A tarn (or corrie loch) is a mountain lake or pool, formed in a cirque excavated by a glacier. A moraine may form a natural dam below a tarn

https://en.wikipedia.org/wiki/Tarn_(lake)

By DooFi 14:03, 18 May 2008 (UTC) - Own work based on this and this, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=4070165

Di JJ Harrison (https://www.jjharrison.com.au/) - Opera propria, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8689844