Reactivation of temporarily stabilized ice-cored moraines in front of polythermal glaciers: Gravitational mass movements as the most important geomorphological agents for the redistribution of sediments (a case study from Ebbabreen and Ragnarbreen, Svalbard)

Geomorphology 350 (2020) 106952

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Reactivation of temporarily stabilized ice-cored moraines in front of polythermal glaciers: Gravitational mass movements as the most important geomorphological agents for the redistribution of sediments (a case study from Ebbabreen and Ragnarbreen, Svalbard) Marek W. Ewertowski ⁎, Aleksandra M. Tomczyk Faculty of Geographical and Geological Sciences, Adam Mickiewicz University, Krygowskiego 10, 61-680 Poznań, Poland

a r t i c l e

i n f o

Article history: Received 12 March 2019 Received in revised form 6 November 2019 Accepted 6 November 2019 Available online 09 November 2019 Keywords: Glacial geomorphology Moraine GIS Arctic Debris flow Paraglacial Structure-from-motion

a b s t r a c t Ice-cored moraine complexes are prominent landforms characteristic of many polythermal glacial landsystems on Svalbard, usually marking their maximum Little Ice Age extent. Despite being often stable over a period ranging from several years to 50–60 years, the abundance of dead-ice makes them potentially unstable in the future. This study aims to: (1) recognize the different processes that contribute to ice-cored moraine degradation; (2) document the spatial distribution of mass wasting sites within ice-cored moraine complexes of Ebbabreen and Ragnarbreen, Svalbard; (3) investigate the relationship between the distribution of mass-movement sites and the topographic properties of the landscape; (4) develop a model of mass-wasting mechanism in a proglacial area. We performed field-based geomorphological mapping combined with the interpretation of aerial photographs (1960, 1990, 2009), high-resolution satellite imagery (2013), UAV-based images (2014), terrestrial photographs (2011–2014), hillshade models and digital elevation models. In 2013, frontal and lateral ice-cored landforms occupied 3.33 km2 of the forelands of Ebbabreen and Ragnarbreen. The total area with an unstable ice-cored moraine condition was 0.57 km2 (127 sites). Debris falls, slides, rolls and flows were the most important processes contributing to the reworking of glacigenic sediments and altering of their properties. Four different morphological site types were identified: (1) near vertical ice-cliffs covered with debris, transformed mainly by dead-ice backwasting as well as debris falls and slides, (2) steep debris slopes with exposed ice-cores dominated by debris slides, (3) gentle sediment-mantled slopes transformed by debris flows, and (4) non-active debris-mantled areas transformed by dead-ice downwasting alone. The spatial and temporal activation of moraines' reworking was for the most part related to local geomorphic conditions (e.g., slope gradient, occurrence of streams, and meltwater channels). Gravitational massmovements were responsible for the redistribution of debris within the moraine-complex, whereas fluvial transport by meltwater exported sediments outside the Little Ice Age moraines. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Ice-cored moraines are typical of many polythermal glacial landsystems on Svalbard (e.g., Boulton, 1972; Bennett et al., 1996, 2000; Lyså and Lønne, 2001; Sletten et al., 2001; Glasser and Hambrey, 2003; Lønne and Lyså, 2005; Lukas et al., 2005; Midgley et al., 2007, 2013, 2018; Ewertowski et al., 2012). These prominent landforms are often associated with the final advances of glaciers marking their Little Ice Age (LIA) maximum extent (frontal moraines) and ⁎ Corresponding author at: Faculty of Geographical and Geological Sciences, Adam Mickiewicz University, Krygowskiego 10, 61-680 Poznań, Poland. E-mail addresses: [email protected] (M.W. Ewertowski), [email protected] (A.M. Tomczyk).

https://doi.org/10.1016/j.geomorph.2019.106952 0169-555X/© 2019 Elsevier B.V. All rights reserved.

thickness along the valley sides (lateral moraines), hence their frequent use for reconstruction of the LIA extents of glaciers (e.g., Rachlewicz et al., 2007; Ewertowski and Tomczyk, 2015; Małecki, 2016). From a paleogeographical standpoint, it is important to stress that, despite being prominent features in the modern landscape, the main volume of these landforms is usually built of ice-cores (dead ice) (Gibas et al., 2005), whereas the veneer of debris is relatively thin, often b2 m. Therefore, their expression in the deglaciated landscape and geological record will be much less substantial (Evans, 2009). Investigation of ice-cored moraine dynamics yields a much diversified range in the rates of dead-ice melting (Schomacker, 2008; Bennett and Evans, 2012; Ewertowski, 2014; Ewertowski and Tomczyk, 2015; Bernard et al., 2016; Tonkin et al., 2016). Despite the fact that ice-cored moraines are not a final component of the landscape,

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recent studies have indicated that large fragments of ice-cored moraines on Svalbard are in equilibrium with current climatic conditions, having remained stable over a period ranging from several years to 50– 60 years; however, the abundance of dead-ice makes them potentially

unstable in the future (Lukas et al., 2005; Ewertowski, 2014; Ewertowski and Tomczyk, 2015; Tonkin et al., 2016; Midgley et al., 2018; Ewertowski et al., 2019a). Therefore, it is important to stress, that there is a temporal shift between initial creation on the one hand,

Fig. 1. Proglacial areas surrounding Petunibukta, Spitsbergen. Areas exposed after the Little Ice Age are indicated. (Source: Ewertowski and Tomczyk, 2015.)

