A cirque is a bowl shaped depression carved out, in the bedrock, at the head of an alpine glacier. It is not completely visible until the glacier retreats to a new area above the cirque. When this happens the remaining depression often fills with water, from glacial runoff, snowmelt, and rain, and becomes an alpine lake.
Cirques begin in protected areas on a mountain slope where snow can accumulate more than it melts. Freeze/thaw conditions under the snow start to hollow out larger and larger holes in the slope.
Eventually this process forms a large bowl shape. As the snow continues to accumulate it becomes packed into glacial ice and begins to flow out of the bowl and down the mountain using the freeze/thaw debris to further gouge out the hole through the glacial process of abrasion.
When the ice flows out over the lip of the bowl, it cracks, forming a large
known as a bergshrund, which separates the ice in the cirque from the rest of the glacier downslope.