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An esker is a special sort of hill. Eskers develop underneath a glacier, so in Ohio they formed during the last ice age. The sediment that eventually creates an esker starts its life as the sand, gravel and rock deposited on the bottom of a riverbed. However the unusual thing about the associated river is that it flows under great pressure beneath a massive glacier. Instead of having normal riverbanks made of earth, this kind of river flows through a crevasse or icy tunnel at the base of a glacier.
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