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and subsequent transformation and final de-icing of the ice-cored moraines on the other. The key question is also related to factors which contribute to the switch between stable and unstable conditions. The main geomorphic agents causing the degradation of ice-cored moraines on Svalbard are mass movement processes (Lukas et al., 2005; Schomacker and Kjær, 2008; Ewertowski and Tomczyk, 2015; Pleskot, 2015). The primary glacial deposits released from the ice are subsequently transferred by these mass movement processes until they finally reach a more stable position. The continued redistribution of sediments causes several phases of debris transfer from higher to lower topographic points, leading to relief inversion in some locations. The moraine distribution in palaeo-environmental settings is often used to infer the character and dynamics of past ice masses (Evans, 2009; Kirkbride and Winkler, 2012; Barr and Lovell, 2014). Similarly, the sedimentary properties of past glacial deposits are often used to infer the character of the depositional environment and indirectly the glaciological properties of ice masses. Therefore, knowledge of the mechanisms of ice-cored moraine degradation and the level of change in sediment properties by post-depositional processes is of utmost importance for understanding the sedimentary and geomorphological record of Quaternary glaciations. Among the most interesting questions are the following: (1) What are the main processes responsible for sediment redistribution during ice-cored moraine transformation? (2) What is the spatial distribution of these processes? (3) What is the extent of instability where ice-cored moraines are subject to degradation in front of modern glaciers? This contribution aims to provide a better understanding of the degradation of ice-cored moraines using modern analogues from Svalbard. The main objectives are: 1) To recognize the different processes that contribute to ice-cored moraine degradation. 2) To document the spatial distribution of mass wasting sites within the ice-cored moraine complexes of Ebbabreen and Ragnarbreen. 3) To investigate the relationship between the distribution of massmovement sites and the landscape's topographic properties. 4) To develop a model of mass wasting mechanism in a proglacial area. 2. Regional settings Spitsbergen Island is part of the High Arctic Svalbard archipelago, characterised by extensive (~60%) glacial coverage. Our research focuses on the central part of the island with a much drier climate than coastal areas (Rachlewicz and Styszyńska, 2007; Rachlewicz, 2009) with mean annual precipitation at Svalbard Lufhavn station (~60 km from the study area) for 1981–2010 equalled to 189.1 mm (Bednorz and Jakielczyk, 2014). Mean air temperatures in the Petunibukt area are above 0 °C between June and September (Rachlewicz, 2003; Rachlewicz and Styszyńska, 2007; Láska et al., 2012). Geology of the vicinity of the Petuniabukta is related to the Billefjorden fault zone (a north-south orientation). In consequence, different types of bedrock related to Precambrian (various crystalline), Devonian (mainly sandstones and mudstones), Carboniferous-Permian (conglomerates, sandstones, mudstones, limestones, coal, gypsum, anhydrites, and dolomites), and Quaternary (alluvium, colluvium, and landslide debris, as well as glacial, fluvioglacial, marine, intertidal and beach deposits) are exposed (Harland et al., 1974; Lamar and Douglass, 1995). The permafrost is continuous in Svalbard with a thickness ranging from 400 to 500 m in the upland area to b100 m in the valley bottoms; presumably thinning toward the seashores (Humlum et al., 2003; Etzelmüller and Hagen, 2005; Keating et al., 2018; Rouyet et al., 2019). Therefore, active geomorphic processes are limited to the thickness of the active layer, which in the vicinity of Petuniabukta varies from 0.5 to 2.5 m (Gibas et al., 2005; Rachlewicz and Szczuciński, 2008). Previous geomorphological studies in proglacial areas of Petuniabukta (e.g., Kłysz, 1985; Kłysz et al., 1989; Karczewski et al., 1990; Rachlewicz, 2009, 2010; Szuman and Kasprzak, 2010; Evans et al.,

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2012; Ewertowski et al., 2012, 2016, 2019a; Ewertowski and Tomczyk, 2015; Pleskot, 2015; Strzelecki et al., 2015, 2018; Nehyba et al., 2017) as well as glaciological observations (e.g., Rachlewicz et al., 2007; Rachlewicz, 2009; Małecki, 2013, 2016; Małecki et al., 2013) have provided an excellent background for investigating the processes responsible for ice-cored moraine degradation. In this study, we focused on the mechanism of degradation based on study sites located in the proglacial areas of Ebbabreen and Ragnarbreen (Fig. 1), whose dynamics have already been quantified using repetitive laser scanning (see Ewertowski and Tomczyk, 2015 for details). Aerial photographs from 1936 and 1938 shown that margins of both glaciers had already retreated from their maximum LIA positions (Fig. 2). Studied sites are located in areas, which were exposed from under the ice in different periods (LIA – 1936; 1936–1960; 1960–2009; after 2009); However, it has to be stressed that both moraine complexes are composed of thick ice cores covered by relatively thin veneer of debris (Ewertowski, 2014; Ewertowski and Tomczyk, 2015). 3. Methods Fieldwork in the proglacial areas of Ebbabreen and Ragnarbreen was conducted in the period 2011–2014. Geomorphological mapping involved a field-based approach combined with the interpretation of aerial photographs (1960, 1990, 2009), high-resolution satellite imagery (WorldView-2 from 2013), UAV-based images (2014), as well as digital elevation models (DEMs) and hillshade models generated from aerial and UAV images. UAV surveys were conducted in 2014 using a quadcopter (DJI Phantom) equipped with a 14MP camera. UAV images were processed through the standard structure-from-motion workflow (cf. Evans et al., 2016; Ewertowski et al., 2019b). The general distribution of mass movement sites was mapped from remote sensing data and verified via ground-truthing. The types of mass movement and the most probable cause of their activation were also recorded based on observations and terrain characteristics. Detailed terrestrial photographic monitoring of five selected sites (Fig. 2) were performed in 2012, 2013, and 2014, and included taking a series of overlapping pictures from various points alongside the studied sites. This enabled the use of the Structure-from-Motion approach to create point clouds and 3D models from ground-based images. Pictures were taken using a DSLR camera (16MP) with fixed focal length. The characteristic points were then selected from laser scans and used for georeferencing of the 3D point clouds. This ground based photography allowed for identification and observation of the most important processes responsible for transformation of the studied ice-cored moraine complexes. Description of the diamicton followed the approach proposed by Krüger and Kjaer (1999), using lithofacies code, where diamicton was marked with the D letter and following letters characterised type of the matrix and relationships between matrix and clasts (Krüger and Kjaer, 1999): 1) General appearance: m – massive; s – stratified; 2) Granulometric composition of matrix: C - coarse grained sandygravelly; M - medium-grained matrix, silt-sandy; F - fine grained, clayey-silty; 3) Clast/matrix relationship: (c) – clast supported; (m1) matrix supported, clast poor; (m2) matrix supported, clast moderate; (m3) – matrix supported, clast reach. 4. Results 4.1. Mechanisms of the delivery and emergence of the debris onto the glacier surface in sub-marginal locations Geomorphological fieldworks revealed several ways which can be responsible for the elevation of debris-rich ice layers from a basal or

