The glacial geomorphology of the Mackenzie Mountains region, Canada

ABSTRACT During the Last Glacial Maximum, the Mackenzie Mountains region was glaciated by three distinct ice sources; the Laurentide Ice Sheet, the Cordilleran Ice Sheet, and independent montane glaciers. Rapid ice sheet thinning of the Laurentide-Cordilleran ice saddle in the south of this region contributed to rapid sea level rise events and influenced the style of deglaciation to the north. The current understanding of the glacial history of the broader region has been established through mapping from aerial imagery and early surveys between the early 1970s to the 2010s. The central portions of the Mackenzie Mountains have not yet been mapped. We present a new glacial geomorphological map for the Mackenzie Mountains region covering over 220,000 km2. This updated geomorphological map will form the basis of future work to reconstruct the former maximum ice extents, flow dynamics, and retreat pattern.


Introduction
Recent numerical modelling studies have identified the northwest Laurentide Ice Sheet (LIS) as a significant contributor to the Meltwater Pulse-1a sea level rise event (Gregoire et al., 2012;Gregoire et al., 2016;Menounos et al., 2017).The collapse of the saddle between the NW LIS and the Cordilleran Ice Sheet (CIS) provides a mechanism to understand the rapid retreat of this ice sheet sector and its contribution to sea level rise (Gomez et al., 2015;Stoker et al., 2022).Improved empirical constraints on the maximum ice extent, flow dynamics, and retreat pattern will allow us to better understand the drivers of rapid ice sheet retreat.
The Mackenzie Mountains were glaciated by three distinct ice sources during the local Last Glacial Maximum.The LIS abutted the Canyon Ranges, which form the eastern mountain front of the Mackenzie Mountains, and extended almost 50 km west up the major valleys (Figure 1) (Duk-Rodkin & Hughes, 1991).The CIS advanced from the southwest, through the valleys and into the Backbone Ranges of the central Mackenzie Mountains (Figure 1) (Duk-Rodkin & Hughes, 1991).Finally, independent montane glaciers grew in the Backbone Ranges and contributed to the CIS, acting as an accumulation zone (Turner et al., 2008a(Turner et al., , 2008b)).The geomorphological record documents the interaction and relative patterns of advance and retreat of these ice masses during the last glaciation.In multiple locations, landforms from montane glaciers are clearly cross-cut by landforms from the LIS (Duk-Rodkin & Hughes, 1991).In the Liard region, to the south of our study area, the deflection of glacially streamlined landforms indicates the coalescence of the LIS and CIS (Bednarski, 2008b;Margold et al., 2013;Smith, 2003aSmith, , 2003b)).Cross-cutting relationships indicate an early advance of the LIS into the lower valleys of the region, followed by coalescence and buttressing with the CIS, which allowed the ice sheets to thicken and overtop the mountain summits (Smith, 2003a(Smith, , 2003b)).
The Mackenzie Mountains have experienced multiple cycles of glaciation of variable extents (Duk-Rodkin & Barendregt, 2011;Duk-Rodkin & Hughes, 1992).Duk-Rodkin and Hughes (1992) suggested that the penultimate montane and CIS glaciation, which they term the Mountain River glaciation, was the most extensive.They correlate this glaciation with the Reid glaciation of the CIS in neighbouring Yukon.They suggest the last glaciation was less extensive and term it the Gayna River glaciation, correlated with McConnell glaciation (Duk-Rodkin et al., 1996;Duk-Rodkin & Hughes, 1992).These age classifications are based on moraine morphology, palaeosol development, and magnetochronology, with no numerical dating constraints to differentiate the exact timing of these glaciations (Duk-Rodkin et al., 2004;Duk-Rodkin & Hughes, 1992;Trommelen & Levson, 2008).In contrast, the LIS only extended into the North American Cordillera during the last glaciation and so was most extensive in the Mackenzie Mountains region during the Last Glacial Maximum (Duk-Rodkin et al., 1996;Jackson et al., 2011;Zazula et al., 2004).
The development of remote sensing methods and the increase in freely-available, high-resolution digital elevation models provides an opportunity to revisit the geomorphology of the Mackenzie Mountains region.In particular, the ArcticDEM provides an advantage in visualising low-relief and large-scale features.We present a new glacial geomorphological map which complements the pre-existing surficial geology maps and provides robust, empirical evidence for future studies to reconstruct the glacial history of this region.

