View source for Esker

{{short description|Long, winding ridge of stratified sand and gravel associated with former glaciers}}
{{Other uses}}
{{pp-semi-indef}}
[[File:Fulufjalletesker.jpg|thumb|300px|Esker at [[Fulufjället]], western Sweden]]
[[File:Esker (PSF).png|thumb|Esker used as a hiking path]]
[[File:Billudden1.JPG|thumb|Aerial view of a partially drowned esker at [[Billudden]] in northern [[Uppland]], Sweden. The shape is modified by coastal processes.]]

An '''esker''', '''eskar''', '''eschar''', or '''os''', sometimes called an ''asar'', ''osar'', or ''serpent kame'',<ref>[http://www.collinsdictionary.com/dictionary/english/eskar Collins English Dictionary]</ref><ref>{{Cite web |url=http://encyclopedia2.thefreedictionary.com/esker |title=McGraw-Hill Dictionary of Scientific & Technical Terms |access-date=2015-06-30 |archive-url=https://web.archive.org/web/20150420032852/http://encyclopedia2.thefreedictionary.com/Esker |archive-date=2015-04-20 |url-status=dead }}</ref> is a long, winding [[ridge]] of [[stratum|stratified]] [[sand]] and [[gravel]], examples of which occur in [[glacier|glaciated]] and formerly glaciated regions of Europe and North America. They are most often formed by the deposition of sediment at the bottom of ice-walled channels within or beneath a former glacier. As a glacier retreats, the channel walls melt while the accumulated sediment remains, slumping into ridges as the only remaining evidence of the former channel path.<ref name=dkp>{{cite book|title=The Ultimate Family Visual Dictionary|publisher=[[DK (publisher)|DK Pub]]|chapter=Geology, Geography, and Meteorology|year=2012|pages=286–287|language=en|isbn=978-0-1434-1954-9}}</ref> They represent an unusual form of [[sedimentation]] where the location of the former channel is higher than the surrounding landform. Eskers are frequently several kilometres long and, because of their uniform shape, look like [[railway]] [[Embankment (transportation)|embankment]]s.<ref>{{cite web|last=Gedney|first=Larry|title=Eskers: The Upside-Down Riverbeds|url=http://www2.gi.alaska.edu/ScienceForum/ASF6/674.html|work=Alaska Science Forum Article #674|access-date=29 September 2011|date=August 1, 1984|url-status=dead|archive-url=https://web.archive.org/web/20120404024221/http://www2.gi.alaska.edu/ScienceForum/ASF6/674.html|archive-date=4 April 2012}}</ref>

==Etymology==
[[File:Denaliesker.JPG|thumb|Portions of the [[Denali Highway]] in [[Alaska]] are built on eskers.]]
The term ''esker'' is derived from the [[Irish language|Irish]] word {{Lang|ga|eiscir}} ({{Langx|sga|escir}}), which means "ridge or elevation, especially one separating two plains or depressed surfaces".<ref>{{cite book
  | last =Quin
  | first =E. G. (gen. ed.)
  | title =Dictionary of the Irish Language
  | publisher =Royal Irish Academy
  | year =1983
  | location =Dublin
  | pages =281
  | isbn = 0-901714-29-1 }}</ref> The Irish word was and is 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 ''[[Esker Riada|Eiscir Riada]]'', which runs nearly the whole width of [[Ireland]] from [[Dublin]] to [[Galway]], a distance of {{convert|200|km|mi|abbr=on}}, and is still closely followed by the main Dublin–Galway road

The synonym ''os'' comes from the Swedish word {{Lang|sv|ås}}, "ridge".

==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>

==Life on eskers==
Eskers are critical to the ecology of Northern Canada. Several plants that grow on eskers, including [[Hedysarum alpinum|bear root]] and [[cranberries]], are important food for [[bear]]s and migrating waterfowl; animals from [[grizzly bear]]s to [[Tundra wolf|tundra wolves]] to [[ground squirrel]]s can burrow into the eskers to survive the long winters.<ref>''National Geographic Almanac of Geography'', 2005, p. 155, {{ISBN|0-7922-3877-X}}</ref>

==Examples of eskers==
[[File:MasonEsker.JPG|200px|thumb|right|A part of the Mason Esker]]
[[File:Mount Pelly 1998-06-28.jpg|200px|thumb|right|Mount Pelly or Ovayok]]

===Europe===
In [[Sweden]], [[Uppsalaåsen]] stretches for {{convert|250|km|mi|abbr=on}} and passes through [[Uppsala]] city. The [[Badelundaåsen]] esker runs for over {{convert|300|km|mi|abbr=on}} from [[Nyköping]] to lake [[Siljan (lake)|Siljan]]. [[Pispala]]'s [[Pyynikki Esker]] in [[Tampere]], Finland, is on an esker between two lakes carved by [[glacier]]s. A similar site is [[Punkaharju]] in [[Finnish Lakeland]].

