Glacial action forming a cirque which may host a tarn
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.
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 closely surrounding it or actually growing into the tarn.