Editing Esker (section)

==Geology==
Most eskers are argued to have formed within ice-walled tunnels by streams that flowed within and under 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. 

Eskers may also form above glaciers by accumulation of sediment in [[Jökulhlaup#Supraglacial and subglacial water flow|supraglacial channels]], in [[crevasse]]s, 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.<ref name=Easterbrook>{{cite book
  | last =Easterbrook
  | first =D.J.
  | title =Surface Processes and Landforms
  | publisher =Prentice Hall
  | year =1999
  | location =New Jersey
  | pages =352
  | isbn = 0-13-860958-6 }}</ref>

[[File:Esker-PB110050.JPG|thumb|Esker in [[Sims Corner Eskers and Kames]] National Natural Landmark, Washington, US. (Trees at the edge of the esker and the single lane road crossing the esker to the right of the photo 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.<ref name=Easterbrook/> They can reach hundreds of kilometers in length and are generally {{convert|20|–|30|m|ft|abbr=on}} 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.<ref>Shreve, R.L., 1985, Esker characteristics in terms of glacier physics, Katahdin esker system, Maine: GSA Bulletin, v. 96, pp. 639–646.</ref>

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 (geology)|Bedding]] may be irregular but is almost always present, and [[cross-bedding]] is common.<ref name=Easterbrook/>

There are various cases where [[inland dune]]s have developed next to eskers after [[deglaciation]].<ref name=Sepp2004>{{Cite book
  | title = Wind as a Geomorphic Agent in Cold Climates
  | first = Matti 
  | last = Seppälä
  | author-link=Matti Seppälä
  | publisher = Cambridge University Press
  | year = 2004 
  | isbn = 9780521564069
  | chapter= Accumulation
  | pages =207–208 
}}</ref> These dunes are often found in the leeward side of eskers, if the esker is not oriented parallel to prevailing winds.<ref name=Sepp2004/> Examples of dunes developed on eskers can be found in both [[Lapland (Sweden)|Swedish]] and [[Lapland (Finland)|Finnish Lapland]].<ref name=Sepp2004/><ref>{{cite journal |last=Seppälä |first=Matti |author-link=Matti Seppälä |date=1972 |title=Location, morphology and orientation of inland dunes in northern Sweden |journal=[[Geografiska Annaler: Series A, Physical Geography]] |volume=54 |issue=2 |pages=85–104 |doi= 10.1080/04353676.1972.11879860}}</ref>

Lakes may form within depressions in eskers. These lakes can lack surface outflows and inflows and have drastic fluctuations over time.<ref>{{Cite book|title=Källor i Sverige|last=Källakademin|publisher=Svensk Byggtjänst|year=2006|isbn=91-7333-104-X|location=Sundbyberg|pages=196|language=Swedish}}</ref>