R J Soare
email: rsoare@dawsoncollege.qc.ca

S J Conway

J-P Williams

A J Hepburn

Possible polyphase periglaciation and glaciation adjacent to the Moreux impact-crater, Mars

Keywords: Mars, atmosphere, climate, surface

The cyclicity and temporal succession of glacial-periglacial periods or epochs are keynotes of cold-climate geology on Earth. Relatively recent work within the Mars community has begun to dissect the mid-to higher-latitudinal terrain of Mars for analogical evidence of similar coldclimate cyclicity and succession.

Here, we carry on with this work by focusing on the terrain immediately to the north of the Moreux impact-crater (40-44 0 N, 43-47 0 E). The crater is located in northern Arabia Terra, to the south of Protonilus Mensae. It lies astride of and postdates Mars' crustal-dichotomy. The latter is a global geological-boundary that separates the ancient southern-highlands from the relatively younger northern-lowland plains.

Using cross-cutting relationships, relative stratigraphy and crater-size frequency distributions (CSFDs) we identify three glacial and two periglacial periods that are temporally intertwined and differentiated by a suite of features unique to each of these periods. For example, we report and discuss clusters of pingo-like mounds amidst ridge and trough terrain or "brain terrain". On Earth, the former are the work of freeze-thaw cycling; on Mars, the latter are thought to be glacial remnants. In turn, the brain terrain is underlain by small-sized polygons possibly formed by thermal contraction cracking and with margins underlain by degraded ice-wedges. Age estimates derived of CSFDs suggest that the polygonised terrain could as much as ~100 Ma, whereas the brain terrain and pingo-like mounds are thought to be ~1 -~10 Ma. Possible terminalmoraines that intercept brain-terrain fragments point to an even more recent period of glaciation If the CSFD age-estimates are valid, then the polygons that underlie the brain terrain and incise the basin floors of our study zone could be an order of magnitude older than most of the age estimates associated with polygonised terrain at other locations on Mars. The fact that there are 3 two distinct periods of polygonization and periglacial activity with a wide offset of time within one relatively small study zone also highlights the extent to which the freeze-thaw cycling of water might be rooted as iteratively and as deeply in Mars' geological history as is its glaciation.

Introduction

Our study zone is immediately to the north of the Moreux impact-crater (40-44 0 N, 43-47 0 E) in northern Arabia Terra (Fig. 1). The latter lies astride of and postdates Mars' crustaldichotomy, a global geological-boundary that separates the ancient southern highlands [START_REF] Mcgill | Origin of the Martian global dichotomy by crustal thinning in the Late Noachian or Early Hesperian[END_REF][START_REF] Frey | Ancient lowlands on Mars[END_REF] from the relatively younger northern lowlands [START_REF] Head | Northern lowlands of Mars: Evidence for widespread volcanic flooding and tectonic deformation in the Hesperian Period[END_REF][START_REF] Tanaka | Geological map of the northern plains of Mars, map 2888[END_REF].

Here, as elsewhere at similar and higher latitudes within both hemispheres, landscape assemblages, landforms and surface textures pointing to the work of: glaciation, i.e. the net and perennial accumulation (aggradation) or degradation of atmospherically-precipitated (water) ice; and, periglaciation, i.e. environments dominated by frost action and often, but not necessarily, the freeze-thaw cycling of water.

The long-term cyclicity and temporal succession of glacial-periglacial periods or epochs are keynotes of cold-climate geology on Earth. Relatively recent work within the Mars community has begun to dissect the mid-to higher-latitudinal terrain of Mars for analogical evidence of similar cold-climate cyclicity and succession (e.g. [START_REF] Dickson | Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases[END_REF]Levy et al., 2009;[START_REF] Baker | Extensive Middle Amazonian mantling of debris aprons and plains in Deuteronilus Mensae, Mars: Implications for the record of mid-latitude glaciation[END_REF][START_REF] Hubbard | Characterisation and interpretation of a mid-latitude glacier-like form: Hellas Planitia[END_REF][START_REF] Souness | An alternative interpretation of late Amazonian ice flow: Protonilus Mensae[END_REF][START_REF] Sinha | Amazonian modification of Moreux crater: Record of recent and episodic glaciation in the Protonilus Mensae region of Mars[END_REF][START_REF] Hepburn | Polyphase mid-latitude glaciation on Mars: Chronology of the formation of superposed glacier-like forms from crater-count dating[END_REF].

Below, we carry on with and extend the range of this work to our study zone. In so doing we have three principal aims:

1) Integrate cross-cutting relationships and relative stratigraphy to posit a possible geochronological ordering of the disparate glacial and periglacial assemblages, landforms and surface textures.

2) Use crater-size frequency distribution to frame the proposed geochronological ordering in absolute terms.

3) Estimate the temporal distance between the latest and earliest onset of glaciation or periglaciation.

Methods

Two HiRISE images (High Resolution Imaging Science Experiment, Mars Reconnaissance Orbiter, MRO, McEwen et al., 2007) (ESP_045349_2235, 43.405 0 N; 44.108 0 E; and, ESP_042105_2235, 43.163 0 N; 43.983 0 E) and one CTX image (Context Camera, [START_REF] Malin | Context Camera Investigation on board the Mars Reconnaissance Orbiter[END_REF]) (J01_045349_2239_XN_ 43N316W, 43.94 0 N; 43.99 0 E) frame our study region geographically and were used to evaluate cross-cutting relationships and relative stratigraphy therein.

Crater counts were conducted using the Cratertools plug-in for the ERSI ArcGIS to measure crater diameters [START_REF] Kneissl | Map-projection-independent crater size-frequency determination in GIS environments -new software tool for ArcGIS[END_REF]. Crater size-frequency distributions (CSFDs) were compared with modeled crater-retention age isochrons from [START_REF] Hartmann | Martian cratering 8: Isochron refinement and the chronology of Mars[END_REF] using Craterstats 2 [START_REF] Michael | Planetary surface dating from crater size-frequency distribution measurements: partial resurfacing events and statistical age uncertainty[END_REF][START_REF] Michael | Planetary surface dating from crater sizefrequency distribution measurements: Poisson timing analysis[END_REF].

Observations

Small-sized (~5-15 m in diameter) and clastically non-sorted polygons (Type-1) are ubiquitous in their coverage of the two basins in our study region (Fig. 2a-c). They also incise some of the terrain near the eastern margin of the northernmost basin and are observed upslope as well on its flanks (Figs. 2a,c). The polygons are shaped irregularly. Some of the polygons show centres elevated slightly above the polygon margins, or high-centred polygons [HCPs]. Polygon margins are metres to sub-metres in width and may or may not comprise sub-metre deep troughs A slightly larger and morphologically-different type of polygon (Type-2) also is observed within the basins (Figs. 3a-b). The polygons tend to be four-sided, not irregular or asymmetrical, show margins that are etched slightly deeper and wider than the basin-surface polygons ,and are sub-divided by secondary cracks. Some of the polygons centres are knobby; many of them are relatively flat. Mirroring the spatially-constrained distribution of the basin-surface polygons, none of the etched polygons are observed beyond the basin borders.

The etched polygons occur amidst terrain superposed on the polygonised basin-surface.

The terrain, in turn, comprises outcrops, ridges and troughs, and small-sized mounds, described below, each of which may or may not be contiguous. Outliers, distal from the principal areas of distribution, are commonplace.

One of the basins in our study region exhibits wave-like and (mostly) equator-facing scarps that span its multiple-kilometer reach (Fig. 4). The scarps display metres to decametres of horizontal separation and, individually, metres of vertical drop. On a smaller scale the polygonised surface-basins also show uneven topography. The latter comprises metres-scale, serialized and sub-parallel undulations (Figs. 5a-b). The undulations are shallow, perhaps comprising a few metres of depth, exhibit kilometres 2 of reach and display a NE/SW vector; their minor axes are scaled to the diameter of individual polygons. Elsewhere, circular to sub-circular and rimless depressions are distributed in two basin-wide arcs (Fig. 5c). The depressions seemingly scale to the same shallow depth as the undulations noted above.

Coverage of the polygonised basins and overprinting by ridge and trough structures and assemblages is widespread (Figs. 6, 7a). Individual ridges and troughs are metres in elevation and metres to decametres in width (Fig. 7c). Some of the ridge-trough assemblages are open; others are closed (Fig. 7c). Long axes often trend longitudinally (Fig. 7c). Numerous basins show a gradual decrease in the mass, height and surface coverage of the ridges and troughs as they edge further into the basin midsts and away from the margins (Figs. 7a-b), also Fig. 6). Spatial outliers also are observed throughout the basin surfaces (Fig. 7b).

As noted above, etched mounds populate the terrain superposed on the polygonised basinsurfaces. (Figs. 3a-b, 8a-d). Typically, long axes are sub-kilometre, elevation is scaled in metres, and shape is circular to sub-circular. Numerous mounds exhibit morphological asymmetry or irregularities at and within their margins, possibly by way or erosion or ablation (Figs. 8a-c). Many but not all of the mounds are clustered and contiguous with the larger-scale distributions of etched terrain on the polygonised basins (Fig. 8b). No mounds or mound remnants are observed on the elevated terrain that flank the basins.

