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 upstrea