A small lake occupying a hollow eroded out by ice or dammed by a moraine; especially common in cirques (q.v.).

Two tarns on the Glyderau range, Snowdonia, North Wales, both occupying cirques. The tarn on the left is Llyn Bochllwyd and that in the centre is Llyn Llyn Idwal ("llyn" is Welsh for "lake"). Photo M. J. Hambrey.