Esker

An esker, also called an eskar, eschar, os, asar, osar or serpent kame, is a long, winding ridge of layered sand and gravel found across regions of Europe and North America that are or once were glaciated. Eskers form from sediment that settles at the bottom of a channel cut into or beneath a glacier by meltwater; once the glacier retreats, the ice walls of the channel melt away and the sediment left behind slumps into a ridge, the only surviving trace of where the channel used to run. A typical esker is several kilometers long and 20 to 30 meters high, resembles a railway embankment in shape, and can extend for hundreds of kilometers, often broken into winding, knobby-crested segments by gaps; the Irish word eiscir, from which the English name derives, means a ridge or elevation separating two plains or low areas. Because the old channel bed now stands higher than the surrounding land, an esker is an unusual, inverted kind of sediment record compared with an ordinary river deposit.