The role of topography in landform development at an active temperate glacier in Arctic Norway

Topography exerts a strong control on how glaciers respond to changes in climate. Increased understanding of this role is important for both refining model predictions of future rates of glacier recession and for reconstructing climatic change from the glacial geological record. In this paper, we examine the geomorphological and sedimentological evidence in the foreland of Fingerbreen, a temperate outlet of the plateau icefield Østre Svartisen. The aim is to investigate the relationship between processes of landform generation and the changing influence of topography as recession progressed. The Fingerbreen foreland is dominated by bouldery Little Ice Age moraines and extensive areas of striated bedrock. A heavily fluted zone occurs in the central part of the foreland that is cross‐cut by annual transverse and sawtooth moraines. Systematic investigations of the structural architecture of moraines at various locations in the foreland provide evidence for a range of moraine‐forming processes, which can be linked to the topographic setting (e.g. deposition on a reverse bedrock slope) and drainage conditions. This includes push and bulldozing of proglacial sediments and squeezing of sub‐glacial sediments and submarginal freeze‐on of sediment slabs. We also identify variations in moraine spacing as a result of topography. This research demonstrates the importance of topography when interpreting moraine records in the context of climate and glacier dynamics.


| INTRODUCTION
Mountain glaciers are sensitive indicators of climate change (Haeberli et al., 2007), and retreat and thinning of these glaciers has accelerated in recent decades (e.g. Carrivick et al., 2019;Hugonnet et al., 2021;Stokes et al., 2018;Zemp et al., 2015). It is predicted that many mountain glaciers will have disappeared entirely by the end of the 21st Century (Rounce et al., 2023;UNESCO & IUCN, 2022;Zemp et al., 2019). These glaciers currently make a substantial contribution to sea-level rise and, in many mountain regions, make crucial contributions to downstream water supply (Immerzeel et al., 2020). They also provide ecosystem services and are important to many economies for tourism (Antoniazza & Lane, 2021;Beniston et al., 2018).
During their recession, mountain glaciers leave a wealth of geomorphological and sedimentological data that document their dynamics and response to climate change (e.g. Bennett, 2001;Evans, 2003;Evans et al., 2019;Le Heron et al., 2022;Weber et al., 2019). These terrestrial archives can record decadal to sub-annual changes in glacier-and climate-related dynamics (e.g. Chandler et al., 2016;Lukas, 2012;Reinardy et al., 2013). Icemarginal moraines are particularly important archives because they mark the position of a former glacier margin. This has allowed moraines to be used to reconstruct the extent and age of palaeoglaciers and to estimate past climate (e.g. Boston et al., 2015).
However, many studies have demonstrated the role that topography plays in moderating the response of glaciers to climate change (e.g. Barr & Lovell, 2014;Boston & Lukas, 2019;Davies et al., 2022;Magrani et al., 2021;Oerlemans, 2012;Pedersen & Egholm, 2013), which may have implications for using moraines to reconstruct palaeoglaciers (Rowan et al., 2022). Processes of glacial landform development are thus affected by the interplay of glaciological, climatic and topographic factors. Documenting this geomorphological imprint, and investigating the associated sub-glacial and ice-marginal processes responsible, offers a detailed insight into changes in individual glacier dynamics over time (e.g. Chandler et al., 2016;Chandler, Lukas, & Boston, 2020;Evans et al., 2019;Hiemstra et al., 2022;Lukas, 2012;Weber et al., 2019), which can help to understand the role of topography in glacial landform generation.
