cirque in
Rila Mountain,
Bulgaria Glacial-erosion 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 is often 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 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. If two adjacent cirques erode toward one another, an
arête, or steep sided ridge, forms. When three or more cirques erode toward one another, a
pyramidal peak is created. In some cases, this peak will be made accessible by one or more arêtes. The
Matterhorn in the European
Alps is an example of such a peak. Where cirques form one behind the other, a
cirque stairway results, as at the
Zastler Loch in the
Black Forest. As glaciers can only originate above the snowline, studying the location of present-day cirques provides information on past glaciation patterns and on climate change.
Fluvial-erosion Although a less common usage, the term cirque is also used for amphitheatre-shaped, fluvial-erosion features. For example, an approximately anticlinal erosion cirque is at on the southern boundary of the
Negev highlands. This erosional cirque or '''' was formed by intermittent river flow in the
Makhtesh Ramon cutting through layers of limestone and chalk, resulting in cirque walls with a sheer drop. The
Cirque du Bout du Monde is another such feature, created in
karst terraine in the
Burgundy region of the department of in
France. Yet another type of fluvial erosion-formed cirque is found on
Réunion island, which includes the tallest volcanic structure in the
Indian Ocean. The island consists of an active shield-volcano () and an extinct, deeply eroded volcano (
Piton des Neiges). Three cirques have eroded there in a sequence of agglomerated, fragmented rock and volcanic
breccia associated with
pillow lavas overlain by more coherent, solid lavas. A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded. with the
Lhotse face of
Mount Everest in the background ==Notable cirques==