Glaciated valley

A glaciated valley is a form of relief characteristic of the areas of Montagne S which were affected by a regional Glaciation. They result from the work of flow in block of the Glacier S, filling up all the bottom of the valley and eroding it by overdeepening. The glaciated valleys are defined by their profile transversely, but especially by profile longitudinally. In the majority of the cases, the glaciated valleys correspond to heritages of the Glaciation S Pléistocène S where powerful icestrœm coming from the high solid masses used these preexistent axes to move towards the Piedmont S. They often met in very longitudinal large valleys like the valleys of the Isere, the top the Rhone, the top the Rhine, of the Inn, the Enns, the Drave, where the Glace concentrated. As it P. & G. Veyret (1967) underlined, it is this exceptional power of transport and refitting which allowed widening, the digging and the overdeepening of these valleys and which confers on the alpine valleys one “surprising maturity” compared to the extreme youth of the solid mass.

Profile transversely

The profile across the glaciated valleys made run much ink since the beginning of the XXe century. Many mistakes in interpretation transversely originate in the abusive generalization of the form of the profile out of trough.

The glacial trough

The most characteristic form thus appears to be the valley in “U”, trough, or glacial trough, with sides precipice and a flat bottom, in particular when it is about the bottom of an old lake. However, as J. Tricart (1981) underlines it, “the glacial trough is actually a bed worked by a concentrated flow of Glace”. Moreover, the shape out of trough does not seem a constant and one can make the following remarks:

Any glaciated valley is not out of trough: the Maurienne and the Tarentaise more present a profile in “V”, others present a dissymmetrical profile like the throats of Manival, in Oisans;
the glacial trough can be reduced with certain sections of a valley. The profile can widen or, on the contrary, narrow in narrow throats and not only in the bolts, like the throats of the Guil (Queyras) or the throats of Servoz to the downstream of the valley of Chamonix.
Any valley in “U” is not obligatorily of glacial origin. Very beautiful valleys having this profile exist in the solid mass of the the Sinai, in Egypt.

Thus, the glaciated valleys present many types of profiles transversely all along their longitudinal development; the troughs exist mainly with the upstream of the glacial large valleys, even if the aspect out of trough is apparent than real. Moreover, certain glacial troughs which appear thus according to a certain profile on the ground, are not to it any more another site or do not resist when one draws this profile on paper. In the glacial troughs “perfect”, the flat bottom related to the accumulation of alluvia is finished or postglacial basic of valley, completely embanking the bed of the old glacial river which can be very deep. There exist shoulders and hanging valleys. Traditional interpretations of these forms depends on the positions of the authors.

For the antiglacialists: The existence of these forms would be related to the maintenance of the profile of the old “ripe” valleys preglacial which would have been preserved. The trough, as-with it, would result from the passage of the Glacier in a “young” valley, encased more in the preceding one. Each projecting ledge would correspond then to the bottom of a trough dug by a former Glaciation.

For the ultraones: “The glacial trough appears to correspond often not to a made current with the measurement of the Glacier, but with the major part of the glacial current” (Blache, 1960). Indeed, in a alpine Glacier, the surface layer, circulating on the level of the projecting ledges, runs out without great effects on the bedrock, whereas the deep layers, subjected to high pressures determine a strong erosion.

Theoretical remarks

According to theoretical calculations, the shape in Roman tile would be the ideal form, corresponding to a compromise between the ideal section to allow the flow of the Glace and the section widened by lateral cutting. According to Embleton & King (1975), the transverse profile of a trough would correspond to a parabola of equation

y = 0,000402 x^ {2,046}

. According to R. Vivian (1975), the base of the slopes are the sites where erosion is maximum for three main reasons:

the presence of great quantities of remains coming from the slopes allows the play of abrasion, in particular at the beginning and the end of the Glaciation, when the slopes are stripped;
the low thickness of Glace supports the penetration of waves of freezing inside the Glacier, allowing cutting up and the gélifraction. The remains unseating and wrenching are thus very powerful;
In this sector of the Glacier, exists also the torrents juxtaglaciaires or subglacial. Those dig channels, even throats and accentuate the values of slip of the Glacier on the bedrock.