The eastern flank (HiRISE image ESP_045349_2235) shows multiple closely-set and bowshaped ridges (Fig. 9). The long-axial distribution of these ridges roughly follows the long axis of the polygonised basin and exhibits kilometres of reach (Fig. 9). Most of the ridges comprise decametre to metre-scale boulders distributed continuously or discontinuously (Fig. 10a). At one location, as many as nine serialised ridges are observed (Fig. 10a). Some of the ridges are subtle, being largely absent of boulders and observable only by means of a gentle decametres-wide and metres-high change of (basin-surface) elevation (Fig. 10a). A much denser distribution of boulders is observed at the topographical transition between the basin surface and the rockier flankingterrain immediately to the east (Figs. 10a-b). Where the ridges intercept the ridge-trough terrain the amplitude and mass of the former increases perceptibly, compared to the ridge segments where no interception occurs (Fig. 11).

Parallel albeit discontinuous lineations normal to these ridges intercept the latter and extend beyond them infrequently (Figs. 10b-c). The lineations show a strong variance in length, from multiple to hundreds of metres, and incise the same type of polygonised terrain that covers the basin surface elsewhere (Fig. 10d). Some of the lineations comprise metre-wide grooves with metre-to decametre-scale boulders nested therein; boulder size, most often, appears to vary randomly (Fig. 10c); others, comprise surface boulders whose size also seems to vary randomly.

A few lineations exhibit smaller boulders on the lee side of larger ones (Fig. 10c).

Two other items are of note. First, much like the distribution of the serialized ridges, the density of boulder alignments and grooves increase with proximity to the basin's edge and the rockier terrain adjacent to the edge (Figs. 10a-c). Second, at some locations the grooved surfacelineations at/near basin edges extend through the positively sloped basin-adjacent terrain (Fig. 9, top right-hand corner).

Periglacial landscapes on Earth

Thermal-contraction polygons

Small-sized polygons are commonplace features of permafrost landscapes on Earth (e.g. [START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF][START_REF] Mackay | Ice wedge cracks, Garry Island, Northwest Territories[END_REF][START_REF] Mackay | Periglacial features developed on the exposed lake bottoms of seven lakes that drained rapidly after 1950, Tuktoyaktuk Peninsula Area, western arctic coast, Canada[END_REF][START_REF] Czudek | Thermokarst in Siberia and its influence on the development of lowland relief[END_REF][START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Rampton | Surficial geology of Tuktoyaktuk, District of Mackenzie[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] French | The periglacial environment[END_REF] (Figs. 12a-b). Typically ≤25 m in diameter, the polygons are produced by the tensile-induced fracturing of frozen sediment. This occurs when the latter undergoes a sharp drop of sub-zero (Celsius) temperatures [START_REF] De | The dominant form of ground-ice on the north coast of Alaska[END_REF][START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF]. Fracturing, or thermal-contraction cracking, opens up shallow, narrow and vertical veins [START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF]. In-filling prevents the ground from relaxing and returning to its pre-cracked state as temperatures rise. Subsequent cycles of cracking and relaxation increase the depth and the width of the veins, eventually forming wedges.

The fill type depends on the ambient boundary-conditions and the availability of: 1) meltwater derived of thawed snow or ice; 2) winter hoarfrost; or, 3) windblown sand, mineral-soil, or a mixture of the two (e.g. [START_REF] De | The dominant form of ground-ice on the north coast of Alaska[END_REF][START_REF] Péwé | Sand-wedge polygons (tessellations) in the McMurdo Sound region, Antarcticaa progress report[END_REF][START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF][START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Sletten | Resurfacing time of terrestrial surfaces by the formation and maturation of polygonal patterned ground[END_REF][START_REF] Hallet | Micro-relief development in polygonal patterned ground in the Dry Valleys of Antarctica[END_REF]) (Figs. 12b-d). The diurnal and seasonal iteration of cracking and filling grows the sub-to centimetre-scale veins into wedges with metre-scale depth and width [START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF]. As the cracks intercept one another they form individual polygons and, eventually, consolidated polygon networks (Figs. 12a-b). The latter may comprise hundreds of square kilometres of continuous or discontinuous coverage and represent hundreds of cycles (e.g. [START_REF] Black | Permafrost -a review[END_REF][START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF][START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Mackay | Ice wedge cracks, Garry Island, Northwest Territories[END_REF].

As ice-and/or sand-wedges aggrade by means of seasonal/annual supplements of fill, the wedges and their sedimentary overburdens may rise above the elevation of the polygon centres; this forms low-centred polygons [LCPs] [START_REF] Péwé | Sand-wedge polygons (tessellations) in the McMurdo Sound region, Antarcticaa progress report[END_REF][START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Harris | Glossary of permafrost and related ground-ice terms[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] French | The periglacial environment[END_REF]. Ice or sand-wedge degradation, by thaw in the case of the former, or aeolian erosion in the case of the latter, degrades and depletes wedge mass. If and as the wedges fall below the elevation of the polygon centres the polygons begin to show high-centres [HCPs] [START_REF] Péwé | Sand-wedge polygons (tessellations) in the McMurdo Sound region, Antarcticaa progress report[END_REF][START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Harris | Glossary of permafrost and related ground-ice terms[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] French | The periglacial environment[END_REF]) (Fig. 12a). Some ice/sand-wedge polygons show neither elevated nor collapsed margins. This is indicative of morphological nascency, evolved insufficiently to uplift marginal overburdens, or of a transitional and topographically-neutral stage between aggradation and degradation.

Ice and sand-wedge polygons show a similar range of morphologies, depending on their stage of aggradation, degradation or neutrality. However, the respective presence or absence of sand-wedge polygons within a permafrost landscape points to climatic, hydrological and aeolian regimes that differ markedly from the regimes associated with ice-wedge polygons. Although the terrain fractured by sand-wedge polygons does require some ice-cementation [START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF], the boundary conditions associated with these polygons tend to be arid and extremely cold [START_REF] Péwé | Sand-wedge polygons (tessellations) in the McMurdo Sound region, Antarcticaa progress report[END_REF][START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF][START_REF] French | Frozen ground phenomena in the vicinity of Terra Nova Bay, northern Victoria Land, Antarctica: a preliminary report[END_REF][START_REF] Murton | Sand veins and wedges in cold aeolian environments[END_REF][START_REF] Marchant | Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica[END_REF][START_REF] Sletten | Resurfacing time of terrestrial surfaces by the formation and maturation of polygonal patterned ground[END_REF][START_REF] Hallet | Micro-relief development in polygonal patterned ground in the Dry Valleys of Antarctica[END_REF][START_REF] Wolfe | Contemporary sand-wedge development in seasonally frozen ground and paleoenvironmental implications[END_REF]. Permafrost landscapes where surface water is bountiful and freeze-thaw cycling is commonplace exhibit greater populations of ice-wedge than sand-wedge polygons [START_REF] Péwé | Sand-wedge polygons (tessellations) in the McMurdo Sound region, Antarcticaa progress report[END_REF][START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF][START_REF] French | Frozen ground phenomena in the vicinity of Terra Nova Bay, northern Victoria Land, Antarctica: a preliminary report[END_REF][START_REF] Murton | Sand veins and wedges in cold aeolian environments[END_REF][START_REF] Sletten | Resurfacing time of terrestrial surfaces by the formation and maturation of polygonal patterned ground[END_REF][START_REF] Hallet | Micro-relief development in polygonal patterned ground in the Dry Valleys of Antarctica[END_REF][START_REF] Wolfe | Contemporary sand-wedge development in seasonally frozen ground and paleoenvironmental implications[END_REF].

The vulnerability of ice-wedge polygons to thaw means that the concurrent observation of low-and high-centred ice wedge polygons, or even a landscape dominated by high-centred polygons and thaw pits, would be expected when ice-rich landscapes undergo thermal stress and disequilibrium [START_REF] Péwé | Sand-wedge polygons (tessellations) in the McMurdo Sound region, Antarcticaa progress report[END_REF][START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Harris | Glossary of permafrost and related ground-ice terms[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] French | The periglacial environment[END_REF][START_REF] Farquharson | Climate Change Drives Widespread and Rapid Thermokarst Development in Very Cold Permafrost in the Canadian High Arctic[END_REF].

By comparison, sand-wedge polygons are insensitive to thermal stress. Aggradation and degradation are based uniquely on a gain of mass by aeolian deposition or mass loss by aeolian erosion. As aeolian processes would not be expected to vary substantially on a highly-localised scale, the closely set presence of low-and high-centred sand-wedge polygons in a permafrost landscape would be unusual (e.g. [START_REF] Péwé | Sand-wedge polygons (tessellations) in the McMurdo Sound region, Antarcticaa progress report[END_REF][START_REF] French | Frozen ground phenomena in the vicinity of Terra Nova Bay, northern Victoria Land, Antarctica: a preliminary report[END_REF][START_REF] Marchant | Formation of patterned ground and sublimation till over Miocene glacier ice in Beacon Valley, southern Victoria Land, Antarctica[END_REF][START_REF] Sletten | Resurfacing time of terrestrial surfaces by the formation and maturation of polygonal patterned ground[END_REF]Hallett et al., 2011).

