"long, narrow ridge or hill of sand, gravel, and boulders," in areas of Ice Age glaciation and formed somehow by the movement of the ice, 1833, a diminutive of earlier drum (1725) "ridge or long, narrow hill," often separating two parallel valleys, from Gaelic and Irish druim "back, ridge." Somewhat similar to, though different in origin (probably) from, an esker, but their exact nature is not quite understood.
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Definitions of esker from WordNet
(geology) a long winding ridge of post glacial gravel and other sediment; deposited by meltwater from glaciers or ice sheets;