A small round lake high up in a mountain area. The lake fills a bowl cut during the Ice Age.

A round rock-basin lake in the Himalayas.

During an Ice Age, the hollows in the flanks of a mountain that once had the headwaters of rivers, become full of snow. The snow compacts into ice. The ice is heavy and begins to slip down the mountainside. As it does so, bits of broken rock stick to its underside and are used like sandpaper to scour the rock.

The scouring action changes the natural hollow into a deeper bowl shape. When the Ice Age ends and the ice melts away, water fills the bowl, creating a small lake.

There are many regional names for these lakes. The word tarn is used in the Lake District of England. Corrie lake is used in Scotland. Cwm is used in Wales. Cirque is used in France.