Thermokarst and ice-rich permafrost

Thermokarst refers to terrain in permafrost regions of excess ice which, depending upon the currency of warming or cooling trends, could undergo frost heave or settlement as the excess ice aggrades or degrades in situ [START_REF] Taber | Historical perspectives in frost heave research: the early works of[END_REF][START_REF] Penner | The mechanism of frost heaving in soils[END_REF][START_REF] Hussey | Tundra relief features near Point Barrow, Alaska[END_REF][START_REF] Hughes | Geology and permafrost in relation to hydrology and geophysics[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF]) (Fig. 12d). Excess ice equals the volume of ice in the ground that exceeds the total pore-volume that the ground would have under natural unfrozen conditions [START_REF] Harris | Glossary of permafrost and related ground-ice terms[END_REF]also, see Taber, 1930;[START_REF] Penner | The mechanism of frost heaving in soils[END_REF][START_REF] Rampton | Massive ice and icy sediments throughout the Tuktoyaktuk Peninsula, Richards Island, and nearby areas, District of Mackenzie[END_REF][START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] French | The periglacial environment[END_REF].

Excess ice may comprise lenses, veins or larger masses of consolidated ice such as tabular ice. All of these ice types originate and develop by the work of ice segregation. Ice segregation, in turn, is the result of cryosuction pulling pore water to a freezing front where the ice consolidates into lenses and, over time, into more substantial bodies of ice (e.g [START_REF] Taber | Historical perspectives in frost heave research: the early works of[END_REF][START_REF] Black | Permafrost -a review[END_REF][START_REF] Penner | The mechanism of frost heaving in soils[END_REF][START_REF] Rampton | Massive ice and icy sediments throughout the Tuktoyaktuk Peninsula, Richards Island, and nearby areas, District of Mackenzie[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] French | The periglacial environment[END_REF]. As this occurs, the overlying terrain undergoes frost heave proportional to the volumetric growth of ice-segregated lenses; frost settlement occurs when these lenses thaw or degrade and the newly voided or de-iced soil settles under its own weight (e.g. [START_REF] Harris | Glossary of permafrost and related ground-ice terms[END_REF][START_REF] Osterkamp | Physical and Ecological Changes Associated with Warming Permafrost and Thermokarst in Interior Alaska[END_REF][START_REF] Farquharson | Climate Change Drives Widespread and Rapid Thermokarst Development in Very Cold Permafrost in the Canadian High Arctic[END_REF].

Fine to medium-grained soils such as silts or silty clays are particularly adept at hosting segregation ice because they have relatively-small interstices (e.g. [START_REF] Taber | Historical perspectives in frost heave research: the early works of[END_REF][START_REF] Black | Permafrost -a review[END_REF][START_REF] Penner | The mechanism of frost heaving in soils[END_REF][START_REF] Rampton | Massive ice and icy sediments throughout the Tuktoyaktuk Peninsula, Richards Island, and nearby areas, District of Mackenzie[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] French | The periglacial environment[END_REF]. Were the grain-size too fine, the migration of pore-water to the freezing front would be overly constricted and inadequate to the requirements of segregation-ice formation; opposingly, were the grain-size too coarse, the migration of pore-water would be overly expeditious and flush out of the system before segregation ice could form.

The oscillation of regional mean-temperatures is one of the principal drivers of frost heave and settlement (e.g. Péwe, 1954;[START_REF] Czudek | Thermokarst in Siberia and its influence on the development of lowland relief[END_REF][START_REF] Murton | Thermokarst sediments and sedimentary structures, Tuktoyaktuk Coastlands, western Arctic Canada[END_REF][START_REF] Grosse | Geological and geomorphological evolution of a sedimentary periglacial landscape in northeast Siberia during the late Quaternary[END_REF][START_REF] Osterkamp | Physical and Ecological Changes Associated with Warming Permafrost and Thermokarst in Interior Alaska[END_REF][START_REF] Schirrmeister | Yedoma: late Pleistocene ice-rich syngenetic permafrost of Beringia[END_REF]Wetterich et al., 2014). This is exemplified, in part, by the hummocky sediments and rolling topography of the Tuktoyaktuk Coastlands of northern Canada (e.g. [START_REF] Rampton | Massive ice and icy sediments throughout the Tuktoyaktuk Peninsula, Richards Island, and nearby areas, District of Mackenzie[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] Murton | Thermokarst sediments and sedimentary structures, Tuktoyaktuk Coastlands, western Arctic Canada[END_REF] and northeastern Siberia (e.g. [START_REF] Czudek | Thermokarst in Siberia and its influence on the development of lowland relief[END_REF][START_REF] Grosse | Geological and geomorphological evolution of a sedimentary periglacial landscape in northeast Siberia during the late Quaternary[END_REF][START_REF] Schirrmeister | Yedoma: late Pleistocene ice-rich syngenetic permafrost of Beringia[END_REF].

The time-frame of excess-ice aggradation and degradation, or of ice-induced heave and settlement, however, need not be synchronous (e.g. [START_REF] Rampton | Massive ice and icy sediments throughout the Tuktoyaktuk Peninsula, Richards Island, and nearby areas, District of Mackenzie[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF][START_REF] Farquharson | Climate Change Drives Widespread and Rapid Thermokarst Development in Very Cold Permafrost in the Canadian High Arctic[END_REF]. For example, the wide-ranging presence of thermokarst lakes and alases (thermokarst-lake basins absent of water) throughout the Tuktoyaktuk Coastlands is rooted in the relatively recent Holocene Era (e.g. Rampton andMackay, 1971, Rampton 1988;). Contrarily, the radiocarbon dating of wood ensconced in segregation-ice lenses and beds that are metres to tens of metres deep point to region-wide ice-enrichment having taken place thousands and possibly tens of thousands of years ago during the middle to late Wisconsinian glacial stade (Rampton and Bouchard, 1988). This means that the geochronological distance or offset of time between ice enrichment and depletion can be substantial.

Perennially ice-cored mounds

Cold-climate and non-glacial landscapes on Earth show disparate mound types, i.e. earth hummocks (e.g. [START_REF] Pettapiece | A hummocky permafrost soil from the subarctic of northwestern Canada and some influences of fire[END_REF][START_REF] Kokelj | The Structure and Dynamics of Earth Hummocks in the Subarctic Forest Near Inuvik, Northwest Territories, Canada, Arctic[END_REF], frost blisters (e.g. [START_REF] Van Everdingen | Frost Blisters of the Bear Rock Spring Area near Fort Norman[END_REF][START_REF] Pollard | The internal structure and ice crystallography of seasonal frost mounds[END_REF], and pingos, hydrostatic (e.g. [START_REF] Mackay | Pingo growth and collapse, Tuktoyaktuk Peninsula areas, western arctic coast, Canada: a long-term field study[END_REF] or hydraulic (e.g. [START_REF] Müller | Observations on Pingos[END_REF]. Of these four types, only the pingos scale to the height and width of the mounds in our study region and are perennial.

Closed-system pingos

Hydrostatic, or closed-system, pingos [CSPs] are uniquely tied to ice-rich periglacial landscapes (e.g. [START_REF] Müller | Observations on Pingos[END_REF][START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Mackay | Pingo growth and collapse, Tuktoyaktuk Peninsula areas, western arctic coast, Canada: a long-term field study[END_REF][START_REF] French | The periglacial environment[END_REF]. They originate and develop in response to the loss of thermokarst lake-water by drainage or evaporation (Figs. 13a-d). Lake-water loss triggers the propagation of a freezing front from the newly-exposed basin sides or floor. Pore water undergoes expulsion ahead of the freezing front into an increasingly compressed area of unfrozen ground, usually at or near the topographical low(s) of the basin. As the hydrostatic pressure of the expelled pore-water increases, the basin floor begins to deform and uplift. This creates a dome-like structure or mound. If and when the underlying water freezes, an ice core forms [START_REF] Mackay | Pingo growth and collapse, Tuktoyaktuk Peninsula areas, western arctic coast, Canada: a long-term field study[END_REF][START_REF] Mackay | Periglacial features developed on the exposed lake bottoms of seven lakes that drained rapidly after 1950, Tuktoyaktuk Peninsula Area, western arctic coast, Canada[END_REF]) (Fig. 13b).

