An esker is a narrow, winding ridge of stratified gravel deposited by meltwater streams within or beneath a glacier.
As the glacier retreats and its ice walls melt away, the stream sediment remains as a raised ridge. Eskers commonly contain sorted layers of sand and gravel, unlike glacial till, which is usually an unsorted mixture of clay, sand, gravel, and boulders.
The name comes from the Irish word eiscir, meaning a ridge or elevated divide. Ireland's Eiscir Riada is a famous example, extending roughly 200 kilometres across the country from Dublin toward Galway. Some eskers in North America and Scandinavia reach hundreds of kilometres.
Eskers are easy to confuse with kames and moraines. A kame is generally a mound or short ridge formed by sediment accumulating against or on melting ice, while a moraine is a debris ridge deposited directly by glacial ice. A drumlin is an elongated hill molded beneath moving ice, not a meltwater deposit.