Cirque and Tarn

These images are from the Alpine Quadrangle in Montana.  The three sided bowl shaped feature in the center of the above image is called a cirque.   A cirque is formed by ice and denotes the head of a glacier.  As the ice goes melts and thaws and progressively moves downhill more rock material is scoured out from the cirque creating the characteristic bowl shape.

Many cirques are so scoured that a lake forms in the base of the cirque once the ice has melted.  These lakes are called tarns.  In the image below the tarn can be seen in the base of the cirque.