Glacial circus

Located at the upstream of the Glaciated valleys, the glacial circuses are depressions of semicircular form, surrounded by stiff slopes, which one finds in Montagne, which is or which was occupied by a glacial storing reservoir, simple Glacier of circus or longer Glacier.

Foreign terms or buildings: “ Kar ” or “ Kessel ” (German), “ hollow ” or “ van ” (Were worth), “ oule ” (the Pyrenees), “ cwms ” (Wales), “ corry ” (Scotland).

The circuses often contain a glacial lake of umbilical point , when the circus is given up by the Glacier or a Glacier of circus when the circus is at the level of the line of glacial balance regional. It is advisable to distinguish the circuses from the snow-shaped notches, smaller and of the glacial amphitheaters, in glacial end of Vallée, which are individuals of size much larger (circus of Gavarnie).

Localization and description of the glacial circuses

The glacial circuses generally present a relationship between the height and the length varying from 1 to 3.

Localization of the circuses

The problem of the localization of the glacial circuses is double: it must take account of their presence of the limit of the persistent Neige S and of their orientation compared to the race of the sun with the top of the horizon.

In extreme cases of the permanent Snow S : The circuses are located more readily in the vicinity of the limit of the Neige S persistent, current for the functional circuses, inherited for the old circuses, i.e. around 2000 m in the the Alps (Veyret, 1967). This is probably due to the important role of melt waters of the Neige S, in the mountains subjected to thermal oscillations. Permanent snowing up depends, like general snowing up, of the temperature and the quantity of snow-covered precipitations.

On a sphere scale, the altitude of the current Neige S persistent is lower in the southern, wetter hemisphere and more than the boreal hemisphere, because of relative distribution of the grounds and the oceans.
On a zonal scale, the altitude of the current Neige S persistent is lower in the equatorial areas, wetter than the dry tropical areas. One considers that in the dry Andes, it is of 6000 m, against 4600 m in New Guinea in the zone of tropical climate wet.
With the regional scales and local, the altitude of the current persistent Neige S is lower in the high slopes directed with the adret than in the slopes ubacs.

This limit varies rather clearly in time. Thus, during higher Pléniglaciaire würmien (Würm), the limit of the persistent Neige S would be localized around 1100 m in the the Alps of North, against 800 m in the the Vosges. In the Peruvian Andes, in the vicinity of Ecuador, the diurnal oscillations are stronger than the seasonal thermal oscillations. The circuses are related to alternations of cold time with in Neige lies and of more lenient time with die Neige lies and evolution of the fine screes.

orientation of the circuses : The circuses have a close connection with the orientation compared to the race of the sun to the top of the horizon. In the Northern hemisphere, the circuses best developed are those which look towards North, where the cast iron of the Neige S is later. Another case of preferential orientation corresponds to the sensitivity to collect precipitations resulting from the wet winds. This capactity can be expressed in two manners, directly or indirectly:
Is directly and it is the general case because the Neige accumulates within the slope more sprinkled;
Is indirectly because the Neige can be driven out by the wind along the slope with the wind, the Neige accumulating with the foot of the wall located under the wind: it is the effect “snow-plow”.

In the Valley of Chamonix, one notes a dissymmetry of the glacial circuses between the Massif of Mont Blanc, looking towards the North-West and that of the Red Aiguilles, looking towards South-east. This dissymmetry is due at the same time to the orientation compared to the sun and compared to the winds of West bringing precipitations and with the “structural determinism”, the Massif of Mont Blanc is higher than the solid mass of the Red Aiguilles. In the Valley Vény, this diagram reproduces in the other direction: the active glacial circuses are localized with the adret, within the Massif of Mont Blanc, higher than the peaks of the slope ubac, whose altitude hardly exceeds 3000 Mr.

Groupings of circuses

The circuses are seldom insulated in a mountainous solid mass, and it is frequent to meet them in rather great number; one can then distinguish two principal combinations, which are mutually not excluded, the second taking over first when altitude decreases:

of the same circuses level : The circuses are about on the same level, aligned on the sides of the high summits, they are touched sometimes and certain circuses have formed undulating walls of successive niches separated by small headlands. These circuses of bigger size are related to the coalescence of simple circuses.

staged circuses : The circuses spread out along same a Vallée, at different altitudes, and a good hierarchisation of the forms is often noticed, as altitude decrease: a whole of small more or less coalescent circuses corroding the base of the tops, the forms larger but fewer dominating the principal Valley and a vast glacial amphitheater, located at the upstream end of a glacial Valley. The hierarchisation of the glacial circuses is not without pointing out the hierarchisation of the thalwegs in the torrential water collecting area.

the case of the glacial amphitheaters : The glacial amphitheater, being able to be named also end of trough or mégacirque, is defined like a vast depression with the birth of the glacial Vallée S, dug or surcreusée by the Glacier S. It is within the glacial amphitheaters that the theoretical hierarchisation of the various circuses susdécrite is observed best. They are generally well preserved in the massive, coherent and homogeneous rocks, in the Calcaire S, like the circus of the Horseshoe (the Alps of Faucigny) or the circus of Gavarnie (the Western Pyrenees) or in the Granite S, like the Vallée of the Lily to the upstream of Luchon (the central Pyrenees).