At temperate non-surging glaciers, ice-marginal moraines can be constructed by seasonal cycles of submarginal squeezing and bulldozing, where water-saturated sub-glacial sediment is squeezed from beneath the margin (in the summer) and subsequently bulldozed during minor winter re-advances (e.g. Chandler et al., 2016;Price, 1970;Sharp, 1984). Bulldozing of previously deposited glaciogenic sediments can also occur (e.g. Chandler et al., 2016;Lukas, 2012;Wyshnytzky et al., 2020). Moraines can also form as ice-contact fans (e.g. Krzyszkowski & Zieli nski, 2002;Lukas, 2005Lukas, , 2012, where sediment is delivered from the ice surface to the ice margin through debris flow and fluvial processes, rather than via a sub-glacial mechanism. Some temperate glaciers are known to have cold-based glacier margins or marginal zones where the margin is sufficiently thin enough to allow the winter freezing front to penetrate beneath the glacier (e.g. Reinardy et al., 2019). This thermal regime can provide favourable conditions for the freeze-on of submarginal sediment slabs/layers to the glacier undersole during winter. Subsequent transport and meltout of these sediment slabs can lead to moraine formation (Evans & Hiemstra, 2005;Krüger, 1994Krüger, , 1995Krüger, , 1996Matthews et al., 1995;Reinardy et al., 2013). The process of slab freeze-on may be further facilitated by the presence of saturated sediments around and beneath the glacier margin prior to freezing (Chandler et al., 2016).
Topography not only controls larger-scale patterns of moraine spacing (e.g. Barr & Lovell, 2014) but it also influences the shape of the ice margin  and foreland drainage conditions (Marren & Toomath, 2014), which in turn affects moraine-forming processes. For example, at active temperate glaciers in Iceland, topographically controlled changes in (a) glacier snout morphology and structural architecture and (b) submarginal to ice-marginal drainage conditions have been argued to be key controls on processes of moraine formation (e.g. Chandler et al., 2016;Evans et al., 2016Evans et al., , 2017Evans et al., , 2018Evans et al., , 2019. Specifically, glacier thinning and snout recession into overdeepenings have resulted in heavily crevassed glacier margins and a switch to constrained, poorly drained submarginal and ice-marginal conditions. Such conditions are favourable for ice-marginal squeezing of sub-glacial tills through radial marginal crevasses (pecten), resulting in the formation of sawtooth and hairpin-shaped moraines during the past 30 years (e.g. Evans et al., 2016;Everest et al., 2017;J onsson et al., 2016).
In this paper, we focus on the Norwegian glacier Fingerbreen, which is an outlet of the plateau icefield Østre Svartisen (Figure 1).
Norwegian glaciers lost 10% of their area between the 1960s and 2010s . Many of the icefields are dominated by low-slope accumulation areas and so are considered particularly vulnerable to the current warming trends (Giesen & Oerlemans, 2010;Stokes et al., 2018;Zemp et al., 2019). Indeed, Østre Svartisen lost the largest area (55 km 2 ) between the 1999-2006 and 2018-2019 Norwegian glacier inventories (Andreassen et al., 2022). Few studies have investigated the relationship between changes in glacier dynamics, topography and processes of ice-marginal landform generation at Norwegian glacier margins, particularly since the 2000s when glacier recession rates have accelerated (e.g. Hiemstra et al., 2015;Weber et al., 2019;Winkler & Matthews, 2010). The aim of this research is therefore to investigate processes of landform generation at Fingerbreen and examine how topography influences the glacier's response to changing climatic conditions.

| STUDY AREA AND PREVIOUS RESEARCH
Fingerbreen is a major outlet glacier of the Østre Svartisen plateau icefield, which is located just inside the Arctic Circle (66 N) in Norway ( Figure 1). Glaciers in the region are considered temperate (Paul & Andreassen, 2009). Fingerbreen has a large, low-slope plateau accumulation area with an ice-surface altitude of over 1000 m, from which the glacier flows down through a narrow valley to around 450 m asl.
In 1996, Fingerbreen was flowing at a maximum horizontal velocity of 0.38 m/day in a narrow zone close to the plateau edge, reducing to 0.05-0.1 m/day near the margin (Sharov, 2003). The area is underlain by Cambrian and Silurian mica schists and intrusive igneous (mainly granitic) rocks (Gurney & White, 2005;Holtedahl, 1960).

