bruarjôkull. The AD 1890 surge moraine is marked (S). B) View across ice-cored hummocky moraine located on the proximal side of the AD 1890 surge moraine at west Bruarjôkull. Note surface pond produced by ice melt-out. C) Example of low-amplitude hummocky moraine produced after completion of ice melt-out, west Eyjabakkajôkull.

of the hummocky moraine at Bruarjokull demonstrates that it occurs in discrete pockets on the foreland. These are located immediately down-glacier from extensive depressions that have been partially filled with proglacial outwash and glacilacustrine sediments since glacier recession. This distribution pattern, together with the sediment textures and structures and evidence of buried ice, strongly supports the contention outlined above that the material comprising the hummocky moraine originated as a drape over parts of a stagnant glacier snout occupying topographic depressions, and that more recent surging displaced both older ice and its sediment drape (Fig. 11.12). The production of high-relief hummocky moraine is more likely to be the result of the displacement of ice-cored outwash and lake sediment rather than the melt-out of englacially thrust or squeezed sediment, the land-forming limitations of which were mentioned above.

11.2.8 Ice-Cored Outwash and Glacilacustrine Sediments

Observations at the margins of Bruarjokull and Eyjabakkajokull indicate that proglacial outwash tracts and glacilacustrine depo-centres have been developed over the shallow stagnant margins of the snouts during quiescence (Evans and Rea, 1999). The outwash occurs as ice-contact fans fed by subglacial and englacial meltwater portals. During and at the termination of the 1982—83 surge of Variegated Glacier, outbursts of water were observed supraglacially (Kamb et al., 1985) and so substantial quantities of sediment may be deposited onto the glacier surface at this time. However, this drainage network is transient and a drainage pattern controlled by the bed topography becomes re-established post-surge. During quiescence Bruarjokull is drained by four major rivers (Kverka, Kringilsa, Jokulsa a Bru and Jokulkvisl) from approximately seven ice-marginal outlets situated in topographic lows. Those parts of the glacier snout that occupy these topographic hollows are prone to burial by large outwash fans and, in more distal locations, glacilacustrine sediments.

Ice-cored outwash fans and glacilacustrine sediment bodies (including ice-contact deltas) are common along the margins of Bruarjokull (Fig. 11.13). An extensive outwash fan deposited over ice on the east side of Bruarjokull (which did not surge in 1963/64) has been modified gradually by melting of the underlying stagnant snout (Figs. 11.13b, 11.13c). This has resulted in the formation of kettles, followed by the collapse of tunnels in the stagnant ice to form ice-walled channels. The final stage in the evolution of the ice-cored outwash involves the production of chaotic sand and gravel hummocks within which sinuous eskers may be recognizable. Subsequent localized reworking of the outwash into terraces by proglacial streams and its draping by proglacial lake sediments may occur. Due to the extensive nature of the underlying ice, such outwash fans will be represented in the ancient landform record as a landscape of chaotic gravel mounds locally modified by fluvial activity. As such they may be difficult to differentiate from the hummocky moraine outlined above, although they will be located in topographic depressions as opposed to down-ice sides of topographic depressions. They may exhibit a fan shape when viewed as a landform assemblage, and will consist of hummocks with largely accordant summits. Additionally, the internal disturbance of the sedimentary structures of fans and lake sediments will be characterized by simple folds and normal faults rather than the compression structures seen in the hummocky moraine described above. A subsequent surge over these glacifluvial and glacilacustrine sediment bodies will result in the production of either thrust-block moraines where the underlying ice has melted out, or hummocky moraine where a substantial amount of buried ice remained.

Fast Ice Streams

block moraine from pre-existing sediments. B) Glacier stagnates and outwash, delta and lake sediments fill in the erosional basin, covering a large remnant of stagnating ice. C) Situation after a further surge and stagnation period (note change in position of the cross-section), showing the construction of another thrust-block moraine beyond the existing example and the transport of ice blocks and contorted lake and outwash sediments from the erosional basin to the top of the overridden thrust blocks (p = pond, h = hummocky moraine).