The development of the glacial amphitheaters is related to the junction of the Glacier S quaternary, even current, descended from the elementary circuses located upstream. Some of these circuses can be occupied by lakes. The thawed amphitheaters often have on their impressive walls, of high cascades (cascades of Gavarnie: 426 m), which underline the steps of junction. The spectacle of the cascades is exacerbated in the circuses Calcaire S, by time of Orage. If the rocks are not sufficiently resistant or if they present a large lithological variety, with the upstream of the glacial Vallée S, the steps of junction between the perched circuses and the bottom of the trough the did not make uneven sufficient one to generate a glacial amphitheater. It is the case for high the Vallée of Ubaye]] where the petrographic range is very broad and high the Vallée S of Guil and the Acute Ewe-lamb (Queyras) where the glossed Schistes dominate the landscape.

Genesis of the glacial circuses

The genesis of the circuses proceeds sometimes of the transformation of a water collecting area Torrent iel into glacial storing reservoir. The birth of the Glacier is accompanied by the formation of rimayes, which plays a crucial role in the genesis of the glacial circuses.

Old interrogations

The altitude of the peak dominating the glacial circuses is rather constant within the same area, it is the “ oberes denudationsniveau ”, from which emergent the most resistant rocks, constituting the horns. The principal problem posed by the glacial circuses is, in fact, that of the altitude of their base. Two assumptions were a long time balances some (Péguy, 1947):

the morphological assumption : According to certain davisiens authors of the Parisian school of the years 1940-1950, the circuses would correspond to projecting ledges of preglacial erosion simply altered by snowing up.
the climatic assumption : According to the Germanic authors and the Grenoble-native school of the years 1940-1950, the base of the circuses would correspond to an old level of in Neige lies persistent. Thus, CH. - P. Péguy (1947) carried out a census of the lakes and small lakes in the high-Durance. It comes out from this study that the lakes occupy indifferently the high adrets and the tops ubacs, but it appears rather clearly which the lakes of circus - highest - low locate with ubac than with the adret, thus consolidating the assumption of the dynamic origin of the circuses.

Torrent with the Glacier

At the beginning of the glaciation, the water collecting area of a torrent can become a zone of puddle pools with Neige, Neige which is transformed then into firn then in ice.

the phase of firn : At the beginning of the cold periods, are born the snow-shaped notches with the favor from favorable conditions: a hollow badly exposed compared to the sun, allowing the conservation of the Snow all the year and/or a convergence point of corridors and cone of Avalanche S, which accumulate Neige hardened beforehand.

Of the firn to the Glacier : With the upstream of a Glacier, the passage of the firn to the glacial storing reservoir can be done in three great stages:

the motionless phase of firn: The firn will remain almost motionless as long as its thickness is not sufficient, so that its mass can overcome the coefficient of friction on the floor.
genesis of the glacial flow: When the mass is sufficient, of the slips per packages with ascending component towards the downstream in the lower part of the circus occur, with a variable intensity. In certain cases, they allow even an eradication of the rock material of the bottom of the circus.
slip by rotation: To pass to the following phase, that of the slip by rotation, in addition to the need for a circus already dug, two requirements are necessary: It is necessary, on the one hand that the movement of the sub-bases of ice is blocked by a Verrou closing the circus with the downstream, or of the considerable asperities. One needs in addition a thickness of sufficient ice, so that it can run out by plasticity.

When the circus is englacé, it thus contains three types of materials: the fresh Snow, firn and ice. In the White Valley (upstream of the Mer of Ice), one adds up 170 m thickness, including 30 m of firn, result obtained by seismic survey (Gaudet, 1973). However, the genesis of a circus starting from a torrential water collecting area is not necessary and the majority derive in fact from the evolution from a simple hollow where the Neige in hot season accumulates. The existence of circuses in the permeable rocks (Calcareous S) or very fissured (crystalline schists of the Massive of Mont Blanc) of it is the best proof.