| METHODS
Geomorphological mapping of the glacier foreland was completed using high-resolution (25 cm) colour digital aerial photographs from 2014 and 2019 (Table 1) and 1:10 000-scale field mapping of the area north of the main meltwater channel, undertaken in August 2016.
Using a standard approach (e.g. Chandler et al., 2018), key erosional and depositional glacial landforms (e.g. moraines, meltwater channels, flutings, ice-smoothed bedrock) were mapped digitally in Esri ArcGIS before field verification. Field data were then combined with further remote mapping to produce the final geomorphological map. In the field, a handheld GPS (accuracy 3 m) was used to provide positional accuracy of smaller features, and the orientations of 25 striae were measured at 41 locations using a compass.
The LIA maximum was identified by the outermost moraines, a reduction in vegetation or a change from weathered to smoothed bedrock. Former ice marginal positions were derived from panchromatic and colour aerial photographs and previously published limits (Knudsen & Theakstone, 1984) (Table 1 and Figure 1b). The panchromatic aerial photographs were georegistered to the colour orthophotos using prominent and stable landscape features visible on both datasets as control points. A spline transformation was used with a large number of control points (>100) due to the high level of radial distortion caused by the large altitudinal range within any one photograph.
To examine processes of landform generation, small sections were excavated within flutes and moraines. Section logging involved recording overall section architecture, particle size, degree of sorting F I G U R E 1 (a) Study area map showing the location of the study site at Fingerbreen, including spot heights in metres above sea level (asl). and bedding plane dip and strike, as described in Evans & Benn (2021) (Figure 2). Samples for clast shape and roundness analysis were also collected at each section and compared with supraglacial and fluvial control samples, using the approach described by Lukas et al. (2013). Each sample contained 50 mica schist clasts, and the data were plotted and analysed in Triplot (Graham & Midgley, 2000).

| Geomorphology
The glacial geomorphology of the foreland is presented in Figure 3a.
We divide the foreland into three zones based on different topographic characteristics and landform-sediment assemblages ( Figure 3b). Zone 1 comprises the LIA limit, the central foreland up to 1.1 km upvalley of the LIA and the lateral margins up to 1.9 km upvalley of the LIA limit. The area is characterised by large bouldery moraines close to the LIA limit, which become smaller upvalley, interspersed by areas of striated bedrock. Zone 2 comprises a flat, heavily fluted area in the central part of the foreland, 1.1-1.9 km upvalley from the LIA limit, which is cross-cut by transverse (straight) and sawtooth (zig-zag) frontal moraines. Zone 3 features latero-frontal moraines deposited on top of a bedrock high and its reverse (ice-proximal) slope and includes the present-day ice margin. This zone begins around 1.9 km upvalley from the LIA limit.

| Zone 1 landform-sediment assemblages
The LIA limit is clearly delimited by an outer perimeter of large bouldery moraine ridges (Figures 3 and 4a,b). Inside the LIA limit, there is a notable reduction in vegetation, smaller lichen diameters (typically    1957, 1968, 1985Widerøe Photogrammetric surveys 1975, 1981Knudsen and Theakstone (1984 Colour   Table 1

