If the cirque is subject to seasonal melting, the floor of the cirque most often forms a tarn (small lake) behind a dam, which marks the downstream limit of the glacial overdeepening.- Cirque (Wikipedia)
A cirque (French: [siʁk]; from the Latin word circus) is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from Scottish Gaelic: coire, meaning a pot or cauldron) and cwm (Welsh for ‘valley’; pronounced [kʊm]). A cirque may also be a similarly shaped landform arising from fluvial erosion.