The important role of the rimaye

These various movements occur thus under the constraint of the firn and the Glacier, in particular thanks to the differences in density between the fresh or already evolved/moved Neige, the Neige of firn and the ice. These differences in density thus allow a notable movement of the Glacier compared to the firn and a light movement of the firn. These movements thus induce broad slits, called rimayes between Glacier and firn on the one hand, between firn and rock on the other hand.

an engine: the gélifraction : The process of erosion dominating for the evolution of the glacial circus is the considerable activity of the gélifraction which is exerted along the rimaye upstream located between rock and firn, largely open during the hot season. This action is particularly intense in consequence of the abundance of the water, which deaf in summer, coming from the surface fusion as well as of the sources. It is much less intense on the abrupt rock face, which dominates the Glacier and which is subjected to direct evaporation, under the action of the sun. However, of the studies carried out in Jungfraujoch (Li, 1965) show that the temperatures of the air in the rimaye oscillate between 0 and -1°C and that only a fast variation of the temperatures (+0,1°C per minute) allows the gélifraction along the rimaye, whereas the walls upstream undergo an intense and quasi-permanent gélifraction. This occurs when the rimayes are largely opened, when the level of the Glacier is very low. It would thus seem that only the rimaye upstream is at the origin of the digging of the circus, by retreat of the walls.

other effects of the rimaye : The descent of water in summer, along the rock face has other consequences: this water can accumulate on the floor of the circus, under the ice and occupy the hollows there. They are compressed by the ice occupying the bottom of the circus, they exert on it a hydrostatic pressure and tend to make it fall apart of the bedrock, therefore to facilitate its rotational movement. The Glacier of circus can thus involve remains coming from the grounds loosened at the time of a phase separation, even of a phase of withdrawal, which explains the retreat of the latter. In fact, it seems that the evolution of the floor of the circuses façonnement even reduced no one while the walls move back actively by forming wall. Thus are born the umbilical point and the Verrou from circus: the umbilical point is located in the surcreusée zone, the firm Verrou the circus.

dynamic rimayes : According to G. Galibert (1965), the dynamic rimaye existing between the nival carapaces of slope and the glacial circus play a great part in the genesis of the circuses and in particular in the formation of notches in the walls of the circus. It is about a privileged site of attack of the cryoclastie which knows a quasi constant moisture up to 4000 m of altitude, except the winter and which is not opposed to the penetration thermal oscillations, mainly in small the rimayes, up to 10 m of depth where the diurnal thermal variations penetrate with a certain delay and increasingly attenuated if the rimaye is deep (Bozonnet, 1981).

Lithography-structural influences

Like any form of erosion, the glacial circuses occupy the structural positions most favorable. These lithography-structural influences guide erosion.

the role of the Lithology : The elementary circuses are particularly well developed in the types of rocks where the demolition of a wall by freezing gives a stiff wall and progresses quickly. The circuses are often located in zones of rocks more easily destroyed (rocks diaclasées thus more porous). Indeed, according to G. Galibert (1965), the consecutive rock mechanics to removal by erosion of a section of slope, loosens the rock according to the former structural directions. Only the hierarchy of the importance of the plans of fractionation is modified in-depth.

the role of the dip : In the areas comprising of the sedimentary rocks, the dip of the geological layers also plays: the circuses which tackle the face of the layers with dip monoclinal have very high walls, and their retreat is opposed by the dip. On the contrary, the circus of reverse, in the direction of the dip, the generally many and small, are poorly staged ones compared to the others. Moreover, when lithological dissymmetry is expressed clearly in the geomorphology, climatic dissymmetry can be expressed. Slopes in dip in conformity, where the slope is less strong than that of the slopes in contrary dip can be affected by the snow-plow effect.

Synthesis

In the basin-slopes of the Drac and Romanche (Oisans), G. Monjuvent (1978) tried a hierarchisation of the favorable conditions and unfavourable to the establishment of the elementary circuses to the Würm.

the favorable conditions : By order of decreasing incidence in the genesis of the circuses, one a:

initial topography: It can be a question of a regular or irregular surface, from which the slope is included/understood between 10° and 30-35°.
altitude: It must be higher or equal to 1900 m, altitude of the persistent Neige S in Würm in the the Alps of North.
the Lithology: The glacial circuses, are generally dug in a site where alternation hard stone/tender rock is the rule, allowing the play of differential erosion.
the structure: The circuses are often dug in crushed or faulted zones, guiding the glacial erosion and determining the density of the glacial circuses.
orientation: The orientation of the circuses does not seem a fundamental factor of their development to the Würm. Indeed, the circuses würmiens of the solid mass of the Jewel cases “seem to be distributed in an indifferent way and not to present any altitude ratio/orientation” (Monjuvent, 1978). However, the orientation in North appears a factor favorable to the conservation of the Glacier S with the Postglacial one.

adverse conditions : There exist two adverse conditions with the establishment of the elementary circuses to the Würm: a vertical topography or subverticale, prohibiting the movement of the ice and an altitude lower than 1900 Mr.

One thus sees the fundamental role of the lithography-structural conditions in the genesis of the circuses. Those also play a prevalent part in the classification of the circuses.

