Eskers are formed by water flow from
glaciers. The
meltwater forms a network of tunnels that are
sub-parallel to the movement of the ice. At the
nose of the glacier, the meltwater and the
sediment load pours out and spreads into the shape of a fan.
As less and less water flows through the tunnels, the waters slow and do not transport as much sediment. The sediment eventually becomes
stuck and fills the tunnels. As a result,
ridges of sorted
sand and
gravel snake across the ground
moraine.
Eskers are
nifty because they show the direction in which meltwater from ancient glaciers flowed. In parts of the
Midwest USA, the sediment was so wet and slurry that the glacier spread out like frosting across the countryside, resulting in the flat plains we know today.
Earth: Portrait of a Planet by Stephen Marshak