Mound shape, usually circular to sub-circular, is determined by the morphology or bathymetry of the lake basins in which the mound is nested (e.g. [START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Mackay | Pingo growth and collapse, Tuktoyaktuk Peninsula areas, western arctic coast, Canada: a long-term field study[END_REF][START_REF] Mackay | Periglacial features developed on the exposed lake bottoms of seven lakes that drained rapidly after 1950, Tuktoyaktuk Peninsula Area, western arctic coast, Canada[END_REF][START_REF] French | The periglacial environment[END_REF]. Mound height (metres to decametres) and long-axis diameters (metres to hundreds of metres) are a function of four variables: 1) the surface area of the basin; 2) the volume of pore water surrounding the basin that is exposed to permafrost aggradation; 3) the stage of development, i.e. nascent, adolescent or growing, and mature or stagnant; and, 4) current (freezingfront) boundary conditions remaining relatively constant [START_REF] Mackay | Pingo growth and collapse, Tuktoyaktuk Peninsula areas, western arctic coast, Canada: a long-term field study[END_REF]. Were thaw temperatures to be lost from this cycling, arresting the loss of lake water and the further exposure of the thermokarst lake-basin pore water, the shape, size and height of the pingo would be fixed. OSPs are less sensitive to this type of temperature-based fixation. Neither the migration of topographically-driven or of deeply seated/geothermal water to the site of mound formation is contingent upon freeze-thaw boundary conditions at/or near the surface.

Metres-wide and deep fissures -dilation cracks -are not unusual amongst larger closedsystem pingos (Fig. 13c). These cracks propagate and trend from the mound summit as the pingo grows and tensile stresses increase (e.g. [START_REF] Washburn | Periglacial Processes and Environment[END_REF][START_REF] Mackay | Pingo growth and collapse, Tuktoyaktuk Peninsula areas, western arctic coast, Canada: a long-term field study[END_REF][START_REF] Mackay | Periglacial features developed on the exposed lake bottoms of seven lakes that drained rapidly after 1950, Tuktoyaktuk Peninsula Area, western arctic coast, Canada[END_REF][START_REF] French | The periglacial environment[END_REF].

Mirroring the degradational pathway of the OSPs (see the discussion below), the summit cracks of CSPs may evolve into depressions as the thermal integrity of the overburden and the underlying ice-core dissipates (Fig. 13c). Subsequent stages of mound degradation possibly comprise fans, slumps and collapse, leaving slightly elevated and irregular ramparts encircling the collapse basin in the wake (Fig. 13d).

Open-system pingos

Hydraulic or open-system pingos [OSPs] are circular to sub-circular in some instances (e.g. [START_REF] Bennike | Pingos in Peary Land, North Greenland[END_REF][START_REF] Worsley | Geomorphology and hydrogeological significance of the Holocene pingos in the Karup Valley area, Traill Island, northern east Greenland[END_REF][START_REF] Scholz | An "open system" pingo near Kangerlussuaq (Søndre Strømfjord), West Greenland[END_REF]Kelly, 2001) and linear, oblong or irregular in others (Cruickshank and Calhoun, 1965;[START_REF] O'brien | Observations on pingos and permafrost hydrology in Schuchert Dal, Northeast Greenland[END_REF][START_REF] Worsley | Geomorphology and hydrogeological significance of the Holocene pingos in the Karup Valley area, Traill Island, northern east Greenland[END_REF]Kelly, 2001). Mound height ranges from metres to decametres and long axes may reach hundreds of metres [START_REF] Cruickshank | Observations on Pingos and Other LandForms in Schuchertdal, Northeast Greenland[END_REF][START_REF] Allen | The chemical and isotopic characteristics of some northeast Greenland surface and pingo waters[END_REF][START_REF] Bennike | Pingos in Peary Land, North Greenland[END_REF]Sholz and Baumann, 1997;Kelly, 2001) (Figs. 14a-d).

Fractures are observed at/or radiating from some mound summits or crests (Fig. 14b); they form, as is the case with CSPs, in response to the tensional stresses within the mound overburden as it undergoes uplift and sedimentary stretch [START_REF] O'brien | Observations on pingos and permafrost hydrology in Schuchert Dal, Northeast Greenland[END_REF][START_REF] Allen | The chemical and isotopic characteristics of some northeast Greenland surface and pingo waters[END_REF][START_REF] Worsley | Geomorphology and hydrogeological significance of the Holocene pingos in the Karup Valley area, Traill Island, northern east Greenland[END_REF][START_REF] Scholz | An "open system" pingo near Kangerlussuaq (Søndre Strømfjord), West Greenland[END_REF]. Further growth may translate these fractures into summit depressions or craters, with the overburden becoming increasingly thin [START_REF] O'brien | Observations on pingos and permafrost hydrology in Schuchert Dal, Northeast Greenland[END_REF][START_REF] Allen | The chemical and isotopic characteristics of some northeast Greenland surface and pingo waters[END_REF][START_REF] Donner | Holocene history of the west coast of Disko, central west Greenland[END_REF][START_REF] Worsley | Geomorphology and hydrogeological significance of the Holocene pingos in the Karup Valley area, Traill Island, northern east Greenland[END_REF][START_REF] Scholz | An "open system" pingo near Kangerlussuaq (Søndre Strømfjord), West Greenland[END_REF][START_REF] Bennike | Pingos at Nioghalvfjerdsfjorden, eastern North Greenland[END_REF]Kelly, 2001) (Figs. 14a-b). Overburden thickness scales to the thermal integrity of the mound and its ice core. Once this integrity is compromised by excessive stretching, ice core thaw could be engendered. Mound collapse is the end-stage of pingo evolution [START_REF] O'brien | Observations on pingos and permafrost hydrology in Schuchert Dal, Northeast Greenland[END_REF][START_REF] Allen | The chemical and isotopic characteristics of some northeast Greenland surface and pingo waters[END_REF][START_REF] Donner | Holocene history of the west coast of Disko, central west Greenland[END_REF]Yoshikawa et al., 1994;[START_REF] Worsley | Geomorphology and hydrogeological significance of the Holocene pingos in the Karup Valley area, Traill Island, northern east Greenland[END_REF][START_REF] Scholz | An "open system" pingo near Kangerlussuaq (Søndre Strømfjord), West Greenland[END_REF]Kelly, 2001).

Three geological-pathways typically are associated with the origin and development of the OSPs, derived largely from field observations in Greenland:

1) Meltwater enters the local hydrological system at points of higher elevation, i.e. mountain slopes or valley walls, and migrates to lower elevations by means of sub-or intra-permafrost channels. The meltwater emerges where the permafrost is sufficiently thin or the hydraulic pressure is sufficiently high to uplift the near-surface/surface terrain into a mound [START_REF] Müller | Observations on Pingos[END_REF][START_REF] Bennike | Pingos at Nioghalvfjerdsfjorden, eastern North Greenland[END_REF]. An ice core forms as the water's near-surface exposure to perennial freezing evolves [START_REF] Müller | Observations on Pingos[END_REF]; or, 2) Meltwater forms basally under the snout of a surging or a receding glacier and migrates down-valley to the mound site(s) by means of near-surface faults, fractures or sub/intrapermafrost channels. Mound formation occurs where the permafrost is relatively thin and most susceptible to the hydraulic pressure generated by the valley-midst topography linking glaciers to mound sites (e.g. [START_REF] Müller | Observations on Pingos[END_REF]Yoshikawa et al., 1994;[START_REF] Yoshikawa | Observations on nearshore pingo growth, Adventalen, Spitsbergen[END_REF]; or, 3) Deeply-seated and possibly geothermally-driven water migrates to the surface through local or regional faults. The pressure is sufficient to uplift near-surface sediments, forming a mound and an underlying ice core as freezing iterates itself annually (e.g. [START_REF] O'brien | Observations on pingos and permafrost hydrology in Schuchert Dal, Northeast Greenland[END_REF][START_REF] Allen | The chemical and isotopic characteristics of some northeast Greenland surface and pingo waters[END_REF][START_REF] Yoshikawa | Observations on nearshore pingo growth, Adventalen, Spitsbergen[END_REF][START_REF] Worsley | Geomorphology and hydrogeological significance of the Holocene pingos in the Karup Valley area, Traill Island, northern east Greenland[END_REF][START_REF] Scholz | An "open system" pingo near Kangerlussuaq (Søndre Strømfjord), West Greenland[END_REF].

Glacial landscapes on Earth

Glaciers

Glaciers comprise perennial and net accumulations of water-ice precipitated episodically from the atmosphere and are classified according to their size and geographical context (e.g. Barry and Gann, 2011) (Figs. 15a-c). Valley glaciers, for example, are relatively small in scale compared to ice sheets and caps and their flow is constrained by the local topography (e.g. [START_REF] Sugden | Glaciers and landscape: a geomorphological approach[END_REF]. Flow movement is the result of gravity and the internal deformation of ice (e.g. Barry and Gann, 2011).

Moraines

Glacial moraines are composite landforms, formed and modified by a range of processes operative at glacial margins; these include glacio-tectonism, bulldozing/pushing, squeezing, freeze-on, melt-out, glaciofluvial, and gravity-driven processes (e.g. Benn and Evans, 2010) (Figs. 15a,c). Irrespective of their genesis, moraines may comprise unsorted glacial-till whose grain-size ranges from silts to boulder-sized clasts (Fig. 15b) and whose uneven distribution may engender topographical irregularity (e.g. [START_REF] Shakesby | Variability in Neoglacial moraine morphology and composition, Storbreen, Jotunheimen, Norway: within-moraine patterns and their implications[END_REF]) (Figs. 15 a,c). The till is formed by the erosion and entrainment of sediment by glacial processes [START_REF] Van Der Meer | Subglacial till: the deforming glacier bed[END_REF].