Data usage and mapping procedure
The map area covers a total of ∼220,000 km 2 , encompassing 21 1:250,000 map sheet areas (Figure 1).When referring to the mapped area, we use the term 'Mackenzie Mountains region' as our map area includes the foothills and Mackenzie Valley to the east.The mapping extent was designed to cover the area of maximum ice margin positions in the Mackenzie Mountains and central area of montane glaciation.Two primary data sources were used in the creation of the glacial geomorphological map.Mapping was performed principally on the 2 m resolution Arctic-DEM elevation dataset (Porter et al., 2018).A hillshade relief model was created within ArcMap v10.6.1 from the ArcticDEM dataset to enhance landform visibility.To prevent any mapping errors from the direction of illumination, we created multiple hillshaded elevation models with different azimuth angles and illumination altitudes (Smith & Clark, 2005).This includes hillshades with an azimuth angle of 315°and 45°and illumination altitudes of 20 and 45.These hillshades were then overlain by a semi-transparent elevation layer.Where the ArcticDEM was not available due to data gaps or poor quality data, satellite imagery at a resolution of 5 m from PlanetLabs was used (Planet Team, 2018).Geomorphological mapping from satellite imagery was limited by image availability and poor lighting conditions, so in these areas only the largest landforms have been mapped.We followed the recommended methodology for the mapping of glacial geomorphology as described in Chandler et al. (2018).To maximise the reliability of the mapping procedure, a repeat-pass method was adopted where each map tile was mapped multiple times with a set scale of 1:25,000-1:50,000.
Secondary mapping datasets were used to supplement the primary data and reduce the misinterpretation of landforms where available.Our final geomorphological map was compared to existing surficial geological maps from the region to evaluate the limitations of our mapping process and identify where new information and understanding has been added.The coverage of pre-existing surficial geology maps across the study region is not complete.Therefore, we only include landforms from the surficial geology maps which are visible in the remote sensing data sources we used to ensure consistency across the study region.Discrepancies between our geomorphological map and pre-existing surficial maps are highlighted throughout the results section.

Map units
In the following section, we briefly define the landforms mapped, their formative mechanisms, and their potential use for palaeoglaciological reconstruction.We explain the procedure for digitising each map unit and provide examples to illustrate some of the mapped landforms.

Glacial lineations
Glacial lineations are elongate, linear ridges formed by basal sliding of ice masses and/or the deformation of subglacial sediments (e.g.till).We include all iceflow parallel subglacial bedforms (streamlined bedrock, drumlins, mega-scale glacial lineations, flutings, crag and tails) in the category of glacial lineations.There is debate about the precise processes responsible for the formation of glacial lineations (Clark et al., 2003;Ely et al., 2016;Fowler, 2000;Möller & Dowling, 2018;Stokes et al., 2013), and these features may form through a variety of erosional and/or depositional processes (Eyles et al., 2016;Hart et al., 2018).Glacial lineations can be used to reconstruct the variations in palaeo-ice flow directions.The elongation of glacial lineations has been hypothesised to reflect the ice flow velocity they were formed under, with mega-scale glacial lineations also being diagnostic of former ice stream positions (King et al., 2009;Stokes & Clark, 2002).We mapped all glacial lineations as polylines along the crestline (Figure 2) and in the Main Map we provide arrows to indicate the former ice flow direction alongside swarms of glacial lineations.

