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How do eskers form in tunnels beneath glaciers?

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esker: Nunavut — glacial landform Eskers, or narrow ridges of gravel and sand left by a retreating glacier, wind through western Nunavut, Canada, near the Thelon River.

Eskers are long, sinuous ridges composed of stratified sand and gravel that form through the activity of meltwater streams flowing within subglacial or englacial tunnels.12

These tunnels, located beneath or within a glacier, often carry pressurized water during periods of high discharge. As these streams travel, they transport significant amounts of debris. When the glacier stagnates and eventually retreats, the sediment that has accumulated within these confined channels is deposited onto the ground surface. Because the surrounding ice walls that once contained the stream melt away, the remaining sediment is left behind as a distinct, linear ridge.213

The resulting deposits are typically sorted by grain size, reflecting their origin as channel-bound stream deposits. While most esker segments are relatively short, some formed by large ice sheets can extend for hundreds of kilometers.123