A cirque is a bowl-shaped, amphitheatre-like hollow carved into a mountainside by a small glacier. It has steep cliffs on three sides, the highest of which is called the headwall, and a lower lip on the downhill side, where the ice once flowed out. Cirques form near the snowline. Snow builds up in a sheltered hollow, usually on a north- or north-east-facing slope in the Northern Hemisphere, and turns to ice. That ice then deepens the hollow by plucking blocks from the walls and grinding the floor. When the glacier melts, the overdeepened floor often holds a small lake called a tarn. A classic cirque is about a kilometre long and a kilometre wide. In Scotland the same landform is called a corrie, from the Gaelic coire (cauldron), and in Wales a cwm, the Welsh word for valley. The French word cirque comes from the Latin circus. The examples below, from the Pyrenees to the Negev, each show a different part of how cirques form and what they leave behind.
Cirque de Gavarnie: Victor Hugo’s “Colosseum of nature” in the Pyrenees
One of the most dramatic cirques in Europe lies in the central Pyrenees, in the commune of Gavarnie in the French department of Hautes-Pyrénées, close to the Spanish border. Victor Hugo called it the Colosseum of nature, and the comparison fits. It is a horseshoe of rock about 800 m wide at its deepest point and about 3,000 m wide at the top, with walls rising up to 1,500 m above the floor. It lies within the Pyrénées National Park and became part of the Pyrénées – Mont Perdu UNESCO World Heritage Site in 1997.
Gavarnie shows every part of a textbook cirque at a huge scale. Its headwall curves round in a near-perfect horseshoe. In spring and summer, meltwater pours over the rim. The largest stream is the Gavarnie Falls, which drops about 422 m in a series of steps, with a tallest single drop of 281 m, and is the highest waterfall in mainland France. The rim is notched by passes, the largest being La Brèche de Roland at 2,800 m. Legend says the hero Roland, nephew of Charlemagne, cut it with his sword. Gavarnie’s exceptional size was most likely caused by repeated cycles of glacial erosion over millions of years, as glaciers returned to the same hollow and cut it deeper and wider. That repetition is the key to why cirques grow. A hollow that holds snow traps more snow the next winter, so the process feeds itself for as long as the climate allows.
The steep upper walls are also where cirques do their fastest work. As the glacier pulls away from the headwall, a deep crevasse called a bergschrund opens between moving and stationary ice. Meltwater running into the bergschrund is chilled by the surrounding ice and refreezes in cracks in the rock. Even small swings in temperature can shatter the rock this way, by a process known as ice segregation or frost shattering. The loosened blocks are then plucked out by the moving glacier. The headwall retreats into the mountain, while debris frozen into the base of the ice scrapes the floor like sandpaper.
Cwm Idwal, Snowdonia: a north-facing corrie and its tarn
Cwm Idwal in the Glyderau range of northern Snowdonia, Wales, is a model corrie, and it shows why aspect matters. The cwm faces north, the kind of slope that is sheltered from most of the Sun’s energy and so lets snow accumulate and turn to glacial ice. Its outflow joins the Afon Ogwen at the head of Nant Ffrancon, a steep-sided glacial valley that drops between the Glyderau and the Carneddau to Bethesda and is now climbed by the A5 road. Llyn Idwal is reached by a 900 m stone-paved path from Ogwen Cottage on that road.
The ice left a full set of glacial features. Dark crags rise around the head of the cwm, scree fans spread below them, moraines lie at the outlet, and ice-smoothed rocks line the floor. The rocks themselves were laid down as volcanic and sedimentary layers in a shallow Ordovician sea and later folded into a trough-shaped fold called the Idwal Syncline, which can still be seen in the cliffs. In the overdeepened floor lies Llyn Idwal, a tarn about 800 m long and 300 m wide. It is named after Idwal Foel, a 10th-century Welsh prince who died fighting the Saxons in 942.
The same cold, north-facing position that let the glacier form still shapes the cwm today. Cwm Idwal is the most southerly place in Britain where Arctic plants such as moss campion and tufted saxifrage grow, and it is home to the rare Snowdon lily. The Snowdonia hawkweed is known only from this cwm. Swimming is not allowed in Llyn Idwal, which is a protected environment and home to several protected species. Climbers have used the Idwal Slabs on its eastern side since they were first climbed in 1897, and in a 2005 Radio Times poll Cwm Idwal was voted the seventh-greatest natural wonder in Britain.
