esker

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 long and, because of their peculiar uniform shape, are somewhat like railroad embankments.

Geology

Most eskers are believed to form in ice-walled tunnels by streams which flowed within (englacial) and under (subglacial) glaciers. After the retaining ice walls melt away, stream deposits remain 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).

Esker in Sims Corner Eskers and Kames National Natural Landmark, Washington, USA. Note trees and the single lane road crossing the esker to the right of the photo which provide scale.

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 between 20-30 metres in height.

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

Portions of the Denali Highway in Alaska are built on eskers

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 about 100 miles, and is still closely followed by the main road from Dublin to Galway.

Examples of eskers

A part of the Mason Esker

In Sweden Uppsalaåsen stretches for 250 km (156 miles) and passes through Uppsala city.

The village of Kemnay in Aberdeenshire, Scotland has an excellent 5km esker locally called the Kemb Hills.

The Mason Esker, about 22 miles long, is one of the longest eskers in the USA.[1] It is in Mason, Michigan. It stretches from DeWitt through Lansing and Holt, ending in Mason. Esker systems in the U.S. state of Maine can be traced for up to 100 miles[2].

Roads are sometimes built along eskers to save expense. Examples include the Denali Highway in Alaska, the Trans-Taiga Road in Quebec, and "The Airline" (Route 9) in Maine[3]. There are numerous long eskers in the Adirondack State Park in upstate New York.

See also

• Glacial landforms
• Glacier morphology
• Eskers Provincial Park (British Columbia, Canada)

References

• Quin, E. G. (gen. ed.) (1983). Dictionary of the Irish Language. Dublin: Royal Irish Academy. pp. 281. ISBN 0 901714 29 1. 

• Easterbrook, D.J. (1999). Surface Processes and Landforms. New Jersey: Prentice Hall. pp. 352. ISBN 0 13 860958 6. 

• Trenhaile, Alan (2007). Geomorphology: A Canadian Perspective. Don Mills, Ontario: Oxford University Press. pp. 188–191. ISBN 0-19-542474-3. 

External links

Wikimedia Commons has media related to: Esker

• Oblique aerial photo of an esker in Waterford, Maine
• Larry Gedney, "Eskers: The Upside-Down Riverbeds," Alaska Science Forum Article #674, August 1, 1984.
• Esker Riada Webpage
• Diagram illustrating (i) tunnel in glacier before retreat of ice, forming (ii) meandering esker in The Ice Melts: Deposition on page 6 of "Pennsylania and the Ice Age" published 1999 by PA DCNR Bureau of Topographic and Geologic Survey
• The Bridgenorth Esker: geomorphology and sedimentology