They may contain tarns, or small lakes, and their 
They may contain tarns, or small lakes, and their widening leads to the formation of sharp arêtes, cols, and horns. Their formation is a positive feedback loop, because shadowing by its back wall causes them to deepen. Generally resulting from erosion beneath a bergschrund, they usually hold recent moraines. For ten points, identify this bowl-shaped basin at the head of a glacial valley whose name is derived from the French word for "circus."
They may contain tarns, or small lakes, and their widening leads to the formation of sharp arêtes, cols, and horns.
{cirque}