Moraines are referenced according to context and composition (e.g. [START_REF] Hambrey | Glacial environments[END_REF]Easterbrook, 1999) (Figs. 15a,c). Terminal or ice-marginal moraines are the outermost ridges which mark the maximum horizontal extent of a glacier (e.g. [START_REF] Hambrey | Glacial environments[END_REF]Summerfield, 1991).

Depending on the orientation of moraines to ice flow, terminal and recessional moraines may be further divided into lateral or frontal moraines, or latero-frontal moraines where the entire tongue is demarcated by ridges [START_REF] Benn | Glaciers and glaciation[END_REF]. Serialised recessional-moraines are not unusual when multiple retreats and pauses of glaciers occur (e.g. Easterbrook, 1999) (Fig. 15a,c).

Lineations (small-large scales)

Centimetre-scale striations, sub-metre-to metres-wide grooves and polished bedrock are the product of glacial abrasion at varying scales of width and depth (e.g. [START_REF] Hoppe | Some Observations on Fluted Moraine Surfaces[END_REF][START_REF] Witkind | Giant glacial grooves at the north end of the mission range, northwest Montana[END_REF][START_REF] Lliboutry | Monolithologic erosion of hard beds by temperate glaciers[END_REF]Fjellander et al., 2006) (Fig. 16). Individual striations are gouged into the bedrock by single and relatively small clasts embedded and entrained in basal ice as it moves downstream [START_REF] Påsse | The origin of polygonal troughs on the northern plains of Mars[END_REF]. Grooves, formed similarly, are the work of larger-sized clasts, cobbles and boulders. The clasts, cobbles and boulders are quarried from nearby rock walls and, as rock contact wears out and blunts their erosive competences, the distance covered by their gouging is proportional to the size of the clasts and cobbles [START_REF] Lliboutry | Monolithologic erosion of hard beds by temperate glaciers[END_REF]. Polished bedrock is the result of a much more comprehensive and evenly distributed type of wear, albeit by finegrained debris or clean ice, across the entire face of the exposed bedrock [START_REF] Påsse | The origin of polygonal troughs on the northern plains of Mars[END_REF].

Fluted ridges or flutes are linear (constructional) bedforms comprised of sub-glacial sediments that have undergone post-depositional deformation [START_REF] Boulton | The origin of glacially fluted surface observations and theory[END_REF]) (Figs. 17a-b).

Bracketed by shallow furrows or grooves, the flutes are low, typically ≤5m in relative elevation above the surrounding sediments [START_REF] Hoppe | Some Observations on Fluted Moraine Surfaces[END_REF]Baranowksi, 1970;[START_REF] Glasser | Styles of sedimentation beneath Svalbard valley glaciers under changing dynamic and thermal regimes[END_REF][START_REF] Benn | Glaciers and glaciation[END_REF], decimetres to metres wide, and tens of metres to hundreds of metres long [START_REF] Hoppe | Some Observations on Fluted Moraine Surfaces[END_REF][START_REF] Roberson | Physical properties and formation of flutes at a polythermal valley glacier: Midre Lovénbreen, Svalbard[END_REF]. When observed in groups, individual flutes often exhibit metres-to multi-metres of separation [START_REF] Hoppe | Some Observations on Fluted Moraine Surfaces[END_REF]; the groups also trend normal to the glacier's terminus and in line with the flow of the glacier [START_REF] Roberson | Physical properties and formation of flutes at a polythermal valley glacier: Midre Lovénbreen, Svalbard[END_REF]. Although not the case in all instances, some flutes exhibit cobbles and boulders whose long-axes also point in the direction of the ice movement [START_REF] Hoppe | Some Observations on Fluted Moraine Surfaces[END_REF][START_REF] Boulton | The origin of glacially fluted surface observations and theory[END_REF].

Flutes develop beneath temperate glaciers and their warm-based ice, as till is pressured into lee-side cavities of sub-glacially transported boulders and cobbles (e.g. [START_REF] Hoppe | Some Observations on Fluted Moraine Surfaces[END_REF][START_REF] Hart | The deforming bed/debris basal ice continuum and its implications for the formation of glacial landforms (flutes) and sediments (melt-out till)[END_REF][START_REF] Glasser | Styles of sedimentation beneath Svalbard valley glaciers under changing dynamic and thermal regimes[END_REF][START_REF] Benn | Glaciers and glaciation[END_REF]. Flute width is consistent throughout the long axes and proportional to the boulders and cobbles that reside upslope of them [START_REF] Glasser | Styles of sedimentation beneath Svalbard valley glaciers under changing dynamic and thermal regimes[END_REF]. Bracketing furrows form adjacent to the ridges as the result of till migration into the lee-side boulder and cobble cavities [START_REF] Boulton | The origin of glacially fluted surface observations and theory[END_REF]also, Baranowski, 1970).

Since flute/furrow genesis is uniquely sub-glacial, assemblages thereof rarely are observed beyond current or past glacial forelands [START_REF] Glasser | Styles of sedimentation beneath Svalbard valley glaciers under changing dynamic and thermal regimes[END_REF].

Boulder trains are linear clusters of erratic boulders that track the flow lines of former glaciers [START_REF] Augustinus | Reconstruction of ice flow across the Bunger Hills, East Antarctica[END_REF][START_REF] Evenson | Enigmatic boulder trains, supraglacial rock avalanches, and the origin of "Darwin's boulders[END_REF][START_REF] Hall | Glacial modification of granite tors in the Cairngorms, Scotland[END_REF]Darvill et al., 2015) (Figs. 18a-b). One of the two principal formation-hypotheses proposes that the constituent boulders are plucked from rock outcrops, cliffs or tors and, subsequently, are dragged and deposited linearly (albeit proximally) by basal ice as it flows downstream [START_REF] Hall | Glacial modification of granite tors in the Cairngorms, Scotland[END_REF][START_REF] Phillips | Cosmogenic 10 Be and 26 Al exposure ages of tors and erratics, Cairngorm Mountains, Scotland: Timescales for the development of a classic landscape of selective linear glacial erosion[END_REF]Darvill et al., 2015). According to this hypothesis, the distal deposition of the boulders is neither expected nor observed [START_REF] Augustinus | Reconstruction of ice flow across the Bunger Hills, East Antarctica[END_REF][START_REF] Hall | Glacial modification of granite tors in the Cairngorms, Scotland[END_REF][START_REF] Phillips | Cosmogenic 10 Be and 26 Al exposure ages of tors and erratics, Cairngorm Mountains, Scotland: Timescales for the development of a classic landscape of selective linear glacial erosion[END_REF]Darvill et al., 2015). Closely-set and cascadingly-smaller boulders downslope of the larger lead boulder upslope is not infrequent, since the lead boulder would protect the lesser-sized boulders from further drag, and would be consistent with this hypothesis [START_REF] Augustinus | Reconstruction of ice flow across the Bunger Hills, East Antarctica[END_REF][START_REF] Hall | Glacial modification of granite tors in the Cairngorms, Scotland[END_REF]).

An alternate formation-hypothesis suggests that boulder trains are vestigial to rock avalanches, comprise markers of supraglacial transportation and are spread linearly by ice flow onto moraines [START_REF] Evenson | Enigmatic boulder trains, supraglacial rock avalanches, and the origin of "Darwin's boulders[END_REF]Darvill et al., 2015). Distal deposition is not inconsistent with this hypothesis [START_REF] Evenson | Enigmatic boulder trains, supraglacial rock avalanches, and the origin of "Darwin's boulders[END_REF]. However, trains composed of cascadingly-smaller boulders downslope of the lead boulder upslope are explained less easily.

Mega-scale glacial lineations comprise a suite of parallel grooves and ridges that are aligned with the antecedent flow of ice [START_REF] Clark | A groove-ploughing theory for the production of mega-scale glacial lineations, and implications for ice-stream mechanics[END_REF][START_REF] Storrar | A Glacial Geomorphological Map of Victoria Island, Canadian Arctic[END_REF][START_REF] Benn | Glaciers and glaciation[END_REF]Fu et al., 2012) (Fig. 19). They exhibit up to tens of metres of height, hundreds of metres of width and as much as 100 km of length [START_REF] Hättestrand | The glacial geomorphology of Kola Peninsula and adjacent areas in the Murmansk Region, Russia[END_REF][START_REF] Fowler | The formation of subglacial streams and mega-scale glacial lineations[END_REF]Fu et al., 2012). The leading formation-hypothesis assumes that entrained boulders or rocks in basal ice act as "keels'' that plough through soft sediments, carve elongate grooves and squeeze sediment into the intervening ridges [START_REF] Clark | A groove-ploughing theory for the production of mega-scale glacial lineations, and implications for ice-stream mechanics[END_REF].

