drumlin n.

"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.