I first learned the word tarn from The Fall of the House of Usher.

From the U.S. Natonal Park Service:

"Cirques are bowl-shaped, amphitheater-like depressions that glaciers carve into mountains and valley sidewalls at high elevations. Often, the glaciers flow up and over the lip of the cirque as gravity drives them downslope. Lakes (called tarns) often occupy these depressions once the glaciers retreat."