Encyclopedia Britannica:
esker
also spelled ESKAR, OR ESCHAR, a long, narrow, winding ridge composed of stratified sand and gravel deposited by a subglacial or englacial meltwater stream. Eskers may range from 16 to 160 feet (5 to 50 m) in height, from 160 to 1,600 feet (500 m) in width, and a few hundred feet to tens of miles in length. They may occur unbroken or as detached segments. The sediment is sorted according to grain size, and cross-laminations that show only one flow direction commonly occur. Thus eskers are considered to be channel deposits (left by streams that flowed through tunnels in and below the ice) that were let down onto the ground surface as the glacier retreated. Esker formation presumably takes place after a glacier stagnates, because movement of the ice would likely spread the material and produce ground moraine.
Mayhew, A Dictionary of Geography:
esker
A long ridge of material deposited from metlwater streams running subglacially, roughly parallel to the direction of ice flow. Eskers range in size from tens of metres, as in north-east Scotland, to several hundred kilometres, as in Finland, and wind up and down hill across the landscape because subglacial streams are under great hydrostatic pressure, and can flow uphill. Since eskers were formerly river beds, they have typical stream channel bedforms, with ripples and dunes. Nearly all eskers have bedding which is slumped at the sides, indicating that the stream was contained within ice walls which then melted. Some have an anastomosing pattern, while others are single features. In many cases, mounds occur along the length of the feature, perhaps where a temporary delta formed.
http://unabridged.merriam-webster.comMain Entry: es·ker
Variant(s): also es·kar or es·char
Function: noun
Etymology: Irish Gaelic eiscir ridge
: a long narrow often sinuous ridge or mound of sand, gravel, and boulders deposited between ice walls by a stream flowing on, within, or beneath a stagnant glacier -- compare KAME
