‘Rock & Ice’
Glacial Landscapes | ‘Svartisen og Engebreen’ in Helgeland, Norway
Fotos : Marc Ihle 2016
The concave shape of a glacial cirque is open on the downhill side, while the cupped section is generally steep. Cliff-like slopes, down which ice and glaciated debris combine and converge, form the three or more higher sides. The floor of the cirque ends up bowl-shaped as it is the complex convergence zone of combining ice flows from multiple directions and their accompanying rock burdens: hence it experiences somewhat greater erosion forces, and is most often overdeepened below the level of the cirque’s low-side outlet (stage) and its down slope (backstage) valley. If the cirque is subject to seasonal melting, the floor of the cirque most often forms a tarn (small lake) behind a dam which marks the downstream limit of the glacial overdeepening: the dam itself can be composed of moraine, glacial till, or a lip of the underlying bedrock.
Glacial-erosion cirque formation
Glacial cirques are found amongst mountain ranges throughout the world; ‘classic’ cirques are typically about one kilometer long and one kilometer wide. Situated high on a mountainside near the firn line, they are typically partially surrounded on three sides by steep cliffs. The highest cliff often is called a headwall. The fourth side forms the lip, threshold or sill, the side at which the glacier flowed away from the cirque. Many glacial cirques contain tarns dammed by either till (debris) or a bedrock threshold. When enough snow accumulates it can flow out the opening of the bowl and form valley glaciers which may be several kilometers long.
Cirques form in conditions which are favorable; in the northern hemisphere the conditions include the north-east slope where they are protected from the majority of the sun’s energy and from the prevailing winds. These areas are sheltered from heat, encouraging the accumulation of snow; if the accumulation of snow increases, the snow turns into glacial ice. The process of nivation follows, whereby a hollow in a slope may be enlarged by ice segregation weathering and glacial erosion. Ice segregation erodes the rock vertical rock face and causes it to disintegrate, which may result in an avalanche bringing down more snow and rock to add to the growing glacier. Eventually, this hollow may become large enough that glacial erosion intensifies. The enlarging of this open ended concavity creates a larger leeward deposition zone, furthering the process of glaciation. Debris (or till) in the ice also may abrade (glacial abrasion) the bed surface; should ice move down a slope it would have a ‘sandpaper effect’ on the bedrock beneath, on which it scrapes.