Erosion by the action of valley glaciers. When the
glacier melts and retreats, it leaves a topography characterized by a broad,
flat-bottomed valley with a river meandering in it. At the edges of the valley,
waterfalls (hanging valleys) flow down into the valley. At the head of the
valley, where there was once the snowfield that produced the glacier, is a deep,
bowl-shaped depression call a cirque. It may be occupied by the kind of lake
called a tarn. The sharp peaks between cirques are called horns; a ridge between
cirques is an arete. The floor of the valley contains erratic boulders
and various
kinds of moraines.