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Fig. 2. Changes in the extent of the glaciers (extent of clean, non-debris-covered ice surface). Locations of the sites described in detail in the Section 4.3 are indicated by points. Locations of the photographs are shown by stars and numbers corresponding to the number of figures.

sub-basal position toward the glacier surface within the studied forelands: 1) Folding of debris-rich ice layers (Fig. 3a), probably due to compressive ice flow caused by the topographic conditions (i.e., narrowing of the valley). Such folds can reach several meters high and can contain a wide variety of debris from fine-grained material to large boulders. 2) Elevation of the debris-reach ice layer along the shear and thrust planes (Fig. 3b). Relatively thick debris-rich ice layers (up to 0.5 m) were characterised by distinct shape and large amount of subglacially derived clasts.

3) Transportation of debris along smaller ice fractures (Fig. 3c), most probably related to high-water pressure which injected debris into the pre-existing ice structures. Regardless of the mechanism, the debris-rich ice layers were elevated from the basal position. In the same time, clean ice surface downwasted until the elevated debris reach the ice surface. As soon as the debris-rich ice layer emerged onto the ice surface, the debris started protecting ice from melting, leading to differential ablation and formation of topographic highs and lows. At this point, initial mass movements started (mostly sliding and debris flowing) leading to the

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Fig. 3. Mechanisms of debris elevation in sub-marginal environment (Ebbabreen, Svalbard): (a) folding of debris-rich ice layers; (b) elevation of debris-rich ice layers along shear planes; (c) injection of the debris into pre-existing fractures in the ice.

development of the ice-cored ablation cones. During the subsequent lowering of the ice surface and increase in debris mobility due to the topography differences and abundance of the meltwater, ablation cones increases in size, become connected to each other and finally transformed into controlled ice-cored moraine (sensu Evans, 2009). 4.2. Spatial distribution of mass movement sites In 2009, frontal and lateral ice-cored landforms occupied 3.33 km2 of the forelands of Ebbabreen and Ragnarbreen. We identified 111 sites covering 0.53 km2 (16% of the total ice-cored area) where mass movement processes have recently degraded ice-cored moraine complexes (Fig. 4). Almost 70% of the total number of mass movement sites were located within an elevation range of 50 to 150 m a.s.l. (Fig. 5a). Most of the sites (90%) were observed on steep-angled slopes with gradients between 20 and 60% (Fig. 5b). More than half of the mass movement sites were located on slopes with Southeast or South aspect (Fig. 5c).

In terms of geomorphology, mass movements were observed within four main settings (Fig. 5d): 1) Along the riverbanks in places where the main drainage route breaks through the frontal moraine complex (11); 2) Near the portals of minor subaerial meltwater channels (10); 3) Along the outer edges of frontal moraine complexes (15); 4) Along the crests of lateral moraines (72), including 29 sites along the lakeside (Ragnarbreen); 5) Other positions (3). Over the subsequent four years (2009–2013), 16 new sites were activated so that in 2013, the total area with an unstable ice-cored moraine condition was 0.57 km2 (with 127 sites). The newly activated sites were observed along the lateral moraine (6), external margins of ice-cored moraine complexes (5), or near meltwater portals (4) and rivers (1). We selected five of these new sites that were representative of

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Fig. 4. Spatial distribution of debris flows in proglacial areas of (a) Ragnarbreen and (b) Ebbabreen.

different types of mass wasting processes, and then monitored their evolution over a two-year period so as to document in detail the processes responsible for ice-cored moraine degradation (Section 4.3). 4.3. Short-term evolution of mass-wasting sites Terrestrial monitoring was carried out on five sites representing different types of mass-wasting processes, thus allowing for investigating their short-term evolution, identifying the most important processes responsible for sediment redistribution, and characterising the landform components where these remobilization processes had taken place:

1) Site 1 (Fig. 6). This site was located on the distal slope of the moraine complex. This part of the snout was covered by debris between 1960 and 1990 as a result of lowering of the ice surface and emerging of the debris concentrated in debris-rich ice layers. The main activation mechanism was probably undercutting of the slopes by meltwater (traces of this drainage route are visible in the field and in the aerial photographs) leading to development of mass movement processes, debris removal and exposure of the buried ice. Aerial photographs and field-investigations indicated the existence of an older (pre2009) mass movement site which was subsequently stabilized, potentially because the re-routing of the drainage. In 2012, the site was reactivated by three smaller debris flows, which remobilized

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Fig. 5. Relationship between frequency of mass movements and: (a) elevation; (b) slope gradient; (c) aspect; (d) geomorphology.

older debris flow deposits. The headwall expanded upslope as well as laterally. At the base of the slope, small lakes were periodically formed and drained, with drainage routed under the moraine, which suggests the presence of dead ice under the deposits. The water level in the lakes fluctuated considerably (up to 2 m) over the years as well as during a single season, which also can potentially influence slope destabilization. The mechanism of debris transfer was complex: a. The initial debris flows (which can potentially be related to the fluctuation of the water in the ponds; however there are no direct observations supporting this fact) developed and created diamictic cliff and exposed ice surface. b. Further movement of debris was related to the degradation of a diamictic cliff – debris was falling down as single, usually large, clasts, as well as conglomerates of diamicton. The cohesive nature of the parent diamicton facilitated block falling. c. After falling, clasts and blocks were temporary stored in the icefloored niche at the bottom of the cliff. d. This temporary debris store was subsequently saturated by water melting from the ice-core. After the amount of water was enough to cause rapid rise in pore-water pressure, reducing shearing resistance to below the factor of safety for material failure, the subsequent flowage of sediments downslope happend. At the foot of the slope, where the topography became flatter, flowing debris was partly sorted: larger clasts were deposited due to the loss of flow competence, whereas a finer fraction (sands and fines) developed into a small alluvial fan protruding into the lake. 2) Site 2 (Fig. 7). The topographic location and chronology was similar to Site 1 – a steep slope covered by diamicton created by previous

