A tarn is a lake or pool formed in a cirque, the bowl-shaped hollow carved by a glacier. Tarns are especially common in mountain regions that were once occupied by small cirque glaciers.
A cirque develops near a mountain’s former snowline as accumulating ice erodes the rock through abrasion, plucking and weathering. Over time, the hollow becomes deeper and more amphitheatre-shaped, with steep walls on three sides. When the glacier melts or retreats, water collects in the overdeepened basin.
The basin may be sealed by a lip of bedrock or by a ridge of moraine and glacial till left behind by the ice. That natural dam helps retain the water. “Tarn” is also used more broadly for small mountain lakes, so not every lake with tarn in its name precisely matches the technical glacial definition. The word comes from Old Norse tjörn, meaning a small mountain lake or pond. A tarn is not a fjord, which is a seawater-filled glacial valley, and it is not a kettle lake, which forms when a stranded block of ice melts.