| Sedimentology
Below, we present a selection of six sections that we consider a representative subsample of the lithofacies (LFs) found within a large number of moraines and flutes examined across the Fingerbreen foreland.
The age of these landforms was estimated using aerial photographs Interpretation: Based on the occurrence of striated clasts, the fine-grained sediment matrix and massive structure of LF 2, we interpret it as a sub-glacial traction till (sensu Evans et al., 2006). A subglacial origin for the diamicton is also broadly supported by the clast shape and roundness data, although the sample also plots closely with the esker control sample in Figure 7. We suggest that the small sand lenses and stringers within the diamicton are sliding bed facies resulting from fluctuating pore water pressures (Ives & Iverson, 2019), possibly combined with sediment washing during ice-bed decoupling events (Larsen et al., 2006) and subsequent shearing within the deforming layer (Kessler et al., 2012).
We suggest that this unit was deposited by debris melt out from the ice, producing a slow-moving slurry down the moraine slopes that had sufficient water content to remove the finer particles (Lawson, 1982).  (Phillips et al., 2007).
The direction of asymmetry within the folds and their dip direction indicates a force from the upglacier side, commensurate with proglacial push moraine formation (Bennett, 2001;Chandler et al., 2016;Krüger, 1994;Lukas, 2012;McCarroll & Rijsdijk, 2003). As LF 2 appears to have been folded and extruded into LF 4, the two diamictons must have been deposited prior to pushing, suggesting that a small readvance occurred after debris flow deposits from the ice margin had already been emplaced. The asymmetric fold within LF 3 demonstrates compression throughout the moraine ridge.
We suggest that the downglacier-dipping structural measurement from Interpretation: LFs 2, 3 and 4 have previously been interpreted, respectively, as a sub-glacial traction till, a glaciofluvial layer and a debris flow deposit. As discussed for FIN-F1, we suggest that the finer sand lenses within LF 2 were incorporated into the sub-glacial till prior to moraine construction. In contrast, the larger sand lenses (LF 3) were likely deposited glaciofluvially in front of the ice margin.
Structurally, the steeply dipping layers of LF 3 resemble slabs of sediment that have been stacked against LF 2. We argue that this indicates a mechanism of submarginal freeze-on for moraine emplacement (e.g. Chandler et al., 2016;Evans & Hiemstra, 2005;Hiemstra et al., 2015;Krüger, 1993Krüger, , 1994Krüger, , 1995Krüger, , 1996Matthews et al., 1995;Reinardy et al., 2013). According to this model, we suggest that LF 3 was initially deposited proglacially in small streams or shallow ponds on top of recently exposed sub-glacial till in the previous melt-season (e.g. Figure 4g). Initial winter glacier advance overrode and incorpo- Interpretation: The silty-clay composition, fissility and occurrence of faceted bullet-shaped boulders within LF 6 are characteristic of a sub-glacial traction till (Evans, 2018;Evans et al., 2006). This interpretation is supported by the clast shape and roundness sample being very similar to the sub-glacial control samples. Given that the moraine is predominantly composed of sub-glacial till and lacking in any largescale structures, we suggest that moraine formation was through squeezing up of a viscous slurry of saturated sediments at and immediately beneath the ice margin during winter ice advance Chandler et al., 2016;Price, 1970;Sharp, 1984). The sand lenses at the base of the moraine were likely incorporated from pre-existing sorted sediments immediately in front of the ice margin (e.g. Figure 4g).  (Figure 4a). This suggests higher glaciofluvial activity on the northeastern side that flushed out finer grained sediment (cf. Winkler, 2021). This assertion is supported by the presence of a still active prominent tributary to the main channel on the northeastern side, 300-400 m from the LIA limit, that follows the shape of lateral-frontal moraines and would have flowed around the former ice margin, efficiently removing glaciogenic debris (Benn et al., 2003).