Meltwater channels
Meltwater channels are carved by drainage of meltwater from ice sheets and glaciers.They can form in a variety of locations relative to ice masses.We have divided meltwater channels into: lateral (marginal and submarginal) meltwater channels, based on the diagnostic criteria of Greenwood et al. (2007), undifferentiated meltwater channels, and lateral meltwater spillways.
2.2.2.1.Lateral meltwater channels.Lateral meltwater channels are eroded by meltwater flow concentrated between the margin of an ice mass and the local topography.Typically, they form as a series of subparallel channels which are subparallel to contour lines and are discordant with the contemporary drainage network (Greenwood et al., 2007).They commonly appear perched on valley slopes with an abrupt initiation and termination and may be associated with kame terraces and deltas (Figure 4).We have grouped both submarginal and marginal channels into one landform assemblage of lateral meltwater channels due to the fact they both form near to the ice margin.Both submarginal and marginal channels are indicators of ice retreat direction and margin location and have similar characteristics which can make them difficult to distinguish from each other (Kleman, 1992).We mapped all lateral meltwater channels as polylines along the channel thalweg (Figure 4a  and 4b); an arrow is provided alongside each swarm of lateral meltwater channels to indicate the former meltwater flow direction.We place lateral meltwater channels into four categories based on the ice source they formed from: Laurentide, Cordilleran, montane, and unknown origin.

Undifferentiated meltwater channels.
Undifferentiated meltwater channels may include all types of channels formed by the drainage of meltwater.This includes: subglacial meltwater channels, proglacial meltwater channels, and some lateral meltwater channels.Subglacial meltwater channels and tunnel valleys are the erosional product of channelised drainage beneath an ice sheet or glacier (Ó Cofaigh, 1996;Shreve, 1972).Subglacial meltwater channels can be part of a larger chaotic network of bifurcating channels or of single channel systems cutting through topography (Sugden et al., 1991).Proglacial meltwater channels are channels formed by the drainage of meltwater, which may cut across interfluves, drain along established fluvial networks, or establish new drainage channels as water flows freely away from the ice margin.In this category, we include: glacial lake spillways, proglacial drainage channels (Greenwood et al., 2007) and over-col spillways (Margold & Jansson, 2012).
Differentiating between different types of meltwater channels can be difficult, especially in areas of low topographic relief.To avoid any misclassification we group subglacial and proglacial meltwater channels into one landform group.The majority of lateral meltwater channels are included in a separate category, but some may be included in this category.We separate undifferentiated meltwater channels into two size categories in our map.We define small undifferentiated channels as less than 1000 m wide and map them as polylines along the channel thalweg.We map large undifferentiated meltwater channels (>1000 m wide) as polygons along the break of slope between the channel base and channel walls, as can be seen in Figure 3.

Lateral meltwater spillways.
As the northwest LIS advanced to the eastern front range of the Mackenzie Mountains it blocked the regional eastward drainage (Bednarski, 2008a(Bednarski, , 2008b;;Lemmen et al., 1994;Matthews, 1980).During deglaciation, a series of glacial lakes formed between the retreating LIS margin and the Mackenzie Mountains.The regional ice sheet configuration meant that the drainage of these lakes was forced northwards, broadly following the former ice sheet margin, resulting in right-angle diversions of the easterly drainage routes.This south-north oriented drainage carved a series of lateral meltwater spillways along the range front of the Mackenzie Mountains which cut across the interfluves.We map these lateral meltwater spillways as a separate feature as they can provide an insight into the former ice sheet margin configuration and the drainage of glacial lakes.Lateral meltwater spillways are mapped as polygons along the channel base (Figure 4c and 4d).

Eskers
Eskers are sinuous ridges of glaciofluvial sands and gravels that are most commonly deposited within conduits at the glacier bed (Brennand, 2000;Storrar et al., 2014).Esker morphology can be highly variable, ranging from simple, single ridges to complex, anabranching esker systems (Figure 5) (cf.Storrar et al., 2015).Eskers are typically oriented subparallel to the former ice flow direction in topographically simple areas (Shreve, 1972;Storrar et al., 2020).These characteristics make them a useful tool for reconstructing the former ice sheet margin retreat patterns in areas of low relief.We mapped eskers as polygons along the break of slope between the landform and the surrounding landscape (Figure 5b).

