What is an Esker?

• A. A dune formed by wind
• B. Steep sided winding ridge of till
• C. A layer of limestone
• D. An underground river

Answer: (B)

Eskers are long, winding ridges that form when meltwater rivers run through or under a glacier. As the ice flows/retreats, these subglacial streams carry and then deposit sand and gravel in their channels, creating a layered, well-sorted ridge that often sticks up above the surrounding landscape after the ice disappears. This is different from wind-blown dunes (which are shaped by air), a limestone layer (a rock type, not a sediment deposit), or an underground river (water that stays beneath the surface without leaving a raised, layered ridge). Eskers consist of sorted sediments laid down by water, not the unsorted debris of glacial ice (till).