Tarns are the result of small glaciers called
cirque glaciers. Glacial cirques (or 'corries') form as hollows on mountainsides near the
firn line. Eventually, the hollow in which a cirque glacier develops may become a large bowl shape in the side of the mountain, caused by weathering, by ice segregation, and as well as being eroded by
plucking. The basin will become deeper as it continues to be eroded by ice segregation and abrasion. A cirque typically will be partially surrounded on three sides by steep
cliffs, with a fourth side a form of
moraine constructed from
glacial till, which forms the
lip,
threshold or
sill, from which either a stream or glacier will flow away from the cirque. Tarns form from the melting of the cirque glacier. They may either be seasonal features as
supraglacial lakes, or permanent features which form in the hollows left by cirques in formerly glaciated areas. ==Etymology==