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How do valley trains form from glacial meltwater deposits?

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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.

Valley trains form when meltwater streams issuing from a glacier's snout carry large amounts of debris, which they deposit as they lose velocity upon exiting the confined ice tunnels. In mountainous regions where valleys are deep enough to contain these sediments, the resulting elongate, planar deposits of sand and gravel are termed valley trains.1

As these streams emerge from the glacier, they are often laden with sediment to their full transporting capacity. Once no longer confined by ice walls, the water spreads out, loses velocity, and is forced to deposit its load. This process frequently creates a braided stream system characterized by shifting channels and bars of sand and gravel. Because the volume of deposited sediment typically decreases with distance from the ice margin, these features are often wedge-shaped in cross-section, sloping gently away from the moraine to which they are graded. If the glacier retreats and stabilizes at a new position, subsequent valley trains may form, often appearing as terraces inset into the older, higher deposits.1