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.
The rate of 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).
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.