A long ridge of material deposited from meltwater streams running subglacially, roughly parallel to the direction of ice flow, and ranging from tens of metres to several hundred kilometres, as in Finland. Eskers wind up and down hill because subglacial streams are under great hydrostatic pressure, and can flow uphill. Brennan (2002) Geomorph. 32, 3–4 identifies and investigates five types of esker. Thomas and Montague (1997) Quat. Sci. Rev. 16 argue that glacial lake systems are crucial for the development of esker systems.