Tarns

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Tarns: The two tarns (lakes) in this image result from "ice cream scoop" like or rotational aspect of glacial erosion. Glacial ice moves down slope parallel to the valley floor surface. But the ice also rotates from where it is deposited in the accumulation zone to where it finally melts in the ablation zone.

The rotation means that the oldest ice on a glacier is currently melting in the ablation zone, and is "upside down" in terms of how it was first laid down as snow long ago. The rotational aspect of glaciers carve out depressions in their mountain valleys, and after the glaciers are gone, the depressions fill with water forming Tarns.

Tarns are most common inside glacial cirques (sometimes called corries). Click here to see an animation of how a tarn forms in a glacial cirque.