Definition of "esker"

Another unusual ice-contact feature is an esker, a long, narrow, steep-sided ridge of glaciofluvial sand and gravel inhabiting a glaciated area. Eskers tend to follow valleys and lowlands, carefully picking a course between obstacles.

1988, Robert Phillip Sharp, Living Ice: Understanding Glaciers and Glaciation‎, page 149

Another objection to the tunnel-valley interpretation comes from a comparison with eskers. […] The esker rivers and the tunnel-channel rivers of Wisconsin therefore seem to have been the result of significantly different meltwater regimes.

1999, L. Clayton, J. W. Attig, D. M. Mickelson, “Tunnel channels formed in Wisconsin during the last glaciation”, in David M. Mickelson, John W. Attig, editors, Glacial Processes, Past and Present, page 77

The multiple deltas must have formed sequentially, which led Thompson (1982) to conclude that the eskers were built in successive segments.

2001, Allan D. Randall, Hydrogeologic Framework of Stratified-drift Aquifers in the Glaciated Northeastern United States, US Geological Survey Paper 1415-B, page B37