Eskers are ridges of glaciofluvial sediment deposited in ice-walled channels or subglacial tunnels. Those deposited supraglacially are closely related to kames. Eskers appear in the postglacial landscape as long sinuous ridges of sand and gravel, and can be used to reconstruct glacial drainage patterns. Because eskers form in a variety of different glacial settings and take a variety of specific forms they are valuable indicators of former glacial conditions.
Subglacial eskers most commonly form in channels running through the ice at the glacier bed and are sometimes referred to as tunnel-fill eskers. Where water flow in the tunnel is at high pressure the routing is controlled by ice surface gradient and the pattern of water pressure beneath the glacier, so eskers can flow uphill as well as down and do not follow the topography. Similarly, supraglacial eskers draped onto the ground when lowered by ice wastage will not necessarily follow the topography. However, water flowing through a subglacial tunnel at atmospheric pressure must follow the topography and eskers of this type will not flow uphill. Eskers can occur as single ridges or in braided patterns, and can also be discontinuous or “beaded.” The morphology of eskers has been related to the hydraulic conditions in which they form, but the origins of many specific eskers remain disputed. For example, some models propose that braided eskers occur subglacially in response to very high water pressures, while others suggest that braiding is a characteristic of supraglacial systems. Eskers are uncommon in areas characterized by widespread subglacial deformation. This is partly because deformation would destroy eskers even as they form and, more importantly, because deforming bed environments are not typically characterized by water drainage through tunnels cut into the ice. Eskers have therefore been cited as evidence against subglacial deformation in areas where they occur.
Eskers comprise a variety of facies ranging from silt and sand to gravel and boulders. Some sedimentary structures are similar to those in open-channel fluvial deposits, but some characteristics of esker sediments are specific to tunnel hydraulics. Many eskers have a core of poorly sorted sands and gravels. It has been suggested that in pressurized tunnels all of the sediment may be in motion as a single mass and that deposition occurs rapidly when the tunnel is blocked or water supply decreases at the end of the melt season. Above the unsorted core many eskers display arched bedding of sorted sands and gravels. These may be deposited during low discharge periods following the deposition of the core.
The largest eskers are hundreds of kilometers in length, hundreds of meters wide and tens of meters high. In some areas they stand prominently above the surrounding landscape and provide convenient route ways for road building. Benn and Evans (1998), Bennett and Glasser (1996) and Hambrey (1994) provide useful reviews.
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