How do tarn lakes form in cirques?

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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.

Tarn lakes form within cirques—amphitheatre-shaped basins carved into mountainsides by glaciers—through a process of glacial erosion.12

As a glacier occupies the head of a valley, it creates a bergschrund, a deep crevasse between the moving ice and the rock headwall. Frost action and ice wedging cause the headwall to disintegrate, providing rock debris that the glacier incorporates into its base. This debris acts as a tool, allowing the glacier to scour and abrade the bedrock floor of the cirque, often creating a concave, over-deepened basin.23

When the glacier eventually retreats or disappears, this scoured-out basin remains. If the depression is deep enough to collect and hold water, it forms a tarn, a small, typically steep-walled mountain lake.13