How do cirques form small lakes called tarns?

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Snow-capped, rugged Atlas Mountains encircle a green valley with scattered trees, terraced fields, and a few small buildings in Morocco. — valley The rugged Atlas Mountains surround a valley in Morocco.

A cirque is an amphitheatre-shaped basin formed at the head of a glacial valley through erosion. The process begins with a glacier scouring the bedrock at the base of the cirque using rock debris that has fallen into the bergschrund—a large crevasse separating the stationary ice from the moving glacier.12

As the glacier moves, these rocks are incorporated into the ice and act as tools that abrade and polish the floor of the cirque. This action often creates a shallow, concave basin that may be over-deepened by several tens of metres. When the glacier eventually retreats or disappears, this scoured basin can fill with water to form a small mountain lake known as a tarn.21