Answer:
An esker is a type of hill made up of glacial debris (sort of a moraine). They are generally found in areas that have had continental glaciation (past ice ages) and differ from the moraines seen at the edges of glacier ice sheet in that they are elongated in the direction of ice sheet flow. Esker is from old Irish meaning "ridge". A esker is commonly long and sinuous.
see : http://en.wikipedia.org/wiki/Esker