mass-wasting processes. The occurrence of former mass-wasting processes is supported by morphological evidences (old, in-active debris flow lobes) visible in the archival aerial photographs. The mechanism of mass-wasting re-initialisation was, however, slightly different – the water flowing from the Site 1 undercut the slope and subsequently flowed under the moraine, with two consequences: (1) development of an overhanging ice wall (including a debris-rich ice layer); and (2) constant removal of debris by meltwater. As a result, the headwall expanded up-slope as well as laterally, but a niche for the mixing of debris was not created, and so all particles either slid or rolled down over the steep ice surface, and were subsequently dumped into the lake or at the base of the overhanging ice. In addition, the clasts that melted from the debris-rich ice layer were dumped as well. As a result, piles of partly sorted debris (i.e. gravels and boulders) developed at the foot of the slope, whereas the finer particles were removed by meltwater. As the water level fluctuated, the fine sediments were washed into these sorted clasts, creating a diamicton containing subglacial clasts and former debris flow deposits. 3) Site 3 (Fig. 8). This part of the moraine was formed between the end of the LIA and 1960. In 2005, 2007 and 2009 there was no meltwater flow here and this part of the moraine was stable. At some point between 2009 and 2012 one of the meltwater drainages changed it course and routed under the moraine developing the meltwater portal and mobilizing debris. The debris involved in the mass movement processes was originally related to the medial moraine and differs markedly from that in Sites 1 and 2 – it consisted mostly of gravitationally derived angular clasts of a single lithology (sandstones). When the portal was created

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Fig. 6. Geomorphology and evolution of Site 1. The site is 25 m high, with average slope of 23°. Arrows in the left column indicate approximate direction of photographs.

(after 2009), meltwater undercut the slope, removing debris and leading to ice-core exposure. However, as debris was loose (not consolidated as in sites 1 and 2), no vertical sediment cliff was created. Instead, particles slid and rolled down over the steep ice wall before being deposited in the streams. The meltwater removed all fine material, therefore no classic diamicton was formed, except for very poorly sorted gravel. 4) Site 4 (Fig. 9). This site was located in the area covered by debris between 1990 and 2009. Mass movements were activated due to the

undercutting of the slope by meltwater flowing in an open, icewalled channel. Debris removal caused exposure of buried ice, which subsequently started melting leading to increase in porewater pressure, slope instability and development of mass movements. The parent material was a matrix-supported diamicton, which formed a vertical, arcuate headwall. Contrary to Site 1, no niche was formed; rather, a debris flow with single channel was activated. Part of the debris was deposited on the flatter sections, creating debris-flow lobes with diamicton. Some of the more extensive

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Fig. 7. Geomorphology and evolution of Site 2. The site is 20 m high, with average slope of 23°. Arrows in the left column indicate approximate direction of photographs.

debris flows were deposited into the river, and debris was sorted and transported further downstream. 5) Site 5 (Fig. 10). Site 5 was located close to the lateral edge of the moraine, covered by debris between LIA and 1960. The debris cover was relatively thick – slightly N2 m. The parent material was a layered, matrix-supported diamicton with a small amount of clasts interpreted as a former debris-flow deposits. In contrast to the previously described sites, Site 5 was relatively flat and stable until 2007. In 2007, the meltwater flowing from the small side valley went through

the fragment of the moraine and started melting buried dead ice by means of thermo-erosion processes. Consequently, despite a lack of favourable topography, the mass-wasting processes started operating. A relatively low amount of water and fine-grained nature of diamicton led to the development of slowly moving, dense-debris flows (type I according to Lawson, 1979). The diamicton was partly mixed during the debris flow; however, it retained much of the parent material's characteristic, such that the final product of mass wasting at this site was a matrix-supported diamicton.

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Fig. 8. Geomorphology and evolution of Site 3. The site is 21 m high, with average slope of 29°. Arrows in the left column indicate approximate direction of photographs.

4.4. Dominant process-form characteristics of mass-wasting sites Mass-wasting sites were characterised by several dominant morphological elements (only some of the sites contained all elements), where the above-mentioned processes operated: 1) Vertical or near vertical headwall (Fig. 11a), cut into sediments (usually a matrix-supported diamicton). The main processes were the falling of clasts and whole blocks of diamicton. 2) Near vertical ice-cliff (Fig. 11a) – cliff locations were transformed mainly due to dead-ice backwasting. Sediments usually slid over a

steep-ice surface, or if the slope was gentler, rolled down. Ice cliffs were observed in the upper part of sites (where they were overtopped by diamicton) or in the bottom part, where ice cliffs were often undercut by meltwater and formed overhanging segments. 3) Transit, ice-floored niche (Fig. 11b) – temporary stabilization and accumulation of debris occurred here. Single clasts as well as blocks of diamicton were stored until water from the melting ice cores caused a saturation of the debris and subsequent debris flow initiation.

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Fig. 9. Geomorphology and evolution of Site 4. The site is 9 m high, with average slope of 16°. Arrows in the left column indicate approximate direction of photographs.

4) Steep ice slopes topped with a thin layer of supraglacial sediments (Fig. 11c) (i.e. clast-supported diamicton or openwork unsorted angular clasts) were dominated by debris slides. 5) Gentle slopes mantled with water-saturated sediments (Fig. 11d), transformed mainly by debris flows. Debris flow channels were often visible. Slopewashes with fine particles occurred almost constantly during summer, whereas larger debris flows transporting larger clasts were observed on a regular, cyclic basis – usually several times per day, with the trigger being saturation of the debris and reduction in pore water pressure. When

the debris flow dewatered and stopped, transported clasts formed a plug in the channel, causing subsequent debris flows to develop new side channels. 6) A small fan (Fig. 11e) (sometimes deltas protruding into ponds) developed at the bottom of the slope. Partly sorted debris accumulated on the surfaces of the fan, whereas water and finer particles were transported further down or stored in the pond. 7) Debris flow lobes (Fig. 11f) – in the case of some debris flow, fine particles formed a broad lobe containing only sands and fine material.