| Zone 2: Inefficient meltwater drainage and changes in glacier structure
In contrast to Zone 1, Zone 2 represents an area of inefficient meltwater drainage (Benn et al., 2003). The location of meltwater portals to the north and south ensured the preservation of this zone in the central interfluve (Figure 5a,b). Later in recession, meltwater on the northeastern valley side appears to have been diverted across the iceproximal side of Zone 3 by the bedrock high (Figure 5c), helping to consolidate drainage within the main channel rather than being distributed across the foreland. Zone 2 was 'water-soaked' in the 1960s and 1970s (Knudsen & Theakstone, 1984, p.379) (Figure 5b), which influenced processes of moraine formation as discussed further below.
The change from transverse moraines to sawtooth moraines within this zone represents a transition in the structure of the ice margin, as previously observed by Knudsen and Theakstone (1984).
In the 1957 and 1968 aerial photographs, the ice margin is arcuate, but by 1985, the frontal zone is more intensely crevassed, forming ice 'pecten' between longitudinal crevasses (cf. Evans et al., 2016) ( Figure 5). This structural transformation has been observed at a number of Icelandic glaciers in recent decades (e.g. Fláajökull, Skálafellsjökull, Fjallsjökull, Skaftafellsjökull and Svínafellsjökull) and has been associated with thinning ice margins receding into overdeepenings (e.g. Chandler et al., 2016;Evans et al., 2016Evans et al., , 2017Everest et al., 2017;J onsson et al., 2016;Lee et al., 2018). Conversely, at Fingerbreen, the transition that occurred is likely the result of the thinning ice margin becoming increasingly influenced by the upglacier bedrock high. The spatial arrangement of the flutes and sawtooth moraines, with flutes occurring on the ice-proximal slopes of the moraines (Figure 4e), suggests that the flutes are genetically linked to the sawtooth moraines, forming during the same advance Evans & Twigg, 2002). Conversely, the transverse moraines were overlain over more continuous flutes as described elsewhere in Norway (e.g. Worsley, 1974).

Ives and Iverson
The combination of a saturated proglacial area and thin ice margin also provided favourable conditions for submarginal freeze-on (cf. Chandler et al., 2016;Reinardy et al., 2019). We suggest that across this area of the foreland, drainage conditions and associated moraine-forming processes varied both spatially and annually ( Figure 13). In colder winters, penetration of the freezing front below the ice margin led to freeze-on of submarginal sediments, sometimes containing overridden glaciofluvial material (e.g. Chandler et al., 2016). In other years, or in other parts of the ice margin, a saturated proglacial area with high porewater pressures led to till squeezing and bulldozing at the margin and around ice pecten (e.g. Chandler et al., 2016;Chandler, Lukas, & Boston, 2020;Evans et al., 2016Evans et al., , 2017. The two welldefined sediment slabs in FIN-M3 could imply that the sawtooth moraine formed over at least 2 years (e.g. Hiemstra et al., 2015) ( Figure 12). Alternatively, multiple slabs that were likely produced during a single year have been recognised at other sites (Chandler et al., 2016). A freeze-on mechanism for moraine formation has been identified at several sites in Norway and Iceland (e.g. Chandler et al., 2016;Evans & Hiemstra, 2005;Hiemstra et al., 2015;Krüger, 1993Krüger, , 1994Krüger, , 1995Krüger, , 1996Matthews et al., 1995;Reinardy et al., 2013). However, it has rarely been recognised as contributing to sawtooth moraine development, where till squeezing around the ice-marginal pecten and within longitudinal crevasses is often advocated (e.g. Chandler, Lukas, & Boston, 2020;Evans et al., 2016).
In the downvalley part of Zone 2, we count more transverse moraines than years between 1957 and 1968, implying that some sub- annual moraines are present. Sub-annual moraines have been rarely documented in the literature (e.g. Chandler et al., 2016; but are thought to occur in situations where squeeze-push mechanisms become decoupled over a seasonal cycle. In these cases, strong summer glacier retreat enables (multiple) squeeze moraines to be formed in between winter push moraines, rather than a single composite push-squeeze moraine being constructed Chandler, Lukas, & Boston, 2020).  Rowan et al., 2022), resulting in a set of closely spaced, bifurcating moraines that probably formed over several years or were reoccupied.
The time frame coincides with a period in the 1990s when glaciers in Norway readvanced (Andreassen et al., 2005;Chinn et al., 2005), which would have also contributed to a slower overall recession rate and reoccupation of moraines. A thin glacier margin over the bedrock high would have provided favourable conditions for the freeze-on of submarginal sediment slabs (e.g. Krüger, 1996) (Figure 14). The bedrock high also contributed to the preservation of these moraines because it diverted meltwater exiting from the northern side of Fingerbreen southwards around the ice margin before joining the main channel.
The clast-supported nature of moraines on the reverse bedrock slope indicates that as glacier recession progressed, sediments deposited in ice-marginal meltwater channels were subsequently bulldozed into moraines, similar to moraines in Zone 1 (e.g. FIN-M1). It is likely that finer grained material from sub-and englacial sources was removed and redistributed by this lateral meltwater. In 2016, the ice margin had reached a flatter area and drainage was again restricted due to the reverse slope acting as a barrier, and because Fingerbreen by then was receding into a larger overdeepening, similar to many glaciers in Iceland (e.g. Evans et al., 2016Evans et al., , 2019Everest et al., 2017;Guðmundsson & Evans, 2022;J onsson et al., 2016;Lee et al., 2018;Schomacker, 2010).
This caused a saturated ice margin, resulting in squeezing up of a silty-clay sub-glacial till into small moraine ridges (Price, 1970). The abundance of ice-cored sand ridges that emanate from longitudinal crevasses also indicates areas of restricted drainage where evacuation of englacial drainage channels within the ice has been restricted, forming small eskers or kames, some of which may have been later bulldozed by the ice margin.

