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Esker
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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 miles in length and, because of their peculiar uniform shape, somewhat resemble railroad embankments.
eskers are believed to form in ice-walled tunnels by streams which flowed within and under glaciers.
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Encyclopedia
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 miles in length and, because of their peculiar uniform shape, somewhat resemble railroad embankments.
Geology
Most eskers are believed to form in ice-walled tunnels by streams which flowed within and under glaciers. After the retaining ice walls melt away, stream deposits remain as long winding ridges. 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.
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).
Etymology
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 virtually the entire width of the island of Ireland from Dublin to Galway, a distance of about 100 miles, and is still closely followed by the main road linking those two cities.
Examples of eskers
In Sweden Uppsalaåsen stretches for 250 km (156 miles) and passes through Uppsala city.
The Mason Esker, at approximately 22 miles, is one of the longest eskers in the U.S. It is located in Mason, Michigan. It stretches from DeWitt through Lansing and Holt, finally ending in Mason. Esker systems in the U.S. state of Maine can be traced for up to 100 miles.
Eskers are sometimes used for construction of highways as an economic measure. Examples include the Denali Highway in Alaska, the Trans-Taiga Road in Quebec, and "The Airline" (Route 9) in Maine. There are numerous, lengthy eskers in the Adirondack State Park in upstate New York.
See also
External links
- Larry Gedney, "," Alaska Science Forum Article #674, August 1, 1984.
- Diagram illustrating (i) tunnel in glacier before retreat of ice, forming (ii) meandering esker in on page 6 of "Pennsylania and the Ice Age" published 1999 by PA DCNR
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