Moraines
Moraines are linear to arcuate ridges composed of till deposited at the margins of glaciers and ice sheets.They exhibit a variety of morphological forms which reflect the variety of depositional processes which form them (Barr & Lovell, 2014;Chandler et al., 2016;Chandler et al., 2020).Simple moraine forms may include lateral moraines deposited between a glacier and a valley wall which form a single linear ridge, or terminal moraines which form linear to arcuate ridges as sediment accumulates at a glacier terminus.Large hummocky moraines are more complex and may include multiple ridges in a broader zone of chaotic, hummocky terrain.There are a variety of theories about their formation, including the melt-out of stagnant ice to the thrusting of sediment (Eyles et al., 1999;Hambrey et al., 1997).Where available, moraines are an essential tool for reconstructing ice margin retreat patterns and former ice standstills (Barr & Lovell, 2014).We place moraines into four categories based on the ice source they formed from: Laurentide, Cordilleran, montane, and unknown origin.
We mapped moraines as either polygons or polylines depending on their size and morphology (Figure 6).Distinct, individual moraines were mapped as a polyline along the ridge crestline.Larger moraine complexes, hummocky moraines, or multi-ridge moraine complexes were mapped as polygons along the break-of-slope with the surrounding landscape.Distinct moraine ridges visible within hummocky terrain and multi-ridge moraine complexes were also mapped as polylines along the ridge crestline.We recognise that the moraine record in the Mackenzie Mountains region may represent multiple glacial periods (Duk-Rodkin & Hughes, 1992), but we do not attempt to distinguish between moraines of different ages here.

Glacial lineations
Glacial lineations are widespread across the Mackenzie Mountains region.We map a total of 11,795 glacial lineations, which range from 0.1 km to 10s of km in length.Lineation morphology is variable across the study region.In general, lineations mapped within the Mackenzie Valley are more elongate than examples in the Mackenzie Mountains.They are  most commonly located at lower elevations, in particular, along the valley floors.Almost half of the total glacial lineations (5565) are located in the Mackenzie Valley, associated with the LIS, oriented along the valley profile, in a broadly SE-NW direction.There are some examples which defy this trend, notably in the major valleys of the Mackenzie Mountains, where lineations record the diversion of ice flow up-valley (Main Map).Some high elevation glacial lineations are observed to the south (∼1300 m) and the west (∼1500-1600 m).Some of the outer summits and ridgelines of the eastern Mackenzie Mountains also display streamlining from the LIS (∼700-800 m around the Carcajou Canyon region), but this is restricted and does not extend far into the mountain range.Our glacial lineation mapping builds upon previous mapping studies.We identify over 3000 lineations in the central Mackenzie Mountains where there are no published surficial geological maps.A major advantage of the ArcticDEM is the ability to more easily discern past ice flow direction.The reconstructed flow directions will provide an insight into the changes in past ice flow dynamics.

Lateral meltwater channels
Lateral meltwater channels (n = 9766) are extensive across the Mackenzie Mountains region and are the most widespread glaciofluvial feature.The largest concentration of lateral meltwater channels are located in the western Mackenzie Mountains, indicating CIS retreat up valleys to the west (n = 9227) (Main Map).Lateral meltwater channels are also carved into the eastern Mackenzie Mountains and on the ridgelines of the Mackenzie Valley associated with the retreat of the LIS (e.g. the Norman Range), but are much less prevalent than the western examples (977) (Figure 4a and b).We also map 778 channels associated with local montane ice masses and 823 meltwater channels of an unknown origin.Over 4000 of these lateral meltwater channels are located in areas of the central and southern Mackenzie Mountains which have not previously been mapped in detail (1:250,000 tiles 95E, 95F, 95L, 95M, 105I, 105P, 106A).Our mapping reproduces the swarms of lateral meltwater channels identified in pre-existing surficial geology maps.However, surficial geology maps based on aerial imagery often map a greater number of lateral meltwater channels in each swarm.This difference would not affect any attempt to reconstruct past ice margin retreat patterns, as our map captures the overall pattern of ice retreat well.

