Glaciers Part 5 - Cirques, aretes and horn peaks

A cirque (or corrie) is a deep, armchair-shaped hollow high up on the mountain side. The high, steep and rocky headwall, which is frequently 1000m or higher, extends around the back and sides of the cirque. The front, however, is open and often marked by a lip of hard rock. The hollow that is left after the ice has melted forms a natural catchment area that is usually occupied by a circular tarn lake. More cirques are located on the northern and north-eastern facing sides of mountains in temperate lands of the northern hemipshere where it is easier for the snow to accumulate away from direct sunlight.

Nivation hollows from which cirques begin are widespread features in mountainous areas. They form under snow patches which grow sufficeintly to enable meltwater to penetrate crackes in the rock to provoke freeze-thaw action. The weathered material is removed by a mixture of flowing meltwater and solifluction which leaves newly exposed rock surfaces are subject to attack from frost action. As nivation hollows are deepned they become potential sites for ice accumulation and the formation of cirque glaciers.

Freeze-thaw weathering is crucial in there formation as it weakens exposed rock surfaces before they are covered by ice and then continues to attack and sharpen up all rock outcrops above the ice. Wherever water seeping under the ice reaches the headwall, such as near the base of the bergschrund (the semi-circular tensional crevasse that is usually present near the top of the cirque glacier), there are oppurtunties for frost action to operate. There is also pressure release, which leads to sheet jointing parallel to rock surfaces. These joints are important in providing weaknesses for the plucking action of moving ice to pull pieces of rock away from the back wall. Both freee-thaw and plucking provide the conditions needed for abrasion to occur. A rotational element in the movement of a cirque glacier is caused by the imbalance between great ice accumulation in the hollow and little wasting at the snout and this results in the force of the glacier being concentrated on the back of the hollow. It is here at the base of headwall that the great abrasive power of ice ice concentrated. Rotational slip not only helps to explain the great height of the back wall of the corrie, but also the presence of a rock lip on the open side where the erosional power of the cirque glacier is much less.

The cirque lip is made of bedrock and its origins are erosional. However, on some rock lips there is a capping of deposited glacial material. There are two explanations for this. One is that the moraine was deposited as the glacier retreated up valley to iits last resting point within the cirque hollow. The second is that it represents the terminal moriane for those ciruque glaciers which do not grow large enough to flow out of the source hollow.

What happens in the cirque basin is the first stage in sharpening the rounded relief from pre-glacial times. As the head walls of two cirques cut back on either side of a ridge, impressively sharp knife-edged ridge tops are created. The arete is kept sharp by frost shattering. Where three or more head walls cut back, the orginal mountain mass is reduced to a three or many sided slab of rock, of which Matterhorn is the classic example.