Explanation:

Esker – A narrow, sinuous ridge of partly stratified coarse sand and gravel of glaciofluvial origin.

Most eskers are argued to have formed within ice-walled tunnels by streams that flowed within and under glaciers. They tended to form around the time of the glacial maximum, when the glacier was slow and sluggish. After the retaining ice walls melted away, stream deposits remained as long winding ridges.