esker

thumb|300px|Esker at Fulufjället, western Sweden thumb|Esker used as a hiking path thumb|Aerial view of a partially drowned esker at Billudden in northern [[Uppland, Sweden. The shape is modified by coastal processes.]]

An esker, eskar, eschar, or os, sometimes called an asar, osar, or serpent kame, is a long, winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America. They are most often formed by the deposition of sediment at the bottom of ice-walled channels within or beneath a former glacier. As a glacier retreats, the channel walls melt while the accumulated sediment remains, slumping into ridges as the only remaining evidence of the former channel path. They represent an unusual form of sedimentation where the location of the former channel is higher than the surrounding landform. Eskers are frequently several kilometres long and, because of their uniform shape, look like railway embankments.