The village of [[Kemnay, Aberdeenshire|Kemnay]] in Aberdeenshire, Scotland has a {{convert|5|km|mi|abbr=on}} esker locally called the Kemb Hills. In [[Berwickshire]] in southeast Scotland is Bedshiel Kaims, a {{convert|3|km|mi|abbr=on|adj=mid|-long}} example which is up to {{convert|15|m|ft|abbr=on}} high and is a legacy of an ice-stream within the [[Tweed Valley]].<ref>{{cite book |last1=Stone |first1=P. |title=South of Scotland (British Regional Geology) |date=2012 |publisher=British Geological Survey |isbn=978-085272-694-5 |pages=191–92 |edition=Fourth}}</ref>

===North America===
[[Great Esker Park]] runs along the Back River in [[Weymouth, Massachusetts]], and is home to the highest esker in North America ({{convert|90|ft|m|abbr=on|order=flip}}).<ref>{{Cite web |date=2017-04-25 |title=Mountain And Glacial Landforms: What Is An Esker? |url=https://www.worldatlas.com/articles/mountain-and-glacial-landforms-what-is-an-esker.html |access-date=2022-07-13 |website=WorldAtlas |language=en-US}}</ref>

There are over 1,000 eskers in the [[U.S. state|state]] of [[Michigan]], primarily in the south-central [[Lower Peninsula]]. The longest esker in Michigan is the {{convert|22|mi|km|abbr=on|order=flip|adj=mid|-long}} Mason Esker, which stretches south-southeast from [[DeWitt, Michigan|DeWitt]] through [[Lansing, Michigan|Lansing]] and [[Holt, Michigan|Holt]], before ending near [[Mason, Michigan|Mason]].<ref>{{cite web |url=http://web2.geo.msu.edu/geogmich/eskers.html |title=Eskers |access-date=2013-12-02 |url-status=dead |archive-url=https://web.archive.org/web/20131203084415/http://web2.geo.msu.edu/geogmich/eskers.html |archive-date=2013-12-03 }}</ref>

Esker systems in the U.S. state of Maine can be traced for up to {{convert|100|mi|km|abbr=on|order=flip}}.<ref name="MaineEskers">{{cite web | url=https://www.maine.gov/dacf/mgs/explore/surficial/facts/surficial.htm | title=Surficial Geologic History of Maine | publisher=Maine Geological Survey | work=Explore Maine Geology | date=6 October 2005 | accessdate=30 August 2022}}</ref>

Thelon Esker is almost {{convert|800|km|mi|abbr=on}} long, straddling the boundary between the territories of [[Nunavut]] and [[Northwest Territories]] in [[Canada]].<ref>{{cite book|last=Gray|first=Charlotte|title=The Museum Called Canada: 25 Rooms of Wonder|year=2004|publisher=Random House|isbn=0-679-31220-X|url-access=registration|url=https://archive.org/details/museumcalledcana00gray|page=56}}</ref>

[[Uvayuq]] or Mount Pelly, in [[Ovayok Territorial Park]], the Kitikmeot Region, [[Nunavut]] is an esker.

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" segment of [[Maine State Route 9]] between [[Bangor, Maine|Bangor]] and [[Calais, Maine|Calais]].<ref>[https://web.archive.org/web/20080113053203/http://www.downeastregion.com/way_downeast.php Down East Region]</ref>

There are numerous long eskers in the [[Adirondack State Park]] in upstate [[New York (state)|New York]].  The [[Rainbow Lake (New York)|Rainbow Lake]] esker bisects the eponymous lake and extends discontinuously for 85 miles (c. 137&nbsp;km). Another long discontinuous esker extends from Mountain Pond through [[Keese Mill, New York|Keese Mill]], passing between [[Upper St. Regis Lake]] and the Spectacle Ponds, and continuing to Ochre, Fish, and Lydia Ponds in the [[Saint Regis Canoe Area|St. Regis Canoe Area]]. A 150-foot-high esker bisects the [[Five Ponds Wilderness Area]].<ref>{{Cite web|title = Sea Serpents in the Adirondacks? You Bet! |url = http://www.adirondackalmanack.com/2009/11/sea-serpents-in-the-adirondacks-you-bet.html|access-date = 2015-07-30|work=Adirondack Almanack| date=7 November 2009 }}</ref>

==See also==
* {{annotated link|Eskers Provincial Park}} 
* {{annotated link|Glacial landform}}
* {{annotated link|Glacier morphology}}
* {{annotated link|Kame}}
* {{annotated link|Tunnel valley}}

==References==
{{reflist|colwidth=30em}}
==Further reading==
* {{cite book
  | last = Trenhaile
  | first = Alan
  | title = Geomorphology: A Canadian Perspective
  | publisher = Oxford University Press
  | year = 2007
  | location = Don Mills, Ontario
  | pages = [https://archive.org/details/geomorphologycan0000tren/page/188 188–191]
  | url = https://archive.org/details/geomorphologycan0000tren/page/188
  | isbn = 978-0-19-542474-4
  }}

==External links==
{{Commons}}
* [https://web.archive.org/web/20070607011324/http://www.maine.gov/doc/nrimc/mgs/explore/surficial/facts/surf-10.htm Oblique aerial photo of an esker in Waterford, Maine]
* [http://homepage.eircom.net/~williamfinnerty/chaplefinnerty/esker/riada1.htm Esker Riada Webpage]
* Diagram illustrating (i) tunnel in glacier before retreat of ice, forming (ii) meandering esker in [https://web.archive.org/web/20070711041432/http://www.dcnr.state.pa.us/topogeo/education/es6/es6.pdf ''The Ice Melts: Deposition''],  p.&nbsp;6 of "Pennsylvania and the Ice Age" published 1999 by PA DCNR [https://web.archive.org/web/20160304002357/http://www.dcnr.state.pa.us/topogo Bureau of Topographic and Geologic Survey]
* [https://web.archive.org/web/20081204092736/http://www.bridgenorthesker.ca/ The Bridgenorth Esker: geomorphology and sedimentology]

{{glaciers}}
{{Continental Glaciations}}
{{sediment transport}}

[[Category:Eskers|*]]