Undifferentiated meltwater channels
We mapped a total of 985 undifferentiated meltwater channels ranging in size from ∼100 to 10,000 s of metres long.The smaller examples form where a small proglacial lakes dammed between an ice mass and topography flowed over a col.The longer examples formed as the LIS blocked the eastward fluvial drainage.In some cases, this resulted in the drainage being rerouted and north-south drainage networks to be established (Main Map).We identify 150 undifferentiated meltwater channels from areas which have not previously been mapped.

Lateral meltwater spillways
We mapped 72 lateral meltwater spillways which form a discontinuous series of subparallel channels cut across the interfluves of the eastern Mackenzie Mountains, stretching from the Ravens Throat River in the south to the Bonnet Plume Basin in the north (Main Map; Figure 4c, 4d and 4e).These channels have previously been well described and mapped, except those in the 1:250,000 map tile 95F (Bednarski, 2008a;Duk-Rodkin & Hughes, 1991;Lemmen et al., 1994).They range from 100 to 350 m deep, 400 to 2000 m wide, and are up to 60 km long and so likely carried large volumes of water during deglaciation (Figure 4e).

Eskers
In total, we mapped 697 eskers across the Mackenzie Mountain region, with a range of complex and simple esker forms observed.Eskers range in size from ∼ 0.2-2 km in length.Longer, more simple esker forms are observed in the Mackenzie Valley compared to the Mackenzie Mountains region (Main Map).This relationship between complex topography and complex eskers is expected as high topographic variability may fragment the drainage system or cause the migration of R-channels in a thin, retreating ice sheet (Stoker et al., 2021;Storrar et al., 2014).Eskers are more commonly located on the valley floors of the Mackenzie Mountains or in the Mackenzie Valley, although some eskers are observed to climb up the topography of broad plateaus in the southern Mackenzie Mountains region.In general, eskers are most common towards the edges of the study area, and are sparse in the central Mackenzie Mountains.Esker orientation is broadly topographically controlled, with ridges oriented subparallel to the valleys they form in.In the Mackenzie Valley, eskers are oriented either SE-NW, subparallel with the valley orientation, with a small amount of eskers oriented W-E across the valley (Main Map).In the Mackenzie Mountains, eskers are more commonly oriented along the valley floor and change orientation to follow the valley, as would be expected for topographically-confined glaciers (Figure 5).While eskers which appear to cross the valley profile or on upland plateaus are less common.

Moraines
Moraines are most common across the Mackenzie Mountains region (n = 3939).The vast majority of moraines were mapped as polylines due to their smaller size (n = 3810), compared to larger moraine complexes or hummocky moraines (n = 129).These larger moraines, up to 2 km wide and 20 km long, are found along the eastern range front of the Mackenzie Mountains associated with the LIS (n = 27) and sporadically in the Backbone Ranges of the Mackenzie Mountains associated with the CIS (n = 46) or montane ice masses (n = 28) (Figure 6).Smaller moraines are widespread in the central and northern Mackenzie Mountains, typically forming a radial pattern of arcuate ridges around the higher peaks and plateaus, indicating the presence of former icefields (Figure 6).Moraines are cross-cut by meltwater channels in many locations across the Mackenzie Mountains (Figure 4a and b).In particular, these cross-cutting relationships are observed along the eastern side of the Mackenzie Mountains (Main Map).Our map includes over 1600 moraines within the Mackenzie Mountains region from areas which have not previously been mapped, improving the moraine record in this area.However, our map does not capture the full record of smaller moraines when compared to the pre-existing maps.This is a result of lower quality data, artefacts, and gaps in the ArcticDEM which make it difficult to identify these smaller features in some areas.The record of larger moraines and valley moraines is largely unaffected by these issues.