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Fig. 10. Geomorphology and evolution of Site 5. The site is 9 m high, with average slope of 7°. Red arrows in the left column indicate approximate direction of photographs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

8) Non-active debris-mantled areas that were stable or transformed by dead-ice downwasting alone. Sediments within ice-cored moraines were observed as surface deposits as well as debris bands incorporated into the ice. The main lithofacies observed within the ice-cored moraines were: 1) DmM(m2) – Matrix-supported diamicton, usually with a moderate amount of clasts and sand or fine matrix (Fig. 12a). This lithofacies was probably related to the subglacial transport (as indicated by

subangular shape) and subsequent transformation by masswasting processes. 2) DmF(m1) - Matrix-supported diamicton with a low amount of clasts and fine-grained matrix (Fig. 12b). This lithofacies was probably the effect of cohesive debris flows (suggested by presence of fine-grained matrix), which were observed in relatively flat areas. 3) Supraglacial deposits related to medial moraine were openwork, angular, usually single lithology gravels (Fig. 12c), or, a more sporadic

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Fig. 11. Morphological elements of mass wasting sites: (a) headwalls; (b) transit ice-floored niche; (c) ice slopes; (d) debris-mantled slopes; (e) small alluvial fan; (f) Debris flows with channels and depositional lobes. Further explanations in the text.

clast-supported diamicton with sand matrix – DmM(c). Angularity was characteristic of these deposits. 4) Openwork gravels (sometimes partly sorted), usually found at the bottom of the slopes as dumped material (Fig. 12d). 5) Sorted fines and sands related to the deposition from low competence debris flows, usually forming a fan at the bottom of the slope or as a debris flow lobe (Fig. 12e).

6) Debris-rich ice layers containing a large amount of subglacially transported clasts (Figs. 12f, g). 7) Debris-rich ice layers with a low number of clasts but with a relatively high concentration of fine-grained particles (Fig. 12h). Based on a time series of detailed field-based observations (2011– 2014), four processes seem to be the most important for the

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Fig. 12. Examples of sedimentary lithofacies: (a) matrix-supported diamiction with moderate amount of clasts; (b) matrix-supported diamicton with low amount of clasts; note: the shovel in the picture is about 1.2 m long; (c) openwork, angular, supraglacial deposits; note: each grid cell is 0.1 × 0.1 m; (d) openwork gravels; (e) fine-grained sediments; (f, g) debris-rich basal ice layers usually found in the lower sections of mass-wasting sites; (h) debris-rich ice layers. Further explanations in the text.

redistribution of debris within the studied ice-cored moraine complexes (Table 1): 1) Debris fall – falling of individual clasts or blocks from vertical walls without continuous contact with ice or sediment surface. Debris falls were active only in the steepest parts of the studied sites (angle N60°), including overhanging ice sections. 2) Debris slide – sliding of clasts over the exposed ice surface without changing its sliding sides. This process required exposed ice surfaces and was observed on different angles of slopes from 20 to 60°.

3) Debris roll – rolling of clasts down over steep sections of a slope (both debris-covered and with an exposed ice core). The rolling of debris was observed on slopes with angles from 20 to 55° 4) Debris flow – flowing of partly saturated mass of debris, usually over exposed ice cores and less often over sediment-mantled slopes. Debris flows were observed on slopes with angles from 6 to 35°. In addition to the processes mentioned above, two other processes (fluvial and aeolian activity) were recorded as being responsible mainly for the evacuation of sediments to more distal foreland (i.e. outside the

M.W. Ewertowski, A.M. Tomczyk / Geomorphology 350 (2020) 106952 Table 1 Main processes responsible for sediment redistribution within ice-cored moraine complex. Efficiency was estimated based on the observed volume of sediments involved in movements. Slope angle was estimated from DEMs. Process type

Type of surface

Angle of surface

Efficiency

Debris fall Debris slide Debris roll

Near-vertical cliffs Exposed ice surface Exposed ice surface or sediment mantled slope Exposed ice surface or sediment mantled slope

N60° 20–60° 20–55°

Medium Low Low

6–35°

High

Debris flow

LIA moraine ridge) and in consequence to the fjord. Therefore, in most cases fluvial activity was related to the removal of sediments outside the moraine complex, whereas mass-movement processes were responsible for the redistribution of sediments within the moraine complex. In some cases, ponds and lakes acted as sedimentary traps, temporarily increasing storage of sediments within the ice-cored moraine complexes. 5. Discussion 5.1. General distribution of active mass-wasting sites The lower number of sites located on higher elevation (Fig. 5a) might be related to the variation in thickness of the permafrost active layer. In general, the active layer thickness decreases as the elevation and distance from the coast increase, i.e. from 1.2 m near the coastline, through 0.5– 2.5 m variation of active layer thickness inland, to lack of active layers in the mountains (Gibas et al., 2005; Rachlewicz and Szczuciński, 2008). The shallow active layer limits the amount of material available to transport by the mass movement processes. It also limits the amount of meltwater, making it harder for mass-wasting processes to operate. Most of the active sites were on slopes with moderate average gradients between 21 and 70% (12–35°). Gentler slopes were not conducive to the development of mass-wasting processes as there is not enough kinematic energy to initiate gravitational movements. On the other hand, slopes steeper than 70% (35°) are too steep to develop debris flows, and in such cases steep, sometimes near-vertical, ice cliffs are usually exposed, thereby limiting the types of mass-wasting processes to falling and sliding. In the studied forelands, as well as in many other examples from Svalbard (Lukas et al., 2005; Schomacker and Kjær, 2008; Midgley et al., 2013, 2018; Ewertowski, 2014; Ewertowski et al., 2016, 2019a; Tonkin et al., 2016), frontal (end) fragments of moraine complexes tend to be more stable than the lateral ones. This is confirmed by the distribution of active sites within the studied forelands - they concentrate along the lateral moraines (Fig. 5d), as this part of the landscape was relatively recently formed and slopes are still unstable, whereas frontal moraine complexes had been usually formed earlier, and thus become more stable. Most of the reaming mass-wasting sites are related to the fringes of the glacial landsystem (i.e. outermost fragments of moraines) and places where the meltwater operates as streams or meltwater portals (Fig. 5d). 5.2. Types and characteristics of mass-movement sites We identified four main types of mass-movement sites based on the dominant processes and topographic characteristics: 1) Type A – water-saturated debris flow. Type A comprised sites characterised by a medium slope, usually between 10 and 35°. They typically consisted of a vertical headwall built of sediments which overtopped the dead ice exposure (steep or near-vertical ice cliff). A small niche developed in the upper part of the slope, where sediments had been mixed together. Further downslope, debris-flow channels with levées were found; sometimes, the

