Cirque

A cirque is a special kind of valley shaped like an amphitheater. It is formed when glaciers erode the land over many years. You can find cirques in mountainous areas where glaciers used to flow. They look like giant bowls carved into the mountainside.

Two cirques with semi-permanent snowpatches near Abisko National Park, Sweden

Cirques have steep walls on three sides and are open on the fourth side, where the glacier once flowed downhill. The bottom of a cirque is often very deep because the moving ice and rocks grind away the earth. Sometimes, after the glacier melts away, a small lake called a tarn forms in the bottom of the cirque.

Besides glacial cirques, there are also cirques formed by rivers. These are called fluvial cirques or makhtesh. They appear in landscapes made of limestone and chalk, where rivers cut through the rock layers, leaving steep cliffs behind. All cirques, whether made by ice or water, are interesting examples of how nature shapes the land.

Formation

Glacial-erosion

Glacial cirques are special bowl-shaped valleys found in mountains all around the world. They are usually about one kilometer long and wide. These cirques are often high up on a mountainside, partly surrounded by steep cliffs. The highest cliff is called a headwall. The open side, where glaciers flow away, is known as the lip or sill.

When enough snow collects in a cirque, it can turn into glacial ice. This ice slowly moves and carves away at the rock, making the cirque bigger over time. As the ice continues to erode the mountain, the cirque grows deeper and wider. Two nearby cirques can erode toward each other, forming a narrow ridge called an arête. If three or more cirques erode toward a common point, they can create a sharp, pyramid-shaped peak. The Matterhorn in the Alps is a famous example.

Maritsa cirque in Rila Mountain, Bulgaria

Eventually, the hollow can grow into a large bowl shape on the mountain's side. The headwall is worn down by ice and erosion. If several cirques form one after another, it creates a series of steps called a cirque stairway.

Fluvial-erosion

The Lower Curtis Glacier in North Cascades National Park is a well-developed cirque glacier; if the glacier continues to retreat and melt away, a lake may form in the basin

Further information: Steephead valley and Makhtesh

Sometimes, the word cirque is also used for valleys shaped by rivers instead of ice. For example, there is a large cirque in the Negev highlands formed by river flow cutting through layers of rock. Another example is the Cirque du Bout du Monde in France, shaped in a type of rock called karst. On Réunion island, there are cirques formed in volcanic rock.

All of these river-shaped cirques share a common feature: tougher rock layers on top of easier-to-erase materials below.