Landscape interpretation

Basin-surface and ridge/trough nested polygons

The morphology and metres-scale of the basin-surface polygons and the ridge and trough nested polygons in our study region are consistent with polygons formed by thermal-contraction in permafrost on Earth (e.g. [START_REF] Lachenbruch | Mechanics of Thermal Contraction Cracks and Ice-wedge Polygons in Permafrost[END_REF][START_REF] Oehler | Origin and significance of decameter-scale polygons in the lower Peace Vallis fan of Gale crater[END_REF] and, possibly, elsewhere on Mars (e.g. Pechmann, 1980;[START_REF] Costard | Outwash plains and thermokarst on Mars[END_REF][START_REF] Seibert | Small scale martian polygonal terrain: implications for liquid surface water[END_REF][START_REF] Soare | Thermokarst lakes and ponds on Mars in the very recent (late Amazonian) past[END_REF]Levy et al., 2009a,b;[START_REF] Oehler | Origin and significance of decameter-scale polygons in the lower Peace Vallis fan of Gale crater[END_REF]. However, where only HCPs or polygons that exhibit no topographical variance between centres and margins are observed, as is the case with the basin-surface polygons, identifying marginal fill is neither simple nor straightforward. Planview observations derived of HiRISE imagery, even at the best resolution and magnification, are equivocal; the aggradational and degradational morphologies of sand-or ice-wedge polygons are relatively congruent when observed remotely.

On Earth, ground truth invariably underlies the discrimination of the two polygon types.

On Mars, by contrast, a landscape-scale evaluation and conciliation of individual features (e.g. [START_REF] Baker | Icy Martian mysteries[END_REF][START_REF] Hauber | Periglacial landscapes on Svalbard: Terrestrial analogs for cold-climate landforms on Mars[END_REF] may comprise the most effective pathway for deducing marginal fill and polygon origin.

Polygonised depressions

The polygonised terrain is our study region is punctuated at some locations by shallow circular to sub-circular depressions (Fig. 5c), lineated and parallel to sub-parallel variances of topography (Figs. 5a-b) and, in one kilometres-scale basin, cascading equator-ward scarps (Fig. 4). In permafrost regions on Earth polygonised terrain with similar variances of topography typically occur where/when ice-rich terrain undergoes thermal degradation, deflation or subsidence, and volumetric loss. These losses are the work of evaporation and/or meltwater migration. Thaw-derived run-off channels often facilitate the latter.

On Mars, contrarily, none of the thermokarst-like landscapes exhibit run-off channels. If and when the atmospheric vapour-pressure at the mid-latitudes is low (relative to the vapour pressure of near-surface ice), and the boundary conditions are well below the triple point of water, the most plausible agent of ice loss would be sublimation (e.g. [START_REF] Morgenstern | Deposition and degradation of a volatile-rich layer in Utopia Planitia, and implications for climate history on Mars[END_REF][START_REF] Lefort | Observations of periglacial landforms in Utopia Planitia with the High Resolution Imaging Science Experiment (HiRISE)[END_REF][START_REF] Lefort | Scalloped terrains in the Peneus and Amphitrites Paterae region of Mars as observed by HiRISE[END_REF][START_REF] Ulrich | Thermokarst in Siberian ice-rich permafrost: comparison to asymmetric scalloped depressions on Mars[END_REF][START_REF] Séjourné | Scalloped depressions and small-sized polygons in western Utopia Planitia: a new formation hypothesis[END_REF][START_REF] Dundas | Modeling the development of Martian sublimation thermokarst landforms[END_REF][START_REF] Dundas | Effects of varying obliquity on Martian sublimation thermokarst landforms[END_REF]. This having been said, hypotheses concerning ice enrichment and ice depletion need not be univocal in identifying plausible environmental triggers. There is no conceptual or geological inconsistency in proposing ice enrichment by the freeze-thaw cycling of water and ice-depletion by sublimation. Inconsistencies are reared, however, if and when ice enrichment is explained by means of sublimation and adsorption cycles.

For example, the heterogenous distribution of excess ice in terrestrial landscapes is the result of highly-localised differences in sedimentary permeability, pore space and tortuosity. In turn, this facilitates or impedes the formation of segregation ice or ice lenses and the heaved deformation of terrain surfaces. Hypotheses favouring the origin and development of excess ice by adsorption-diffusion cycles implicitly assume a latitudinal homogeneity in the composition of near-surface regolith and of its ice richness [START_REF] Mellon | Geographic Variations in the Thermal and Diffusive Stability of Ground Ice on Mars[END_REF]also, Dundas et al, 2015). These hypotheses are weakened by two constraints.

First, and notwithstanding questions concerning the plausibility of regolith homogeneity on the scale assumed here, diffusion-adsorption cycles are self-limiting. Once the pore space of near-surface regolith has become saturated with adsorbed water ice, transport to a sub-adsorbed ice depth is choked off (Clifford, 1993;[START_REF] Mellon | Geographic Variations in the Thermal and Diffusive Stability of Ground Ice on Mars[END_REF]. As such, the decametres of possible thermokarst depth observed in one of our study-region basins, and elsewhere on Mars (e.g. [START_REF] Morgenstern | Deposition and degradation of a volatile-rich layer in Utopia Planitia, and implications for climate history on Mars[END_REF][START_REF] Dundas | HiRISE observations of fractured mounds: possible martian pingos[END_REF][START_REF] Séjourné | Scalloped depressions and small-sized polygons in western Utopia Planitia: a new formation hypothesis[END_REF], lie beyond the explanatory reach of the adsorption-diffusion hypothesis. The highly iterative freeze-thaw cycling of water would not be proscribed similarly.

Ridge and trough terrain

Overlying portions of the polygonised surface of the basins in our study region is terrain comprised of interconnected ridges and troughs, also referenced as brain terrain in the literature (e.g. Levy et al., 2009a, b) (Figs. 6, 7a-c). They are metres in elevation and metres to decametres in width. Basin-ward flanks of the ridge and trough assemblages are less dense and thick than the assemblages closer to the basin margins. Outcrops of these assemblages, possibly remnants of basin coverage by them that was more extensive than today, also are observed (Fig. 7b). The ridges and troughs could be the ablated remnants of an icy mantle (e.g. Levy et al., 2009a, b) precipitated atmospherically and accumulated episodically in the Late Amazonian Epoch (e.g. [START_REF] Head | Recent ice ages on Mars[END_REF][START_REF] Milliken | Viscous flow features on the surface of Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images[END_REF][START_REF] Madeleine | Amazonian northern mid-latitude glaciation on Mars: a proposed climate scenario[END_REF][START_REF] Madeleine | Recent ice ages on Mars: the role of radiatively active clouds and cloud microphysics[END_REF]Shorghofer and Forget, 2012).

Mounds

Decametre-wide, metres-high and circular to sub-circular mounds are observed throughout the polygonised basins in our study region (Figs. 3b,5, Nearby, i.e. the Moreux impact-crater region (Soare et al, in press), and elsewhere, i.e. the mid-latitudes of Utopia Planitia [START_REF] Burr | Young (late Amazonian), near surface, ground ice features near the equator, Athabasca Valles, Mars[END_REF]2013;2020;[START_REF] Dundas | HiRISE observations of fractured mounds: possible martian pingos[END_REF][START_REF] De Pablo | Possible pingo fields in the Utopia basin, Mars: Geological and climatical implications[END_REF], equatorial Athabasca Valles [START_REF] Burr | Young (late Amazonian), near surface, ground ice features near the equator, Athabasca Valles, Mars[END_REF][START_REF] Page | Stratigraphical and morphological evidence for pingo genesis in the Cerberus plains[END_REF][START_REF] Balme | An equatorial periglacial landscape on Mars[END_REF], and in the Argyre impact-crater region [START_REF] Soare | Possible open-system pingos in and around the Argyre impact basin, Mars[END_REF], pingo-like mounds that show similarities of size, shape, polygonization and possible degradation have been observed. Mound origin is deduced from and categorized by mound location. For example, if the mounds are nested in plains, basins and thermokarst-like depressions then a hydrostatic origin is hypothesized (e.g. [START_REF] Burr | Young (late Amazonian), near surface, ground ice features near the equator, Athabasca Valles, Mars[END_REF]2013;2020;[START_REF] Burr | Young (late Amazonian), near surface, ground ice features near the equator, Athabasca Valles, Mars[END_REF][START_REF] Balme | An equatorial periglacial landscape on Mars[END_REF].

A slope-side location in the Argyre region, adjacent to graben-like cavities and possible paleodischarges of water, highlights the one location where a hydraulic origin for the pingo-like mounds has been hypothesized [START_REF] Soare | Possible open-system pingos in and around the Argyre impact basin, Mars[END_REF].

