An esker is a long winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America. Eskers are frequently several kilometres long and, because of their peculiar uniform shape, are somewhat like railroad embankments.
Most eskers are believed to form in ice-walled tunnels by streams which flowed within (englacial) and under (subglacial) glaciers. They tended to form around the time of the glacial maximum when the glacier was slow and sluggish. After the retaining ice walls melted away, stream deposits remained as long winding ridges. (Water can flow uphill if it is under pressure in an enclosed pipe, such as a natural tunnel in ice.)
Eskers may also form above glaciers by accumulation of sediment in supraglacial channels, in crevasses, in linear zones between stagnant blocks, or in narrow embayments at glacier margins. Eskers form near the terminal zone of glaciers, where the ice is not moving as fast and is relatively thin (Easterbrook, 1999).
Plastic flow and melting of the basal ice determines the size and shape of the subglacial tunnel. This in turn determines the shape, composition and structure of an esker. Eskers may exist as a single channel, or may be part of a branching system with tributary eskers. They are not often found as continuous ridges, but have gaps that separate the winding segments. The ridge crests of eskers are not usually level for very long, and are generally knobby. Eskers may be broad-crested or sharp-crested with steep sides (Easterbrook, 1999). They can reach hundreds of kilometers in length and are generally 20–30 metres in height.
The path of an esker is governed by its water pressure in relation to the overlying ice. Generally the pressure of the ice was at such a point that it would allow eskers to run in the direction of glacial flow, but force them into the lowest possible points such as valleys or river beds, which may deviate from the direct path of the glacier. This process is what produces the wide eskers upon which roads and highways can be built. Less pressure, occurring in areas closer to the glacial maximum, can cause ice to melt over the stream flow and create steep-walled, sharply-arched tunnels.
The concentration of rock debris in the ice and the rate at which sediment is delivered to the tunnel by melting and from upstream transport determines the amount of sediment in an esker. The sediment generally consists of coarse-grained, water-laid sand and gravel, although gravelly loam may be found where the rock debris is rich in clay. This sediment is stratified and sorted, and usually consists of pebble/cobble-sized material with occasional boulders. Bedding may be irregular but is almost always present, and cross-bedding is common (Easterbrook, 1999).
The name Esker is derived from the Irish word eiscir (Old Irish: escir), which means: "a ridge or elevation, especially one separating two plains or depressed surfaces" (Dictionary of the Irish Language). The term was used particularly to describe long sinuous ridges, which are now known to be deposits of fluvio-glacial material. The best-known example of such an eiscir is the Eiscir Riada, which runs nearly the whole width of Ireland from Dublin to Galway, a distance of 200 km, and is still closely followed by the main road from Dublin to Galway.
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