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How does frost action help form cirque headwalls?

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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.

Frost action plays a critical role in the development of cirque headwalls through a process of mechanical weathering.12

As a glacier forms at the head of a valley, a large crevasse known as a bergschrund often develops, separating the moving ice from the stationary rock wall. During early summer, this crevasse opens, exposing the underlying bedrock to significant diurnal temperature fluctuations.12

These temperature changes facilitate frost action, where water enters cracks in the rock, freezes, and expands, causing the rock to shatter. This rapid disintegration of the lower rock face leads to the collapse of the rock above, which avalanches down and contributes to the formation of an almost vertical headwall. The resulting rock debris is then incorporated into the glacier, where it serves as abrasive material that helps the glacier scour and deepen the cirque basin.123