The Collaborative International Dictionary of English v.0.48:

Eschar \Es"char\ ([e^]s"k[~e]r or [e^]s"k[aum]r), n. [Ir.]
   (Geol.)
   In Ireland, one of the continuous mounds or ridges of
   gravelly and sandy drift which extend for many miles over the
   surface of the country, deposited by streams in meltwater
   channels under glaciers. Similar ridges in Scotland are
   called kames or kams. The spelling form esker is now
   the most commonly used, and the term is applied in geology to
   similar ridges created by glaciers anywhere in the world.
   Eskers vary in size and extent, but can be 100 feet high and
   up to 100 miles long. [Written also eskar and esker.]
   [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Eskar \Es"kar\, or Esker \Es"ker\ ([e^]s"k[~e]r), n. (Geol.)
   See Eschar.
   [1913 Webster]

WordNet (r) 3.0 (2006):

esker
    n 1: (geology) a long winding ridge of post glacial gravel and
         other sediment; deposited by meltwater from glaciers or ice
         sheets