78
Climatic Geomorphology
cirque enlarges fundamentally by the action of congelifraction and deepens by the effect of
abrasion. The speed of erosion in cirques obtained by different methods is of the order of
500 mm/ka (Benn and Evans, 1998). It seems that the backward movement is faster than
the excavation. On the other hand, the elevations of the bottoms of the cirques have
apparently not changed much during glaciation. Elevation, generally, is used to calculate
approximately the level of permanent snows, because it corresponds with the 0~ isotherm
in summer, in considering the lowest cirques of equal orientation (Flint, 1971).
The origin of cirques, according to most authors, is tied first to the snow accumulation
in a preexisting depression in which congelifraction processes and particle evacuation take
place by snow melting in summer (nivation) (Thorn and Hall, 1980; Evans, 2004c). This
brings as a consequence the widening of the depression and the generation of a nivation
niche. If excavation is deep enough, snow lasts from 1 year to another in the nivation
niche, becoming firn and also turning this material into ice. In this way an embryonic
cirque is formed. The nival accumulation stage is denoted as the "incipient phase" and
when snow begins to last, the "tim phase" (Tricart and Cailleux, 1962).
Once formed, if the circumstances are appropriate, the cirque continues its
development. Its evolution is influenced by different causes. The duration and number
of glaciations exert, without a doubt, a fundamental role in their development. Also, the
glacier has to evolve differently if it is located in temperate and subpolar latitudes, or in
polar areas. Another great incidence variable is the structure and lithology of the rocky
massif on which the cirque is based (Embleton, 1979). Sometimes cirques are located in
relief produced by lithological differences, which in a monoclinal series can give origin to
stepped cirques. The degree of fracturing and the spacing between joint planes play a
fundamental role in mechanical disintegration by gelifraction. Exfoliation can be of
importance in the form of the cirque. Another basic factor in the development of cirques is
their orientation. In middle latitudes of the northern hemisphere, most cirques presenting
good development face towards the north and east, because in that direction they are
protected from solar rays and the snow lasts more easily. One of the analysed factors is the
influence of climate in the variation of the cirque (Derbyshire and Evans, 1976). These
authors indicate that it is not easy to establish generalizations on this aspect, in spite of the
great number of regional studies on form and distribution of cirques.
The development of cirques also includes a backward movement through headward
erosion of its walls, which at the limits with other cirques (Fig. 3.14) can cause knifeedged ridges known as arYtes. The disappearance by growth of the cirque of these ar~tes
can give rise to coalescent cirques (Fig. 3.15). The formation of pyramidal peaks or horns
(Figs 3.16 and 3.17), usually presenting three or four faces, is a result of the backward
movement of the multiple cirque walls.
2.5. Glacial valleys
Deep glacial valleys are one of the most characteristic forms of erosive activity of glaciers.
In contrast, in fluvial valleys, rivers are only in contact with a small part of the valley,
whereas the ice of a glacial valley occupies a greater part of the cross-sectional profile
(Price, 1973). These canalised ice masses excavate deep troughs to hundreds or even
thousands of metres of depth (Figs 3.18 and 3.19). This intense erosion is mainly carried
Climatic Geomorphology
cirque enlarges fundamentally by the action of congelifraction and deepens by the effect of
abrasion. The speed of erosion in cirques obtained by different methods is of the order of
500 mm/ka (Benn and Evans, 1998). It seems that the backward movement is faster than
the excavation. On the other hand, the elevations of the bottoms of the cirques have
apparently not changed much during glaciation. Elevation, generally, is used to calculate
approximately the level of permanent snows, because it corresponds with the 0~ isotherm
in summer, in considering the lowest cirques of equal orientation (Flint, 1971).
The origin of cirques, according to most authors, is tied first to the snow accumulation
in a preexisting depression in which congelifraction processes and particle evacuation take
place by snow melting in summer (nivation) (Thorn and Hall, 1980; Evans, 2004c). This
brings as a consequence the widening of the depression and the generation of a nivation
niche. If excavation is deep enough, snow lasts from 1 year to another in the nivation
niche, becoming firn and also turning this material into ice. In this way an embryonic
cirque is formed. The nival accumulation stage is denoted as the "incipient phase" and
when snow begins to last, the "tim phase" (Tricart and Cailleux, 1962).
Once formed, if the circumstances are appropriate, the cirque continues its
development. Its evolution is influenced by different causes. The duration and number
of glaciations exert, without a doubt, a fundamental role in their development. Also, the
glacier has to evolve differently if it is located in temperate and subpolar latitudes, or in
polar areas. Another great incidence variable is the structure and lithology of the rocky
massif on which the cirque is based (Embleton, 1979). Sometimes cirques are located in
relief produced by lithological differences, which in a monoclinal series can give origin to
stepped cirques. The degree of fracturing and the spacing between joint planes play a
fundamental role in mechanical disintegration by gelifraction. Exfoliation can be of
importance in the form of the cirque. Another basic factor in the development of cirques is
their orientation. In middle latitudes of the northern hemisphere, most cirques presenting
good development face towards the north and east, because in that direction they are
protected from solar rays and the snow lasts more easily. One of the analysed factors is the
influence of climate in the variation of the cirque (Derbyshire and Evans, 1976). These
authors indicate that it is not easy to establish generalizations on this aspect, in spite of the
great number of regional studies on form and distribution of cirques.
The development of cirques also includes a backward movement through headward
erosion of its walls, which at the limits with other cirques (Fig. 3.14) can cause knifeedged ridges known as arYtes. The disappearance by growth of the cirque of these ar~tes
can give rise to coalescent cirques (Fig. 3.15). The formation of pyramidal peaks or horns
(Figs 3.16 and 3.17), usually presenting three or four faces, is a result of the backward
movement of the multiple cirque walls.
2.5. Glacial valleys
Deep glacial valleys are one of the most characteristic forms of erosive activity of glaciers.
In contrast, in fluvial valleys, rivers are only in contact with a small part of the valley,
whereas the ice of a glacial valley occupies a greater part of the cross-sectional profile
(Price, 1973). These canalised ice masses excavate deep troughs to hundreds or even
thousands of metres of depth (Figs 3.18 and 3.19). This intense erosion is mainly carried