Cirque or Cirque Lake: a glacier-carved bowl or amphitheatre in the mountains.
To form, the glacier must be a combination of size, a certain slope and more unexpectedly, a certain angle away from the sun. In the northern hemisphere, this means the glacier must be on the northeast slope of the mountain, away from the suns rays and the prevailing winds. Thick snow, protected in this way, grows thicker into glacial ice, then a process of freeze-thaw called nivation, chews at the lower rocks, hollowing out a deep basin. Eventually a magnificently circular lake is formed with steep sloping sides all around.