11.2.9 Complex Till Stratigraphies

Observations from Trapridge Glacier have prompted Clarke et al. (1984) and Clarke (1987) to suggest that a deformable substrate may play a significant role in cyclical surging activity, although exact causal mechanisms for surging are still unclear (Raymond, 1987). Similar to the flow of non-surging glaciers over deformable beds, the advance of surging glacier snouts into areas of soft sediment produces glacitectonic structures and the thickening of stacked glacitectonites, deformation tills and intervening deposits at the glacier margin (e.g. Boulton, 1996a, b; Alley et al., 1997). This has been observed at the margin of Sefstrombreen, a surging glacier on Svalbard (Lamplugh, 1911; Boulton et al., 1996; Fig. 11.14a), a contemporary setting that has been used as an analogue for till deposition in eastern England. The latter location is illustrative of the use of stratigraphic sequences comprising several tills and associated stratified sediments as diagnostic criteria for palaeo-surging (e.g. Eyles et al., 1994). A stratigraphic section exposed by river erosion at the margin of Eyjabakkajokull, Iceland in 2000 is reproduced in Fig. 11.14b. The sediments and structures in this exposure record the most recent surge of part of the glacier margin in 1972. Glacifluvial gravels, including a stranded iceberg, have been glacitectonized by glacier advance (Fig. 11.14c) and capped by a surge till. The till is a massive diamicton that forms crevasse-squeeze ridges on the present day ground surface. In ancient settings, short periods of till deposition, identified by radiocarbon dating of organics lying between individual till layers in multiple till sequences, has been presented as compelling evidence for surging of the southern margin of the Laurentide Ice Sheet (e.g.

Outwash Fans

Figure 11.13 Ice-cored outwash fans and glacilacustrine sediments at the margin of Bruarjokull, Iceland. A) Proximal glacilacustrine sediments displaying evidence of extensive collapse due to melt-out of the 1964 surge snout. B) Ice-contact fan developed over the 1964 surge snout (photographed in 1995 with glacier towards the left), showing evidence of kettle production due to melt-out of buried glacier ice. C) The same ice-contact fan viewed from the glacier snout in 2000. Note that the pitted outwash has developed into a landform characterized by large icewalled channels and chaotic mounds of gravel.

Figure 11.13 Ice-cored outwash fans and glacilacustrine sediments at the margin of Bruarjokull, Iceland. A) Proximal glacilacustrine sediments displaying evidence of extensive collapse due to melt-out of the 1964 surge snout. B) Ice-contact fan developed over the 1964 surge snout (photographed in 1995 with glacier towards the left), showing evidence of kettle production due to melt-out of buried glacier ice. C) The same ice-contact fan viewed from the glacier snout in 2000. Note that the pitted outwash has developed into a landform characterized by large icewalled channels and chaotic mounds of gravel.

DISTANCE FROM DATUM (m)

Figure 11.14 Examples of tills and associated stratified sediments from the forelands of contemporary surging glaciers. A) Glacitectonically folded and thrust sediments on Coralholmen, Svalbard, produced by a surge of Sefstrombreen. Facies A is red silt and clay-rich diamicton, Facies B is red silty-sandy diamicton and Facies E is green sand and gravel with large quantities of Lithothamnium molluscs. Modified from Boulton et al. (1996). B) Stratigraphic section at the margin of Eyjabakkajokull showing a sequence of poorly sorted gravels overlain by glacitectonized gravels and sands, containing a former grounded iceberg, and then massive diamicton (till) that forms the surface crevasse-squeeze ridges. Till deposition and glacitectonic disturbance was initiated by a surge in 1972 when the glacier overrode proximal proglacial outwash containing a stranded iceberg. C) Glacitectonically disturbed sand and mud in the fine-grained outwash to the right of the section in Figure 11.14 B.

Figure 11.14 Examples of tills and associated stratified sediments from the forelands of contemporary surging glaciers. A) Glacitectonically folded and thrust sediments on Coralholmen, Svalbard, produced by a surge of Sefstrombreen. Facies A is red silt and clay-rich diamicton, Facies B is red silty-sandy diamicton and Facies E is green sand and gravel with large quantities of Lithothamnium molluscs. Modified from Boulton et al. (1996). B) Stratigraphic section at the margin of Eyjabakkajokull showing a sequence of poorly sorted gravels overlain by glacitectonized gravels and sands, containing a former grounded iceberg, and then massive diamicton (till) that forms the surface crevasse-squeeze ridges. Till deposition and glacitectonic disturbance was initiated by a surge in 1972 when the glacier overrode proximal proglacial outwash containing a stranded iceberg. C) Glacitectonically disturbed sand and mud in the fine-grained outwash to the right of the section in Figure 11.14 B.

Clayton et al., 1985; Dredge and Cowan, 1989b). However, a sequence of glacier advances and retreats could feasibly produce a similar stratigraphy, so this alone is not diagnostic of palaeo-surging.

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