An esker is a long, winding ridge of stratified sand and gravel deposited by meltwater streams flowing beneath a glacier. Eskers are significant features in glacial geomorphology, providing evidence of past glacial dynamics and meltwater processes. They are also important in hydrogeology as potential aquifers.
Reference: Shreve, R. L. (1985). “Eskers, Kames, and Subglacial Drainage Channels.” Geological Society of America Bulletin, 96(5), 639-646.