An esker, eskar, eschar, or os, sometimes called an asar, osar, or serpent kame, 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 railway embankments.
The term esker is derived from the Irish word eiscir (Old Irish: escir), which means "ridge or elevation, especially one separating two plains or depressed surfaces". 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 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 Dublin-Galway road
The synonym os comes from the Swedish word for an esker, ås.
Most eskers are argued to have formed within ice-walled tunnels by streams which 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. 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.
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. 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.
Eskers are critical to the ecology of Northern Canada. Several plants that grow on eskers, including bear root and cranberries, are important food for bears and migrating waterfowl; animals from grizzly bears to tundra wolves to ground squirrels can burrow into the eskers to survive the long winters.
The village of Kemnay in Aberdeenshire, Scotland has a 5 km esker locally called the Kemb Hills.
There are over 1,000 eskers in the state of Michigan, primarily in the south-central Lower Peninsula. The longest esker in Michigan is the 22 mile long Mason Esker, which stretches south-southeast from DeWitt through Lansing and Holt, before ending near Mason.
Esker systems in the U.S. state of Maine can be traced for up to 100 miles. 
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 and Calais.  There are numerous long eskers in the Adirondack State Park in upstate New York.
Brunkebergsåsen was an esker that once reached over much of Stockholm's Norrmalm district. Geologically, it is a part of the much larger Stockholmsåsen. It formed a considerable obstacle to traffic, effectively dividing Norrmalm into a western and an eastern part. Consequently, most of it has been dug away over the centuries to make room for the development of that district. The pedestrian tunnel Brunkebergstunneln and, since the 1910s, the eastern part of Kungsgatan cut through Brunkebergsåsen's southern part. Conspicuous remnants of the esker can be seen in the vicinity of Johannes kyrka, at Observatorielunden, and Vanadislunden.Cochrane District
Cochrane District is a district and census division in Northeastern Ontario in the Canadian province of Ontario. It was created in 1921 from parts of Timiskaming and Thunder Bay districts.
In 2016, the population was 79,682. The land area of this district is 141,270.41 square kilometres (54,544.81 sq mi), making it slightly smaller than the US State of Michigan and the second largest district in Ontario after Kenora District. The district seat is Cochrane.
Bennet Lake Esker Kame Complex Conservation Reserve is located in Cochrane District.Doon, County Offaly
Doon (Irish: An Clochán) is a small hamlet in the north-west corner of County Offaly, Ireland. The Doon landscape is dominated by the ruins of the castle of Esker, which stands on a sandy ridge north of Doon crossroads. A short distance to the east of the crossroads stands the ruin of an ancient building know locally as 'the monastery'. It was the private religious house attached to the castle. The hamlet today thrives principally thanks to its friendly, mostly farming community, whose needs are serviced in no particular order by the pub "Martins" of Doon and by its Gaelic football pitch, the home pitch of Doon GAA, and training ground for those within the surrounding catchment area.Esker, Iran
Esker (Persian: اسكر, also Romanized as ‘Askar; also known as ‘Asgar) is a village in Rabor Rural District, in the Central District of Rabor County, Kerman Province, Iran. At the 2006 census, its population was 2,080, in 494 families.Esker, Wisconsin
Esker is an unincorporated community in Portage County, Wisconsin, United States.Esker Riada
The Esker Riada (Irish: Eiscir Riada) is a system of eskers that stretch across the middle of Ireland, between Dublin and Galway.Glenamaddy
Glenamaddy (Irish: Gleann na Madadh) is a small town in County Galway, Ireland. It lies at the crossroads where the R362 and R364 regional roads meet. Glenamaddy became the musical capital of Connacht during the 1960s when the showband craze swept the country.
To the east of the town lies Loch Lurgeen, a raised bog. The origins of the parish lie in the village of Boyounagh, which lies to the northwest of Glenamaddy.Great Esker Park
Great Esker Park consists mostly of a geological formation known as an esker (a winding ridge of stratified sand and gravel) formed by a glacier 12,000 years ago. It is along the Weymouth Back River in Weymouth and across the river from Bare Cove Park in Hingham.The park features the highest esker in North America (90 ft), reversing falls, salt marshes, six miles (10 km) of trails, a playground and picnic area. Bird watching is a popular activity as osprey, great blue heron, red-tailed hawks, owls, and other species of birds are abundant seasonally. There are man-made shelters on the river where Osprey nest in the spring and summer months.
In the summer months the park is used by Wey-Rec for children's programs.
