The term esker was first used by Close (1867) in his review of Irish glacial geology, then introduced in North America by Upham (1876). The term, which comes from the Irish word eiscir (or Welsh escair) and means ridge, has long been used in the Anglo-Saxon world without distinction from the term kame (Levasseur, 1995), which has a different meaning nowadays. The Scandinavians prefer the terms ås or ôs and their plural derivatives osar or åsar. French geographers used the term os until the 1950s.
The term esker is generally used to describe a thin straight or sinuous ridge in positive relief from the surrounding topography, which has been deposited by fluvioglacial processes. Within a single fluvioglacial corridor, numerous ridges can be seen, locally intersecting one another to form networks of anastomosing appearance.
Eskers are either continuous or discontinuous and are a few hundred metres to several hundred kilometres long, a few hundred metres wide and up to 200 m high (Wilson, 1939; Flint, 1971; Shilts et al., 1987; Warren and Ashley, 1994; Benn and Evans, 2010). The height and width are normally proportional to the length of the structure (Levasseur, 1995). The sides of ridges are generally steep, but may be less steep as a result of gravity processes, reworking, or due to certain inherent properties of the topography and ice substrate (Allard, 1974; Shreve, 1985a; 1985b). In some cases, they may be quite flat, such as those formed late in the deglaciation process.
Sediments found in an esker may consist of fluvioglacial silt, sand, gravel and boulders (Sugden and John, 1976; Warren and Ashley, 1994; Brennand, 2000; Benn and Evans, 2010), and are generally sorted and stratified. In general, the material is rounded to subrounded and indicates extensive meltwater transport. Eskers typically display normal sorting, while the core consists of coarser particles (pebbles and boulders) and the surface is made of stratified sands and gravels. Normal faults as well as crossbedding structures (Terwindt and Augustinus, 1985) could be observed in the sediment. They are respectively associated with glaciotectonic processes and episodic meltwater inflow.
Eskers may be isolated, but are most commonly arranged in swarms fed by a dendritic and complex network of subglacial channels (Flint, 1971). They are generally oriented subparallel to the ice flow direction (Tanner, 1937; Sugden and John, 1976) and located in areas of low relief (Flint, 1971; Lundqvist, 1979). The arrangement and spatial distribution of eskers generally reflect the retreat pattern of the ice margin, as they are generally arranged perpendicular to it.
Where valleys are more incised and the ice is thinner, the drainage pattern marked by eskers is arranged according to the relief (Mannerfelt, 1945), unless the ice flow direction is strongly perpendicular to these valleys (Flint 1971). The profile of the eskers is generally gently sloping down-ice, but some ridges can show moderate relief (Flint, 1971; Shreve, 1972).
Kettles typically border esker strings or observed within wider segments (Flint, 1971). Eskers occur in association with several other fluvioglacial bedforms, such as kames, proglacial moraines or proglacial or subglacial channels (Sugden and John, 1976). Frontal moraines are typically larger where esker strings meet.
Few eskers could be observed at the time of their formation for obvious reasons of access, but the first reports of direct observations (Lewis, 1949) confirmed the previously existing theories (Hummel, 1874; De Geer, 1897; Mannerfelt, 1945; Flint, 1947).
Eskers may be deposited in a context of active ice (Synge, 1950; Barnett and Holdsworth, 1974; Shreve, 1972; 1985; Gorrell and Shaw, 1991) or decaying and relatively thin ice (Flint 1930; 1971; Shaw, 1972; Cheel, 1982; Hebrand and Åmark, 1989; Ashley et al., 1991). Sugden and John (1976) suggest that all eskers are preserved under inactive ice.
In the glacial environment, supraglacial, intraglacial or subglacial drainage systems may coexist (Röthlisberger, 1972; Sugden and John, 1976; Benn and Evans, 2010). These systems are constantly evolving, as water can move spatially within the glacier either by primary permeability (ice permeability) or secondary permeability (karst system, crevasse, moulin). For example, a supraglacial channel may transit to a subglacial tunnel via a moulin. The water comes from ice melting in the walls of these glacial tunnels. Subglacial channels, in which eskers are generated, have a tunnel shape and sit at the base of a glacier where meltwater is channelled under high pressure, overdeepening the overlying ice.
Eskers are tunnel fillings made of granular material that form sinuous ridges after the ice retreats (Benn and Evans, 2010). The high hydraulic pressure in subglacial tunnels ensures that they do not close under cryostatic pressure (Röthlisberger, 1972; Shreve, 1972; Sugden and John, 1976). This high hydrostatic pressure also allows for the transport of large quantities of sediment (Benn and Evans, 2010), which are deposited in the subglacial tunnel during a drop in hydraulic flow, typically following spring floods (Flint, 1971). This high pressure also explains the construction of esker segments that are not controlled by topography (Shreve, 1972; 1985; Sugden and John, 1976; Syverson et al., 1994; Benn and Evans, 2010).
The direction of esker flow is controlled by cryostatic pressure and hydraulic potential. Cryostatic pressure is controlled by the slope of the ice surface and the topography of the ice bed (Flint, 1971; Shreve, 1972). This explains why glacial drainage favours low-pressure areas, such as interlobar ice margin or ice margin confluence zone contexts (Veillette, 1986; Gustavson and Boothroyd, 1987; Warren and Ashley, 1994; Huddart et al., 1999). The direction of long esker trains is more representative of the late flow direction of a glacier than that measured from glacial bedforms such as drumlins (Embleton; 1964; Sugden and John, 1976).
At the ice margin, hydraulic pressure rapidly decreases and sediments are deposited, which results in subaerial or subaqueous outwash depending on the context of the margin, i.e. whether it is in contact with a proglacial water body (Banerjee and McDonald, 1975; Brennand, 2000). In the case of subaqueous outwash, several ice-proximal deltas could be recorded along an esker, reflecting the successive retreat of the glacial margin in contact with a body of water and producing eskers with a beaded appearance. Typically, the coarse fraction is deposited in the tunnel, while the finer fraction is deposited at the outlet of the tunnel as an outwash (Warren and Ashley, 1994). The subsequent retreat of the margin, and consequent removal of the granular source, will allow for the deposition of fine material onto the previously deposited esker ridge.
As well-defined sediment transport vectors, eskers are commonly used in glacioprospecting. Petrographic counts and identification of indicator minerals allow source rocks with economic potential for different metallogenic settings to be traced (Lee, 1965; McClenaghan and Kjarsgaard, 2007; Cummings et al., 2010).
In addition, eskers are a source of good quality, inexpensive aggregate used in the construction industry (Locat et al., 1990). Finally, eskers constitute a high-quality groundwater reserve for several Quebec municipalities and businesses. In fact, fine-grained deposits on the flanks of eskers are not very permeable and therefore conducive to the formation of aquifers (Nadeau, 2011).
Eskers are ubiquitous in areas that have been glaciated (Storrar et al., 2013). Their numbers are greater in areas of rigid bedrock, for example in the Canadian Shield (Shilts et al., 1987; Clark and Walder, 1994), but are also found on other substrates (Benn and Evans, 2010). Current esker formation has been observed in Antarctica (Drews et al., 2017) and some similar forms have even been identified on Mars (Kargel, 1992).
In Quebec, eskers are listed in almost every regional work dealing with Quaternary geology, except in uninhabited areas affected by the existence of a postglacial lake or sea. In the Abitibi and James Bay Lowlands as well as the St. Lawrence Lowlands, eskers are generally buried, partially or entirely, under a layer of glaciomarine or glaciolacustrine clays or sands (Veillette et al., 2007; Nadeau, 2011; Lamarche and Dubé-Loubert, 2017).
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