Bow-like ridges and boulder/cobble lineations

Unlike periglacial landscape features such as small-sized polygons and pingo-like mounds whose form does not necessarily constrain one and only one possible origin, the connexion between process and form is less equivocal with regard to the bow-like ridges that populate the (inner) eastern margin of the two main basins in our study zone. For example, the ridges are topographically-irregular through their fronts and flanks, are sometimes serialized and are distributed proximally to possible valley-constrained flows (Figs. 9, 10a). These traits are commonplace amongst glacial moraines on Earth (Figs. 15a,c). On Mars, the moraine-like ridges are thought to be no less diagnostic of glaciation, especially when they are observed amidst other glacier-like forms such as lineated valley fill and sinuous esker-like channels (e.g. Arftstrom and Hartmann, 2005;[START_REF] Banks | High Resolution Imaging Science Experiment (HiRISE) observations of glacial and periglacial morphologies in the circum-Argyre Planitia highlands, Mars[END_REF][START_REF] Dickson | Late Amazonian glaciation at the dichotomy boundary on Mars: Evidence for glacial thickness maxima and multiple glacial phases[END_REF][START_REF] Head | Northern mid-latitude glaciation in the Late Amazonian period of Mars: criteria for the recognition of debriscovered glacier and valley glacier land-system deposits[END_REF][START_REF] Hubbard | Characterisation and interpretation of a mid-latitude glacier-like form: Hellas Planitia[END_REF][START_REF] Hubbard | Glacier-like forms on Mars[END_REF][START_REF] Souness | An inventory and population-scale analysis of martian glacier-like forms[END_REF][START_REF] Brough | Former extent of glacier-like forms on Mars[END_REF][START_REF] Hepburn | Polyphase mid-latitude glaciation on Mars: Chronology of the formation of superposed glacier-like forms from crater-count dating[END_REF]. Speculation about the origin of boulder-alignments on Mars is wide-ranging and stretches from periglacially induced self-sorting through to mass wasting or englacial flow (e.g. [START_REF] Banks | High Resolution Imaging Science Experiment (HiRISE) observations of glacial and periglacial morphologies in the circum-Argyre Planitia highlands, Mars[END_REF][START_REF] Hubbard | Characterisation and interpretation of a mid-latitude glacier-like form: Hellas Planitia[END_REF][START_REF] Hubbard | Glacier-like forms on Mars[END_REF]. A number of variables weigh against (periglacial) self-sorting. First, decametre-scale boulders are not observed in self-sorted landscapes on Earth; convectional forces and frost heave substantially greater than those expressed terrestrially would be required for stripes comprised of these large boulders to form. Second, some of the boulder alignments and spatially-associated grooves are contiguous with grooves in the basin-adjacent terrain. This would be more consistent with an erosive origin, proximally, and to mass movement or entrainment than with freeze-thaw cycling and the work of periglaciation. Third, the density of boulder distribution and of boulder alignments decreases as the distance to the basin's edge increases. This too points to the spatial proximity of source material and away from in situ selfsorting. With regard to mass wasting, it is not clear that the gravity-induced flow or slide of boulder material in and of itself could engender the tightly distributed lines of boulders and grooves observed here.

By way of scale, the basin-margin grooves are too narrow and shallow to fit the terrestrial definition of mega glacial-lineations, as well as too wide and deep to be deemed striations (Fig. 10b). Setting aside these scale-based definitions, the grooves could have been gouged by glaciallyentrained boulders whose size befits the actual width and depth of the former. This hypothesis also would be consistent with the contiguous gouging observed upslope of the grooves in the basinadjacent terrain. Where the mega-lineation explanation falls short, however, is with regard to the presence of groove-nested or -adjacent boulders at various basin-margin locations throughout our study region. This suggests deposition, not erosion, and the possibility that the lineated grooves and boulder lineations are the result of two separate and distinct glacial regimes.

As noted above, some boulder alignments do exhibit smaller boulders downslope of larger ones and, in as much the distribution and density of boulder alignments increases with its proximity to an upslope source on the basin-adjacent terrain, they could be boulder trains (Figs. 10b-c). At the same time, the entrainment of boulders and the gouging of erosive grooves or furrows is not a keynote characteristic of boulder trains. Once again, a separate and distinct glacial regime would have to be inferred as a complement to the boulder-train hypothesis for its plausibility to be warranted.

Of the glacially-associated explanations of boulder alignment, the glacial-flute hypothesis befits the geological context of our observations most closely For example, the hypothesis concurrently encompasses the formation of small-scale grooves and of adjacent terrain that is slightly elevated and bouldered, as seen in Fig. 10b. The adjacency of boulder sources proximal to the possible flutes (Fig. 10d) offers tertiary support of the hypothesis, although this would be equally supportive of the boulder-train hypothesis.

The ridge and trough terrain is thought to comprise the sublimated remnants of surface ice on Mars that is precipitated atmospherically (e.g. Levy et al,, 2009a, b); as such, it could be considered artefactual to glaciation. However, this terrain has no observed analogue on Earth and questions concerning the origin of its distinctive and unique morphology remain largely unanswered.

Age-estimates and the temporal-ordering of local periglaciation and glaciation

Glacial Stade I (inferred not observed)

The clastic composition of the terrain incised by the small-sized polygons and the thermokarst-like depressions is unknown. However, for ice enrichment and wedging to occur the terrain would have needed to incorporate relatively fine-grained material. Then, as now, the removal, transport and deposition of regolith by the work of wind is a key player in Mars' dynamic surface-environment. Volcanic ash or loess, for example, could have been brought to the region by aeolian activity sufficient to form a thick surface to depth horizon or sets of horizons (e.g. [START_REF] Greeley | Dust deposits on Mars: the "Parna" analog[END_REF][START_REF] Bridges | Eolian bedforms, yardangs, and indurated surfaces in the Tharsis Montes as seen by the HiRISE camera: evidence for dust aggregates[END_REF][START_REF] Séjourné | Evidence of an eolian ice-rich and stratified permafrost in Utopia Planitia, Mars[END_REF][START_REF] Bridges | Duststones on Mars: source, transport, deposition and erosion[END_REF][START_REF] Skinner | Widespread loess-like deposit in the Martian northern lowlands identifies Middle Amazonian climate change[END_REF][START_REF] Soare | Possible open-system pingos in and around the Argyre impact basin, Mars[END_REF][START_REF] Smalley | Desert loess: a selection of relevant topics[END_REF].

The possibility that variances of obliquity and eccentricity have facilitated and induced the atmospheric precipitation of dusty snow/ice and its surface accumulation at the northern midlatitudes throughout the Late Amazonian Epoch is well documented in the literature (e.g. [START_REF] Jakosky | Chaotic obliquity and the nature of the martian climate[END_REF][START_REF] Head | Recent ice ages on Mars[END_REF][START_REF] Laskar | Long term evolution and chaotic diffusion of the insolation quantities of Mars[END_REF][START_REF] Madeleine | Amazonian northern mid-latitude glaciation on Mars: a proposed climate scenario[END_REF][START_REF] Madeleine | Recent ice ages on Mars: the role of radiatively active clouds and cloud microphysics[END_REF]Shorghofer and Forget, 2012). Were this dusty/ice snow to have undergone thaw, with meltwater migrating into regolith composed of volcanic ash or loess, this would have framed an ideal set of boundary conditions for the freeze-thaw cycling of that meltwater [START_REF] Soare | Possible open-system pingos in and around the Argyre impact basin, Mars[END_REF]. We define this hypothesized period of glaciation and deglaciation as Glacial Stade-I, and suggest that it preceded the Type-1 polygonization of the currently-exposed basin surfaces in our study region.

Periglacial Period I

Age estimates of these Type-1 polygonised basins derived from the evaluation of CSFD raise the possibility that the polygons are as old as 100 Ma (see Appendix A, Figs. 20a-e). We define this formation period as Periglacial Period-I (PP-I). The 100 Ma age estimate, if valid, is roughly an order of magnitude greater than the relatively recent age estimate of the fragmented or degraded terrain, possibly icy in origin, that overlies the Type-1 polygons at some locations (Fig. 7c).

Glacial Stade II (possible remnants observed); Periglacial Period II

Fragmented-terrain members include: networked ridge and trough structures (i.e. brain terrain) pingo-like mounds and mound remnants; and, etched Type-2 polygons that incise the mounds, mound remnants and adjacent terrain (Figs. 7a-c, 8a-d All of the observed thermokarst-like depressions in the valley basins are incised by Type-1 polygons, many of which show high-centres. We suggest that the devolatilized depressions formed prior to their polygonization during PP-I. Whether devolatilization was the work of thaw or sublimation is moot, as long as one differentiates between periods of ice enrichment, which require water to undergo freeze-thaw cycling, and periods of ice loss, which do not. Arguably, devolatilization could have postdated GS-II, been concurrent with PP-II and not involved the freeze-thaw cycling of water. Similarly large offsets of time between enrichment and depletion are commonplace, for example. amidst late Quaternary Epoch and Holocene era permafrost landscapes on Earth (e.g. [START_REF] Rampton | The influence of ground ice and thermokarst upon the geomorphology of the Mackenzie Beaufort region[END_REF][START_REF] Rampton | Quaternary geology of the Tuktoyaktuk Coastlands, Northwest Territories[END_REF]. On the other hand, devolatisation of metres-deep ice-rich terrain could have occurred within a relatively short period of time during PP-II, as has happened throughout the very late Holocene era (e.g. Farquaharson et al., 2019), when and if conditions of water meta-stability arose in our study region.

