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