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bottom of the channel was ice-floored. At the foot of the debris flow, small fans developed, containing mostly fine-grained sediments. This type of flow corresponds with type IV according to Lawson (1979) classification, i.e. flows characterised by channelized morphology, high bulk water content and high surface flow rates controlled aby the amount of water within sediments. 2) Type B – steep ice cliffs topped with a thin debris layer (b1 m). Due to the steep topography (N32°), most of the clasts went downward by sliding, therefore no classical debris flow evolved. Fine particles flowed down, while coarser clasts slid or rolled down. At the base of the slope, small piles with clasts were formed. 3) Type C – low-water content debris flows. Developed on relatively low angled surfaces. The headwall consisted of sediments (up to 2 m thick), and dead ice was often visible at the bottom of the headwall. Debris flows were of type I or II according to Lawson (1979), i.e. were characterised by lobate, non-channelized morphology with marginal ridges (Lawson's type I) or lobate to channelized morphology (Lawsons's type II) and small bulk water content. Displacement of debris was slow, and the main mechanisms of clast support was gross strength. 4) Type D – classic debris flows characterised by the occurrence of head scarp, debris flow channel, and depositional lobe. In many cases, deposition was in a stream; therefore, material forming the lob was subsequently removed by the meltwater. This type of debris flow corresponds with Type III according Lawson (1979), i.e. with clearly channelized morphology and water content up to 25%. 5.3. Factors affecting the development of a specific type of mass-movement The most important factors that controlled the type of mass movement were: 1) Slope gradient – the slope controlled the main mechanism of particle movement with a steep gradient favouring debris fall, slide and roll, while more gentle slopes enhance debris flow. 2) Exposure of dead ice and the availability of meltwater– when the ratio of the available meltwater and debris was low, highly viscous and slow debris flow occurred (debris flow I and II – Lawson, 1979). Increasing the amount of meltwater leads to the creation of channelized debris flows with the characteristic geomorphology including channels, levees, and depositional lobes. 3) Type of debris – the character of the material governs the primary phase of mass movement and influences the morphology of masswasting sites. Where a fine-grained, matrix-supported diamicton was available, degradation occurred mostly as whole blocks – cracks developed and whole blocks fell. In contrast, if mostly coarsegrained clasts with a low amount of fine-grained matrix were observed (mostly related to supraglacial sediments, e.g., medial moraines), single particles were falling and rolling down, behaving more like landslides or small rockfalls rather than debris flows. 5.4. Evolution of debris flows In most cases, degradation of the moraine slopes propagated from the bottom, in the upslope direction. The starting point for mass wasting processes was a slope created by previous direct glacial processes (in case of initiation of paraglacial processes), or by previous masswasting processes (in case of rejuvenation of previously modified surfaces). Destabilization of the slope was in most cases related to external factors such as meltwater streams undercutting the slope (which was directly observed in our study area) or excessive rainfalls causing an increase in the rate of dead-ice melting and mass movement activities on slopes in general as reported from other areas in Svalbard (see de Haas et al., 2015). In most cases, slope tension cracks started developing in more cohesive sediments when the meltwater undercut them, and whole blocks of diamicton collapsed, causing the exposure of the ice

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core and the development of a steep, almost vertical, sediment cliff (usually 1–2 m high). Where only loose material was available at the moraine, single clasts tend to roll and slide down, with no significant cliff being created. In some cases, a niche was created where debris was temporarily stored. Debris was stored in the niche until the amount of meltwater was sufficient to develop a debris flow, which subsequently formed a debris-flow channel and levées.

debris flows have again enriched them. Therefore, a diamicton can be the effect of both a single-phase and multi-phase transport. However, usually this cannot be explained by the sedimentary characteristics (which look similar) but rather by direct field-based investigations. Through the direct observations of the same sites over three field seasons our research confirms that occurence of successive cycles of sediment gravity flows is a common situation (Lawson, 1979, 1982; Lønne and Lyså, 2005; Lukas et al., 2005).

5.5. Formation of redeposited diamicton Based on our observations, in most cases the product of masswasting processes was diamicton. This redeposited diamicton is a “transition” product – in many cases it will undergo further transformations by subsequent generation of mass wasting processes or fluvial activity. However, in some situations (e.g., isolation of the moraine from active meltwaters) such diamicton can survive which can be confirmed by occurrence of glaciogenic debris flow records in the Pleistocene glacial environments (e.g., Zielinski and van Loon, 1996; Pisarska-Jamroży, 2006; Benn and Evans, 2010). The redeposited diamicton can be produced in two different ways. As a classic product of debris flow, where only one transport phase was observed, the debris flowed as one mass of water and sediments. The second way was a two-phase process (Fig. 13): at first, debris was sorted by gravitational processes and meltwater. The larger clasts fell down, creating a dumped pile of sediments – poorly sorted, openwork gravels (Fig. 13b, c). With subsequent increase in meltwater volume, the fine-grained sediments can be washed into (Fig. 13d, e) this openwork deposit, creating a clast-rich diamicton. Alternatively, water had depleted the debris-flow sediments of fine material, but subsequent

5.6. A simplified model of paraglacial degradation of ice-cored moraine complexes Paraglacial processes are known to operate over various temporal and spatial scales, and there are several models describing temporal changes in their activity (Church and Ryder, 1972; Church and Slaymaker, 1989; Harbor and Warburton, 1993; Ballantyne, 2002a, 2002b, 2005). However, still unknown is the detailed relationship between time passed since deglaciation and the volume of available sediment, as well as the occurrence and quantification of time-lag between deglaciation and the start of a paraglacial activity, as such are hard to quantify based on available datasets. Recently, Knight and Harrison (2018) suggested that paraglacial systems experience periods of dynamic behaviour interlaced with periods of temporary stabilization. Such model of transience in paraglacial systems is more complicated than traditional exhaustion models (Church and Ryder, 1972; Church and Slaymaker, 1989; Ballantyne, 2002b) and their further modifications (Ballantyne, 2003; Cossart and Fort, 2008; Etienne et al., 2008; Mercier, 2008).