| Topographic controls on glacier dynamics and landform development
The sequence of events described for the three zones highlights the importance of topography in moraine spacing, process of moraine formation, drainage conditions at the ice margin and preservation poten- There are some similarities between the landform-sediment signatures of Fingerbreen and glaciers in southern Iceland, which are also currently receding into overdeepenings. However, unlike at the Icelandic glaciers (e.g. Evans et al., 2016), the bedrock high at Fingerbreen has also been strongly influential. (2022) that the largest moraines form at pinning points and that moraine spacing is larger on flatter slopes (Oerlemans, 2012). Our work at Fingerbreen demonstrates the need to take even relatively small-scale bedrock undulations in the glacier foreland into consideration in any moraine-based palaeoclimate studies.
Processes of moraine formation have also been used in palaeoglacial research to infer glacier dynamics (e.g. Chandler, Lukas, & Boston, 2020;Lukas, 2005). Here, we demonstrate spatial variability in moraine-forming processes within a relatively small area due to changes in sub-and ice-marginal topography and drainage conditions.
We show large differences in moraine composition, from clastsupported sandy diamicton to silty-clay diamicton, over a short distance as a result of changing drainage efficiency at the ice margin caused by changes in slope. We recommend that bedrock topographic factors that may affect ice margin morphology and structure and icemarginal drainage conditions should be considered in any assessment of glacier dynamics.

| CONCLUSIONS
We present the LIA to present day geomorphological and sedimentological record of Fingerbreen, an outlet glacier of the Østre Svartisen plateau icefield. We divide the foreland into three zones. Zone 1 includes the LIA limit and is characterised by bouldery moraines interspersed by areas of striated bedrock. Zone 2 is a flat area of heavily fluted terrain, with cross-cutting annual, and some sub-annual, transverse and sawtooth moraines, that became exposed between 1957 and 1985. Here, a switch occurred during the 1970s from transverse to sawtooth moraines as a result of changes in the morphology and structure of the ice margin. Since 1985, Fingerbreen has been receding over and down the reverse slope of a bedrock high on the northern valley side (Zone 3).
Section logs for five representative moraines and one flute, The spatial and temporal variability in moraine-forming processes at the glacier margin demonstrates that valley topography exerts a strong influence on ice margin shape and structure, and meltwater flow pathways, which in turn controls moraine formation processes.
This work demonstrates the key role of foreland topography in moraine spacing and processes of moraine formation, which needs to be taken into consideration when using moraines to make inferences about palaeoclimate and glacier dynamics.

AUTHOR CONTRIBUTIONS
CMB and BJD conceived the project. CMB, HL, BMPC and PW conducted the fieldwork. PW provided access to aerial photographs held at NVE. CMB, BMPC and HL analysed the sedimentary log data. All authors contributed to writing the manuscript.