How are eskers formed?

Eskers are narrow ridges of fine sediment and gravel formed by subglacial rivers. Eskers remain long after glaciers retreat, sometimes running for several miles and standing over hundreds of feet tall.

Meltwater trickling underneath a stagnant glacier eventually forms subsurface tunnels or channels through which subglacial rivers flow. These rivers carry debris and sediment from the glacial bed. When the river exit is clogged or glacial ice falls and blocks the tunnel, the sediment carried by the river slowly builds up and back into the tunnel. When the glacier eventually retreats, it leaves behind the ridges of built up sediment.