Fig. 13. An example of formation of redeposited diamicton as a two-phase process: (b, c) open-work gravels are formed as a result of sediments falling down from the ice cliff; (d, e) subsequent slopewash enriched gravels with finer fractions.

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Based on the existing paraglacial models we proposed a simplified model, which can explain the development of ice-cored moraines within our studied forelands as one of the examples of paraglacial degradation of landscape (Fig. 14): 1) Stage I – Creation. In stage I, debris-rich ice layers lead to differential melting of the ice surfaces (Fig. 3). In the high-Arctic condition, a thin veneer of debris is enough to protect ice from melting, resulting in the formation of ablation cones and finally in the development of ice-cored ridges. 2) Stage II – Initial adjustment to topography. Shortly after the retreat of the clean ice margins, the overall steepness of the topography, the abundance of debris distributed as a generally thin veneer, and the presence of meltwater related to the melting of exposed ice-

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cores all caused an intense activity in the reworking processes. This is interpreted based on a large amount of older debris-flow deposits (cf. Ewertowski et al., 2012). 3) Stage III – Temporary stabilization (equilibrium with current climatic conditions). This stage is representative of most current proglacial areas on Svalbard. Investigated ice-cored moraines are in general stable with the current environmental conditions and have not shown any significant changes as a whole complex over the last 50 years (Ewertowski, 2014; Ewertowski et al., 2019a). However, there are several situations wherein some parts of the icecored moraines temporarily switch from stable to active conditions: a. Increase in meltwater activity flowing from the side valleys. b. Exposure of ice cores due to undercutting by flowing water.

Fig. 14. Simplified model of ice-cored moraine formation and degradation. Further explanations in the text.

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c. Steep topography, usually around the margin of moraine complexes or within segments where larger streams cut through the moraine complex. d. Undercutting by the streams and rivers routed through or along the margins of the ice-cored moraine complex. 4) Stage IV – Active degradation by mass-wasting processes. In the cases described above, mass-wasting processes will lead to serious degradation of the moraines' active fragments. The rate of degradation varies but can be as high as 1.8 m/a of elevation lowering (Ewertowski and Tomczyk, 2015). Active degradation is an iterative stage which may again lead to Stage III (temporary stabilization) or State V (full depletion). 5) Stage V – Full depletion of sediment storage. After several cycles of mass-wasting degradation, any dead-ice in a given fragment of the moraine will be depleted. As a result, it will lead to the stabilization of landscape units unless the meltwater will not remove the remaining sediments, i.e. unless change in a drainage pattern reroute the meltwater through the moraine – see an example from Hørbyebreen (Ewertowski et al., 2019a). However, it is not possible to assess the time period necessary to achieve full depletion of the system, as the observed forelands still responds to the glaciers' recession from their LIA maximum. Moreover, at least part of the sediment storage transported by LIA glaciers originated from older glaciations, therefore, LIA can be seen as “rejuvenation” of older paraglacial conditions. In terms of Pleistocene record the final landscape component of a relict ice-cored moraines will therefore most likely be broad belts of low-relief hummocks (Evans, 2009). However, even such landscape might be considered as unstable in some certain condition (i.e. not “fully”-depleted), for example in case of major rainstorms, which potentially can mobilize the sediments again. Forelands of the investigated glaciers represent a spatio-temporal continuity of the stages I-IV of the model proposed above; however in this study we focussed on the transformations from the Stage III (Temporary Stabilization) to Stage IV (Active degradation by masswasting processes), i.e. on transition from temporarily stable to unstable conditions (sensu Lukas et al., 2014). In the modern Arctic environment, areas which can be treated as depleted (or at least stable in longer temporal scales) are still uncommon, as the last glacial disturbance of the system (i.e. LIA) terminated only 120 years ago. Therefore, in terms of general paraglacial models (Church and Ryder, 1972; Church and Slaymaker, 1989; Ballantyne, 2002b) investigated sites represent an early stage of sediment exhaustion cascade (Mercier, 2008), illustrating transience of paraglacial systems (sensu Knight and Harrison, 2018). As we demonstrated here and in other studies about the glaciers in the Petuniabukta (Ewertowski and Tomczyk, 2015; Ewertowski et al., 2019a), gravitational mass movements (and debris flows in particular) were the most important geomorphological agents for the redistribution of sediments during this early stages of paraglacial topography adjustment. These observations confirms results of earlier studies from Arctic regions (e.g., Lawson, 1979, 1982; Bennett et al., 2000; Sletten et al., 2001; Lønne and Lyså, 2005; Lukas et al., 2005; Schomacker and Kjær, 2008; Pleskot, 2015), and high mountain settings (Curry et al., 2009a; Kirkbride and Deline, 2018; Ravanel et al., 2018). It is probable, that as switches between Stages III and IV continue in the future, other processes will become more dominant, for example paraglacial gullying of sediments described for several sites in Norway which were deglaciated c. 9 ka BP (Curry, 1999; Curry and Ballantyne, 1999), but also for post-‘Little-Ice-Age’ paraglacial reworking (Curry et al., 2006, 2009b; Mercier et al., 2009). Another aspect, which will probably influence the degradation of ice-cored moraine in the future, will be stabilization of the moraine surfaces by “engineering” species of plants (Draebing and Eichel, 2018; Eichel et al., 2018).

