ESKER, a name for long and often winding ridges of gravel and sand in regions of former glaciation, notably in Ireland, Sweden (where they are called Isar), and in the state of Maine, U.S.A. Their origin has given rise to much controversy, but they are generally considered to represent the material formerly filling the channels of streams under, within, or even upon the ice. This material is left behind as the ice melts. (See also GLACIAL PERIOD.) (R. H. RA.)