Great types of circuses

Typology suggested is based on the shape and the depth of the circuses, narrowly dependant on the context lithostructural.

Circuses in armchair

They are comparatively numerous in the the Alps, from where the name of “alpine” circus that one gives them sometimes. As their name indicates it, these types of circuses are with the image of the armchair, with three main features:

They are deeper than they are not broad;
They have vertical or subverticaux edges escarpés called walls of circus including/understanding rather often a high pyramid dominating the level of the peaks, called horn;

When they are released by the ice, they often contain a deep lake, with the ponding to the downstream of the Verrou, this last being in general rather imposing.

The circuses in armchair often meet in the rocks of homogeneous and resistant facies, like the Granite S, the metamorphic gneisses and rocks compact, the Calcaire S solid masses where a vertical clearing overrides the erosion of the floor. They are often dominated by the horn. With the image of Cervin (4478 m), it is about a high rock pyramid generally dominating the average level altitudinal of the peaks of stepping within one modelled glacial of high mountain. The edges of the horns are concave and their tops tend to becoming increasingly acute. These pyramids are related to the progressive retreat of the glacial circuses, primarily by cryoclastie, within the rimaye when the circuses are englacés or with the free air.

Circuses in van

The term of “van” corresponds an instrument sevant to winnow the grain. Sometimes relatively frequent in the the Pyrenees, they could be named “Pyrenean circuses”. They are rather broad, their bottom is flat or moderately corrugated, the peaks which dominate them are relatively low and the walls less sloping than in the circus in armchair. The slopes are covered with remains which regularize them: they are thus definitely less stiff; these accumulations can be, with high-altitude, of the even creeping falls true Glacier S rock, as around the Ale lake (Jorda, 1976). The lake contained in a circus in van is thus much less deep than in the circus in armchair. The floor of the circus in van making up of rocks more resistant than the walls, the Surcreusement is much more difficult there, the glacial language which in fate is less thick, erosion is thus less and side clearing dominates compared to the deepening.

Less clear forms

Sometimes named “circus in funnel of upstream”, in fact soft forms, rather bastard generally neither are surcreusées nor Verrou illées. Postglacial erosion - torrentiality, solifluction and gélifluxion of all types, eboulisation… - largely contributed to the degradation of these not very clear forms right from the start. One finds them in the rocks outputting itself in plates (flyschs and schists in particular…), which does not allow a clear action of erosion by the firn. Thus, in schistous Queyras (approximately 75% of the surface of Queyras), one observes glacial drafts of circuses, with the broad forms, widened and encircled ruined or very blunted peaks.

Snow-shaped notches

The snow-shaped notches correspond to cells more or less large, convex, where the Neige accumulates in winter, and where the cast iron, even partial remains relatively difficult in summer, which allows a Diagenèse partial of the Neige towards the Neige of firn. The period of cast iron and regelation of the Neige is longer in the nivhes of nivation of the tropical areas. They are forms definitely smaller than the glacial circuses s.s. They are current in the areas Périglaciaire S (high-altitudes and high latitudes), sometimes inherited as in the areas medium mountains or the north-Mediterranean field.

Sites of the snow-shaped notches

The leading causes of the persistence of the puddle pools of Neige S under a more or less high rock face are the repetition of the Avalanche S within corridors of Avalanche converging towards a precise site and snow-covered overfeeding, by effect of “snow-plow”, along a slope with the wind, the Neige accumulating with the foot of the wall located under the wind. insi, the preferential sites of the snow-shaped notches are being the shaded hollows, a convergence of corridors of Avalanche S or a projecting ledge, unspecified foothills of origin, supporting the accumulation of the Neige by the wind within the slope. In Iceland, one can observe the ground cold under the puddle pools of Neige, because the melt water, percolant through the Neige, comes to freeze superficially in summer. The two major processes of erosion are thus the fragmentation of the material by the gélifraction and its export by melt waters which carry out a washing of the material.

The downstream part of the snow-shaped notches

With the downstream of the plate of Snow, associated with the snow-shaped notches, the concentration of the fine products by the streaming supports the action of an intense gelivation, which generates the formation of geometrical grounds, the resumption of the material by the gélifluxion - the shapes in pads -, then, towards the downstream, one notices languages spread out in steps. Once the hollows of nivation are sufficiently deep, the puddle pools of Neige persist one year on the other and can become true firns, with physical transformation of the Neige by Diagenèse.

Fossil notches snow-shaped into Low Provence

Often, in low-Provence, of the small valleys have several snow-shaped notches inherited the Würm, even at very low altitudes: in the link of Nerthe, in the South of the pond of Berre, one can meet some to 70 m above the current marine level. Their orientation towards the South - in position of adret - appears a priori surprising. She is explained nevertheless by the effect of snow-plow related to the mistral.

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