The main parking area is located at the end of Elva Road with additional parking off of Bridge Street (3A) and Puritan Road.Hunstanton Park Esker
Hunstanton Park Esker is a 17.3-hectare (43-acre) geological Site of Special Scientific Interest east of Hunstanton in Norfolk. It is a Geological Conservation Review site.This is a 1.5-kilometre (1-mile) esker, a long winding ridge of stratified sand and gravel dating to the glacial Devensian period, between 115,000 and 11,700 years ago. This is an uncommon landform in central and southern England.It is private land and there is no public access.Keese Mill, New York
Keeses Mills, a.k.a. Keeses Mill and rarely Keese Mill, is a hamlet west of Paul Smiths, New York in the Town of Brighton, Franklin County, New York in the Adirondacks. It is named for a sawmill that was located on the Keeses Mills dam on the Saint Regis River. Keeses Mills road, which starts at Paul Smiths, is the only road in the hamlet; it provides access to Black Pond and Long Pond, trails to Saint Regis and Jenkins Mountains and the Saint Regis Esker Trail, and the middle branch of the Saint Regis River.Mount Pelly
Mount Pelly or Pelly Mountain (Inuinnaqtun: Ovayok or Uvajuq) is an esker in Kitikmeot, Nunavut. It is located in the Canadian North on Victoria Island within the Ovayok Territorial Park. The hill, which is more than 200 m (660 ft) high, is located 15 km (9.3 mi) north east of the hamlet of Cambridge Bay, Nunavut.Ontario Highway 672
Secondary Highway 672, commonly referred to as Highway 672, is a provincially maintained secondary highway in the Canadian province of Ontario. The 47.6-kilometre (29.6 mi) route lies within Timiskaming and Cochrane district, connecting Highway 66 — part of the Trans-Canada Highway — in the south with Highway 101 in the north. It is the only highway to provide access to Esker Lakes Provincial Park. Though the highway was first assumed by the province in 1990, the existing road had been built north from Highway 66 to the provincial park in 1977 and extended to Highway 101 in the late 1980s.Sassen–Bünsow Land National Park
Sassen – Bünsow Land National Park (Norwegian: Sassen–Bünsow Land nasjonalpark) lies on Spitsbergen island in the Svalbard archipelago, Norway. The park was opened in 2003 and includes both glaciers and several glacially carved valleys. The sealer/whaler Hilmar Nøis built the hunting station Fredheim on the north side of the Sassen river outlet in this area. One of Spitsbergen's highest waterfalls lies in Eskerdalen (Esker Valley) inside the park.Siegenthaler-Kaestner Esker State Nature Preserve
Siegenthaler-Kaestner Esker is a 37-acre (150,000 m2) state nature preserve located in Champaign County, Ohio, United States. It contains an esker, kame, and kettle, all glacial landforms.South Dublin County Council
South Dublin County Council (Irish: Comhairle Contae Baile Átha Cliath Theas) is the authority responsible for local government in the county of South Dublin, Ireland. It is one of three local authorities that comprised the former Dublin County Council before its abolition and one of four councils in the Dublin Region. As a county council, it is governed by the Local Government Act 2001. The council is responsible for housing and community, roads and transportation, urban planning and development, amenity and culture, and environment. The council has 26 elected members. Elections are held every five years and are by single transferable vote. The head of the council has the title of Mayor. The county administration is headed by a Chief Executive, Daniel McLoughlin. The county town is Tallaght, with a civic centre at Monastery Road, Clondalkin. It serves a population of approximately 192,000.
The council is the third largest local authority in Ireland with a population of 265,205 (Census 2011), 90,000 households, and 6,000 businesses, covering an area of 222.74 square kilometres. There are 183,336 local government electors and 174,349 Dáil electors registered to vote in the County Council administrative area.Stockton, Wisconsin
Stockton is a town in Portage County, Wisconsin, United States. The population was 2,896 at the 2000 census. The unincorporated communities of Arnott, Custer, Fancher, Esker, and Stockton are located in Stockton.Weymouth Back River
The Weymouth Back River, sometimes called Back River, is a short, primarily tidal river in Hingham and Weymouth, Massachusetts, about 10 miles (16 km) south of Boston. It arises from a number of tributaries in ponds and swamps, most notably Whitmans Pond, flows northward, and empties into Hingham Bay (just south of Grape Island and Slate Island).Weymouth Back River Reservation
Weymouth Back River Reservation is a protected coastal reservation in Hingham and Weymouth, Massachusetts. It contains parks on the west and east sides of the northern end of Weymouth Back River. On the west side in Weymouth, Abigail Adams Park is adjacent to and north of Route 3A Bridge and Great Esker Park is south of the bridge. On the east side in Hingham, Stodder's Neck is north of the bridge and Bare Cove Park is south of the bridge. It features Weymouth Back River views, walking trails and landscaped areas.The reservation is part of the Metropolitan Park System of Greater Boston.Wiveton Downs
Wiveton Downs is a 28.9-hectare (71-acre) biological and geological Site of Special Scientific Interest west of Sheringham in Norfolk. Part of it is a Geological Conservation Review site, and an area of 6.5-hectare (16-acre) is a Local Nature Reserve It is in the Norfolk Coast Area of Outstanding Natural Beauty.This is a classic example of an esker, a glacial crevasse which has been filled in until it forms a narrow winding ridge. It has been very important for teaching, research and demonstration.