Glacial Stade III

On the eastern margin of the southernmost Type-1 polygonised basin in our study zone some of the bouldery and debris-covered ridges, possibly moraines, intercept ridge and trough remnants. The amplitude and mass of the bouldery ridges is greater at these contacts than where the ridge and trough remnants are absent. This apparent push-morphology suggests that the remnants predate the bouldery ridges (Fig. 11). As such, the moraine-like structures could be artefacts of a separate and distinctly successive period of glaciation, i.e. Glacial Stade III (GS-III).

To the east and upslope of the bouldery ridges, lineations reminiscent of glacial flutes and boulder-trains are observed (Figs. 10a-b). Some of them extend almost to the bouldery and moraine-like ridges themselves (Fig. 10c). They too could be features associated with GS-III.

Interestingly, the possible flutes and boulder trains superpose polygons whose shape, size, texture and networked distribution matches the Type-1 polygons that populate the adjacent basin to the west. At some locations, the two sets of polygons intersect and we interpret the sub-flute polygons to be formationally concurrent with the Type-1 polygons that incise the basin polygons (Fig. 2c).

Conclusion

Using cross-cutting relationships, relative stratigraphy and crater-size frequency distributions (CSFDs) we have identified landscape features within our study region that point to the existence of three disparate albeit temporally intertwined glacial stades and two periglacial periods. Age estimates of the earliest polygons, Type-1, and of the polygonally-incised thermokarst-like depressions in our basin-based landscapes suggest an origin that could be 100 Ma. This is millions of years older and almost an order of age magnitude beyond that of the more youthful Type-2 polygons.

Heretofore, much of the discussion about periglaciation on Mars has focused on its relative recentness and the glaciation that preceded it (e.g. [START_REF] Mustard | Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice[END_REF][START_REF] Head | Recent ice ages on Mars[END_REF]Levy et al., 2009a, b;[START_REF] Séjourné | Scalloped depressions and small-sized polygons in western Utopia Planitia: a new formation hypothesis[END_REF][START_REF] Schon | Recent high-latitude resurfacing by a climate-related latitude-dependent mantle: Constraining age of emplacement from counts of small craters[END_REF][START_REF] Sinha | Amazonian modification of Moreux crater: Record of recent and episodic glaciation in the Protonilus Mensae region of Mars[END_REF]. The older estimation of periglaciation in our study region and the intertwined chronology of two (possible) periglacial periods and three glacial stades documented here suggests that the cycling of the two environment types could be much more Earth-like and extend much more deeply into Mars' history than had been thought hitherto.

Appendix

Crater counts provide constraints on the exposure and crater retention ages of land surfaces.

Craters measured on the CTX image (Fig 20a .), below, across all surfaces generates a broad craterretention age for our study region immediately to the north of the Moreux impact-crater. The CSFD for crater diameters larger than 200 m are well approximated by a 1.5 Ga model isochron, assuming the [START_REF] Hartmann | Martian cratering 8: Isochron refinement and the chronology of Mars[END_REF] model. This is broadly similar to the crater-count age estimate derived by [START_REF] Sinha | Amazonian modification of Moreux crater: Record of recent and episodic glaciation in the Protonilus Mensae region of Mars[END_REF].

Crater counts also were conducted on the polygonised basin-surface (area #1) and ridgetrough terrain (area #2) in HiRISE image ESP_045349_2235 (Fig. 20b) and the polygonised basin-surface (area #3) in HiRISE image ESP_042105_2235 (Fig. 20c). The CSFDs of these areas do not conform to a production function and the shallower power-law slope is indicative of a loss of smaller diameter craters from the populations. This is consistent with and similar to the age estimates derived of the polygonised basin surface on the eastern margin of the Moreux impactcrater (Soare et al., 2020, in press).

Various processes may alter a crater population on Mars: burial, erosion and exhumation.

Each of these processes preferentially remove smaller-diameter craters from the landscape. A shallower power-law slope results from the size-dependent deficit of smaller craters (see reviews by [START_REF] Williams | Dating very young planetary surfaces from crater statistics: A review of issues and challenges[END_REF]Rubanenko et al. 2020 and references therein for further discussion).

We note that the larger craters are shallow, possibly infilled, and polygonized consistent with older, more heavily modified craters compared with the smaller diameter craters which have more pristine, bowl-shaped morphologies due to the relatively rapid loss of craters at these diameters.

The CSFDs of the polygonised terrains in areas #1 and #3 (Figs. 20d-e) extend across isochrons ~1 Ma at the small diameters and extend to >10 Ma at the large diameters with the largest craters in area #3 overlapping the 500 Ma isochron. This suggests that the polygonised terrains are older than 10 Ma, possibly as old as ~100 Ma or more. The CSFD in area #2, composed of ridges and troughs, extends from ~1 Ma to ~10 Ma. Areas #1 and #3 generally contain larger diameter craters than area #2. This points to the polygonised terrain being older than the latter, possibly by 10s to 100s Ma, and is consistent with the observed overprinting of the polygonised basin-surface by the ridge and trough terrain in HiRISE image ESP_045349_2235. The similar density of the more pristine, bowl-shaped craters on all three areas at the smaller sizes indicates that the characteristic time of the retention of these smaller craters is more uniform across terrains.

  ). Mound summits show fracturing and possible depletion. Mound surfaces, as noted above, exhibit polygonization. Mound distribution, typically, is clustered. Isolated outliers are infrequent. No mounds are observed on the terrain adjacent to the basin.

  ). Based on this relative stratigraphy and the CFSD of the brain terrain, we suggest that the Type-2 polygons, possibly the mounds as well, regardless of whether they are open or closed, formed during a second and more recent periglacial period (PP-II), i.e. ~1 -~10 Ma, than PP-I. Moreover, if brain terrain comprises glacial residue whose distribution varies between dense to sparse throughout the Type-1 polygonised basins, then the antecedent presence of an icy mass that covers the basins completely (Glacial Stade-II) does not seem implausible. Left open is the question of whether the thermokarstic depressions formed during PP-I or PP-II.

FiguresFig. 1 .

 1 Figures

Fig. 2 .

 2 Fig. 2. (a) Context image of Type-1 polygon distribution in one of the two basins studied by us.

Fig. 3 .

 3 Fig. 3. (a) Etched polygons incise terrain fragments and (b) isolated mounds which, antecedently,

Fig. 4 .

 4 Fig. 4. Basin-covering scarps, possibly thermokarstic in origin, with equator-ward orientation. The loss of elevation trends to the south (CTX image J01_045349_2239_XN_43N316W, 43.94 0 N; 43.99 0 E, 5.94 m/pixel). Image credit: Malin Space Science Systems.

Fig. 5 .

 5 Fig. 5. (a) Context image of undulating, polygonised terrain, comprised of high-centred polygons.

Fig. 6 .

 6 Fig. 6. Landscape-scale image (CTX J01_045349_2239_XN, 43.94 0 N; 43.99 0 E, 5.94 m/pixel) of

Fig. 15 .

 15 Fig. 15. Landforms and deposition associated with glaciation on Earth. (a) Aerial view of Mueller

Fig. 16 .

 16 Fig. 16. Parallel scratches in bedrock are glacial striations (western Manitoulin Island, Ontario,

Fig. 17 .

 17 Fig. 17. (a) Glacial flute in Iceland, 75 m from the margin of Múlajökull glacier. The flute has

Fig. 18 .

 18 Fig. 18. (a) Clach Bun Rudhtair, eastern Cairngorms mountains, Scotland: The summit of the

Incision of ice wedge by sand wedge and vice versa [START_REF] Murton | Sand veins and wedges in cold aeolian environments[END_REF]. (d) Panoramic and cross-sectional view of ice wedges (white arrows) in the midst of massive-ice outcrops.

The small, surface depressions above the ice-wedge in (c) and the ice-wedge to the right, here, mark the location of bilateral polygonal troughs associated with a field of highcentered polygons. The depressions, along with the hummocky terrain above and surrounding the massive-ice, comprise thermokarst. Photo credit: R. Soare. upper tower has been removed by ice flowing in the direction of the lineated train of boulders; the summit of the third tower is unmodified and carries weathering pits over 1 m deep [START_REF] Hall | Glacial modification of granite tors in the Cairngorms, Scotland[END_REF]. (b) Boulder train highlighted by aligned and closely-set members, cascading clast-size lee of the largest boulder and in the direction of glacial flow.

Photo credit, V. Bjarnadóttir. [START_REF] Michael | Planetary surface dating from crater sizefrequency distribution measurements: Poisson timing analysis[END_REF]. CSFDs extend across model isochrons >10 Ma suggesting terrains are older than 10 Ma. The crater diameters extend to larger sizes in areas 1 and 3 consistent with the polygonised terrain being older than the ridge and trough terrain. Image credit: Malin Space Science Systems, NASA/JPL/University of Arizona.