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.

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.

2001, Allan D. Randall, Hydrogeologic Framework of Stratified-drift Aquifers in the Glaciated Northeastern United States, US Geological Survey Paper 1415-B, page B37, The multiple deltas must have formed sequentially, which led Thompson (1982) to conclude that the eskers were built in successive segments.