A tarn (or corrie loch) is a mountain lake or pool, formed in a cirque excavated by a glacier. It is formed when either rain or river water fills the cirque. A moraine may form a natural dam below a tarn. A corrie may be called a cirque.
The word is derived from the Old Norse word tjörn meaning pond. Its more specific use as a mountain lake emerges as it is the commonly used term for all ponds in the upland areas of Northern England. Here, it retains a broader use, referring to any small lake or pond, regardless of its location and origin (e.g. Talkin Tarn).
In Scandinavian languages a tjern or tjärn, tärn or tjørn is a small natural lake, often in a forest or with vegetation close around it or growing into the lake.
Robert Tarn, Mackenzie Tarn and Johnston Tarn (foreground to background), Tarn Shelf, Mt Field National Park, Tasmania, Australia. The tarns were formed by glacial scouring.