A tarn, also known as a corrie loch, (from the Scottish or Gaelic) is a mountain lake or pond formed in a depression excavated by glacial action. It is created when either rain, river water, or other runoff fills the basin. Sometimes glacial debris may form a natural dam below the tarn. This is known as a moraine created by rock and soil. A corrie might also be called a 'cirque', a French word meaning an arena or amphi-theatre. They have this name because the downhill side of the tarn is sometimes open and a bit lower than the upper end or side.
The word tarn is from the Old Norseword tjörn which means pond. In Scandinavian countries a tjern or tjärn, tärn or tjørn is a small natural lake, usually in a forest with flora growing close around it or growing in the lake itself.
In the upland areas of Northern England and Wales almost all of the ponds are known as tarns. The Lake District is a prime example.