Implications and conclusions
Glaciation of the Mackenzie Mountains region left a geomorphological record marking the maximum extent, changes in ice flow dynamics, and the retreat pattern.We present a glacial geomorphological map based on high-resolution DEM data which updates our knowledge of the glacial geomorphology of the region.Our new map replicates and builds upon pre-existing maps in areas where they are available, providing greater detail for some features (e.g.glacial lineations) and slightly less detail for other features (e.g.meltwater channels), albeit over a larger area.The mapped area also extends into areas where no pre-existing maps are available.The observed crosscutting relationships in the eastern Mackenzie Mountains are key to deciphering the relative glacial sequence between the ice masses which glaciated the Mackenzie Mountains.The lateral marginal spillways at the eastern range front potentially record valuable information on the drainage of large volumes of meltwater from the LIS along the Mackenzie Mountains during the last deglaciation.This map will underpin future work to create an empirical palaeoglaciological reconstruction of the maximum ice extent, as well as ice flow dynamics and retreat patterns.

Software
The preparation and interpretation of landforms from the ArcticDEM elevation dataset was performed within ESRI ArcGIS v10.6.1, including the creation of hillshades.Landforms were identified and digitised as polylines or polygons in ArcGIS with the Canada Lambert Conformal Conic projection.The map data were exported as SVG files from ArcGIS and used to create the final map panel in Affinity Designer v1.9.

Data
The ESRI shapefiles produced for each glacial landform are provided in the supplementary material to accompany the paper.This includes both polylines and polygons.The description and classification for each shapefile is contained in the methods section of this paper.

Figure 1 .
Figure 1.Location map of the Mackenzie Mountains region, Northwest Territories and Yukon.Reconstructed former glacial limits from Dalton et al. (2020) are shown by the blue shaded area.The NTS tiles mapped in this study are labelled in black text.Grey diamonds indicate the location of the figures within this paper.In tile 095N and 095 K/NE, D-R = Duk-Rodkin and H = Huntley.

Figure 2 .
Figure 2. A hillshade image overlain by elevation data from the ArcticDEM showing glacial lineations in the western Mackenzie Mountains.Image location is shown in Figure 1.(a) Closely-spaced lineations in a valley bottom, glacial lineations are especially pronounced on bedrock outcrops.(b) The associated geomorphological mapping of glacial landforms.

Figure 3 .
Figure 3.A hillshade image overlain by elevation data from the ArcticDEM showing a series of large subglacial meltwater channels carved into a ridgeline in the Mackenzie Valley.The image location is shown in Figure 1.(a) Subglacial meltwater channels carved through the elevated topography indicate water flow uphill, to the west.The channels are broadly oriented in the same direction as the mapped glacial lineations.A complex esker system is associated with the subglacial channels at the downflow end.(b) The associated geomorphological mapping of glacial landforms.

Figure 4 .
Figure 4. Examples of the range of meltwater channels observed across the study region.Locations are shown in Figure 1.(a) Lateral meltwater channels in the Backbone Ranges of the Mackenzie Mountains, note the cross-cutting relationship with the lateral moraines from nearby montane glaciers.(b) The associated geomorphological mapping of glacial landforms.(c) Lateral meltwater spillways mapped along the eastern slopes of the Mackenzie Mountains.Similar channels are observed all along the eastern range front of the Mackenzie Mountains.(d) The associated geomorphological mapping of glacial landforms.The black line shows the location of a cross-profile highlighting the channel dimensions.(e) Cross-profile of the lateral meltwater spillways.

Figure 5 .
Figure 5. (a) A hillshade image overlain by elevation data from the ArcticDEM showing a complex esker system in the Mackenzie Mountains.(b) The associated geomorphological mapping of glacial landforms.Image location is shown in Figure 1.

Figure 6 .
Figure 6.A hillshade image overlain by elevation data from the ArcticDEM showing a series of moraines formed by montane glaciers in the Mackenzie Mountains.Image location is shown in Figure 1.(a) A range of moraine morphologies are observed, including large moraines and more distinct, sharp-crested moraines.(b) The associated geomorphological mapping of glacial landforms.