5.7. Implications for high-Arctic paraglacial landform dynamics and landscape evolution Occurrence of ice-cores in landforms has an important implication for future evolution of formerly glaciated landscapes: 1) On one end of the spectrum, there are many ice-cored moraine complexes in the high-Arctic environment of Svalbard, which remain stable landscape components when a temporal scale of decades is considered (Ewertowski, 2014; Ewertowski and Tomczyk, 2015; Tonkin et al., 2016; Midgley et al., 2018; Ewertowski et al., 2019a). Such “temporarily stable” conditions occupied N80% of the moraine complexes of the studied glacial forelands (Section 4.2; Fig. 4). However, they still comprise large amount of dead ice, and is very unlikely, that they represent the final geomorphological record (Midgley et al., 2013). Further transformation by paraglacial processes (especially gravitational mass movements) and topographic inversion will likely transform their future morphological expression to belts of low-relief hummocks, i.e. controlled moraines (sensu Evans, 2009); however, formation of fully stabilized and only partially de-iced moraine, which is in equilibrium with its environment, was also suggested as the possible “end point” in the high-Arctic landscape evolution (Tonkin et al., 2016). 2) When ice-to-sediment ratio is relatively low (b40%), the preservation potential of such landforms is higher. For example, for the outer-frontal part of Midtre Lovénbreen moraine and for the part of the moraine complex at Austre Lovénbreen which contains smaller volume of ice (20–40%), only minor topographical modifications as a result of complete de-icing were inferred (Midgley et al., 2013, 2018). 3) On the other end of the spectrum, there are moraines built without dead-ice component, characterised by high preservation potential. Such examples were described from Alpine settings, where prominent lateral moraines were produced mostly of sediments and the paraglacial modification will led only to moderate lowering of crests and production of more symmetrical crossprofiles (Lukas et al., 2012). It is therefore a clear relationship between the amount of dead-ice within the landforms and their preservation potential; which is especially important in the context of dating efforts of older glacial moraines – interpretation of the obtained ages has to consider the initial character of the moraines, as the volume of dead-ice (if any) can seriously change understanding of the resultant age. 6. Conclusions We reported observations of degradation of ice-cored moraines in front of two polythermal glaciers, Ebbabreen and Ragnarbreen on Svalbard. The most important findings include the following: 1) In 2009, frontal and lateral ice-cored landforms occupied 3.33 km2 of the forelands of Ebbabreen and Ragnarbreen, of which 0.53 km2 (16% of the total ice-cored area) was actively degraded by mass-wasting processes. Over the period 2009–2013, the total area with an unstable ice-cored moraine condition increased to 0.57 km2. 2) Debris falls, slides, rolls, and flows were the most important processes behind the reworking of glacigenic sediments and the altering of their properties. The relative contribution of each process to overall landform dynamics was controlled by local topographic conditions and the character of parent material. 3) Four different morphological site types were identified: (1) nearvertical ice cliffs covered with debris, and transformed mainly by dead-ice backwasting as well as debris falls and slides; (2) steep debris slopes with exposed ice cores dominated by debris slides; (3) gentle sediment-mantled slopes transformed by debris flows;

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and (4) non-active debris-mantled areas transformed by dead-ice downwasting alone. 4) The spatial and temporal re-activation of moraines' degradation was for the most part related to local geomorphic conditions (e.g., slope gradient, occurrence of streams, and meltwater channels). 5) Gravitational mass-movements were responsible for the redistribution of debris within the moraine-complex, whereas fluvial transport by meltwater exported sediments outside Little Ice Age moraines. In some places, small ponds have created a temporary local storage for the sediments. The continuous recession of Svalbard's glaciers after termination of the LIA has exposed proglacial areas containing a large amount of dead ice covered by relatively thin - usually less than a couple of metres - veneer of debris. Large areas of these ice-cored moraines are in dynamic equilibrium with current topographic conditions. However, in places where unstable conditions occur where key thresholds of slope stability are breached by intermittent exposure of dead ice cores triggered by external factors (fluvial undercutting and extreme rainstorm events, etc.), the landscape can be very dynamic, mainly due to the mass-movement processes and dead-ice melting. The continuous redistribution of sediments causes several phases of debris transfer and relief inversion. Hence, the primary glacial deposits released from ice are subsequently transferred by the mass-movement processes until they finally reach a more stable position. Funding This research was funded by the National Science Centre, Poland, Grant Number DEC-2011/01/D/ST10/06494. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Fieldwork would not be possible without the logistic support provided by AMUPS (Adam Mickiewicz University Polar Station). We are very grateful to the reviewers and the editor, Achim A. Beylich, for their constructive comments. This research was funded by the National Science Centre, Poland, Grant Number DEC-2011/01/D/ST10/06494. References Ballantyne, C.K., 2002a. Paraglacial geomorphology. Quaternary Sci Rev 21 (18), 1935–2017. Ballantyne, C.K., 2002b. A general model of paraglacial landscape response. The Holocene 12 (3), 371–376. Ballantyne, C.K., 2003. Paraglacial landform succession and sediment storage in deglaciated mountain valleys: theory and approaches to calibration. Z Geomorphol Supp. Bd. 132, 1–18. Ballantyne, C.K., 2005. Paraglacial Landsystems. In: Evans, D.J.A. (Ed.), Glacial Landsystems. Arnold, London, pp. 432–461. Barr, I.D., Lovell, H., 2014. A review of topographic controls on moraine distribution. Geomorphology 226 (Supplement C), 44–64. Bednorz, E., Jakielczyk, M., 2014. Cyrkulacyjne warunki występowania ekstremalnych opadów atmosferycznych na Spitsbergenie. Badania Fizjograficzne, Seria A – Geografia Fizyczna 65 (39–53). Benn, D.I., Evans, D.J.A., 2010. Glaciers and Glaciation. Hodder Education, London. Bennett, G.L., Evans, D.J.A., 2012. Glacier retreat and landform production on an overdeepened glacier foreland: the debris-charged glacial landsystem at Kvíárjökull, Iceland. Earth Surf Proc Land 37 (15), 1584–1602. Bennett, M.R., Huddart, D., Hambrey, M.J., Ghienne, J.F., 1996. Moraine development at the high-arctic valley glacier Pedersenbreen, Svalbard. Geografiska Annaler: Series a. Phys. Geogr. 78A (4), 209–222. Bennett, M.R., Huddart, D., Glasser, N.F., Hambrey, M.J., 2000. Resedimentation of debris on an ice-cored lateral moraine in the high-Arctic (Kongsvegen, Svalbard). Geomorphology 35 (1–2), 21–40.

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