An esker is a long, narrow ridge predominantly composed of sand and gravel which is somewhat sorted and stratified. These features usually appear as longitudinal forms which curve and meander similar to a stream channel. Eskers usually conform to local terrain and parallel the direction of previous ice flow. Lateral moraines in a valley trough are often mistaken for an esker, but the stratification and sorting of the material will distinguish the two. The cross section of the esker is usually a pronounced triangle rather than a convex triangle of the lateral moraine.
Eskers are believed to result from the filling of stream channels. Most evidence indicates that eskers form by deposition in subglacial channels. Some eskers do not conform to the local terrain and cross uphill over ridges indicating superglacial or englacial deposition.