The Matterhorn and the Western Cwm: where cirques meet
A single cirque makes a bowl, but neighbouring cirques together make some of the world’s sharpest mountains. When two cirques cut back towards each other from opposite sides of a ridge, the rock between them is narrowed into a knife-edged ridge called an arête, or worn down into a saddle-shaped col. When three or more cirques bite into the same summit from different sides, they leave a pyramidal peak, or glacial horn.
The Matterhorn, 4,478 m high on the border between Switzerland and Italy, is the standard example. Its four steep faces point roughly north, south, east and west, and they are separated by the Hörnli, Furggen, Zmutt and Lion ridges. Its shape is the result of cirque erosion by several glaciers diverging from the peak, such as the Matterhorn Glacier at the foot of the north face, each one cutting its own headwall into the mountain. The Swiss town of Zermatt lies to the north-east and the Italian town of Breuil-Cervinia to the south.
Climbers on Everest pass through a glacial basin named after the Welsh word. The Western Cwm is a broad, flat basin at about 6,000–6,800 m that ends at the foot of the Lhotse Face. George Mallory named it in 1921, during the British reconnaissance expedition, borrowing the word from Wales. Climbers reach it through the Khumbu Icefall on the south-east route to the summit. Its bowl-shaped, snow-covered walls reflect and concentrate sunlight, which warms the basin despite its altitude. The shape of a cirque, in other words, governs its climate as well as its look.
Blue Lake, Kosciuszko: one of four cirque lakes in mainland Australia
Cirques also record where glaciers once were. Because glaciers form only above the snowline, the height of old cirques shows where the snowline lay during past ice ages. Australia, which today has no glaciers on the mainland, has only a few small glacial features. Blue Lake in Kosciuszko National Park, New South Wales, is one of only four cirque lakes on the mainland. The other three, Cootapatamba, Albina and Club, are shallower and held back entirely by moraines.
Blue Lake sits at 1,890 m, about 3.5 km north of Charlotte Pass and 28 km west of Jindabyne. It covers 16 hectares and is 28 m deep. It formed where glaciers flowing from the Great Dividing Range converged and carved a basin into the granite, and it has the best-developed glacial features in the Kosciuszko alpine area. Fed by Blue Lake Creek from Mount Twynam and by snowmelt, it stays frozen for about four months of the year. On 17 March 1996 a 320-hectare area including the lake and nearby Hedley Tarn became Ramsar Site 800, a wetland of international importance. Threatened species there include the mountain pygmy possum and the critically endangered Kosciuszko galaxias, a small fish.
Makhtesh Ramon, Negev: a cirque cut by water, not ice
The word cirque is also used, less often, for amphitheatre-shaped hollows cut by rivers. These fluvial cirques form where a hard cap rock lies over softer, more easily eroded layers. The largest is Makhtesh Ramon in Israel’s Negev desert, about 85 km south of Beersheba. It is 40 km long, 2–10 km wide and 500 m deep. Despite its appearance, it is neither an impact crater nor a volcanic one.
Its history began under the Tethys Ocean, which once covered the Negev and left behind a hump-shaped ridge as it retreated. About 5 million years ago the Arava Rift Valley formed and changed the course of the rivers, which began cutting into the softer rock inside the ridge. The floor deepened faster than the surrounding walls, exposing rocks up to 200 million years old. The small town of Mitzpe Ramon stands on the northern rim, and the hollow is now part of the Ramon Nature Reserve, Israel’s largest national park.
Similar water-cut cirques appear elsewhere. The Cirque du Bout du Monde in Burgundy’s Côte-d’Or was carved into karst, and three great cirques have been eroded into the extinct Piton des Neiges volcano on Réunion in the Indian Ocean. The glacial cirques at Gavarnie and Cwm Idwal, though, remain the classic examples. A cirque is usually only the starting point of a much larger glacial landscape, with moraines, U-shaped valleys and sometimes a drowned fjord further downstream. You can follow that sequence in our guide to glacial landforms, see what a glacier leaves at its snout in what is a moraine, and find where a cirque glacier’s valley can end up in what is a fjord.