Eskers form as a result of a stream flowing on or inside a glacier.  Like ordinary streams, the stream on or in the glacier will be transporting broken rock debris as its load.  Some of this material will accumulate along the stream on the bottom of the channel as it is moved by saltation and traction.  When the glacier melts, this material, will be dropped onto the surface below forming a long, narrow, winding (snake-like) ridge added to the topography.

This esker is in southeastern Wisconsin.