These types of modellings widen by the base the Talweg of the glaciated valley and dig sections of troughs. In fact, “the calibration and the pace of the profile across the valley are primarily due to the glacial forms of accumulations” (Vivian, 1975), i.e. the platings of lateral moraines and deposits juxtaglaciaires acquire a concave profile at the time of the passage of the Glacier, while the inequalities of the bedrock are masked by fluvio-glacial and/or fluvio-lake accumulations.

Shoulders

Classically, one defines the shoulders as continuous and symmetrical side projecting ledges located on the sides of a glaciated valley.

traditional assumptions: According to E. of Martonne (1951), the shoulders correspond to successive phases where fluviatile erosion and glacial erosion alternate, each projecting ledge corresponding to a vaster Glaciation former; it evokes “encased troughs then”. However, it will be noticed that the majority of the projecting ledges located in the valleys of average altitude are discontinuous, asymmetrical or of structural origin. For F. Taillefer (1966), the shoulders are a direct consequence of glacial dynamics: within the alpine Glacier S, only the lower layers of Glace subjected to forrtes pressures have a great capacity of erosion. A contrario, the surface layers run out in a passive way, without causing erosion. The shoulders would correspond then to the part of the valley affected by this type of Glace.

shoulders related to the work of the side Glacier S: Work of P. Veyret (1968) in the valley of Chamonix, confirmed by the observations of G. Monjuvent (1978) in the basins of the Drac and the Romanche, showed that the true shoulders of the high valleys seem the result of glacial dynamics starting from the Glacier S of circus side, at short distance of the hanging valleys:

When the shoulders are jointed, one notices that the tributary circuses are very brought closer;
When the shoulders are located at an equivalent altitude, one notices that the circuses have comparable dimensions.

“The shoulders do not form part of the glaciated valley; they make on the contrary left the slope of which they represent façonnement particular” (Veyret, 1969). The two described phenomena appear clearly in the valley of Chamonix, between the Mer of Ice and the Glacier of the Bossons or the small valley of the Props (high Vénéon, Oisans).

shoulders related to the retreat differentiated from the walls: However, certain shoulders appear due to the presence of more frost susceptible rocks on the level where the shoulder was registered. This one is thus more dependant on the retreat of the walls by the gélifraction and on the maintenance of a strong slope above and below the projecting ledge (Vivian, 1975).

In the case of the Eastern side of the Great Breakage and the Grande Motte (Vanoise): The base of this shoulder is cut by phylliteux marbles of Sorted Average, while the retreat of the walls has affected the Calcaire S frost susceptible of the tablecloth of the Grande Motte. In the case of the small valley of Leisse (Vanoise), the shoulder is built in the Gypse S and the Cargneule S, dominated by the glossed Schiste S.

Hanging valleys

The majority of the tributary valleys of a glaciated valley emerge above the bottom of the principal valley: it is said that they are hanging valleys: the course of the postglacial rivers crosses this step of junction by a throat known as gorges with connection or by a cascade (valley of Lauterbrunnen, in the Bernese Oberland), when lithology is homogeneous. It is even possible that the valley which appears today as the principal valley was a secondary valley during the Glaciation S Pléistocène S, with the image of the Romanche and the Vénéon, of the Arc and Doron de Termignon or the Durance and the Gyronde. These hanging valleys definitely are dug than the principal valleys and often have a profile “out of V”. It is also possible that a large valley is completed on a whole of valleys hanging like the valley of the Arve to the upstream of Passy where the valleys of the Bon Nant and the high valley of Arve are suspended. One will notice initially, who the hanging valleys are related to Glacier S less powerful than the principal Glacier. However, there exist several cases of hanging valleys:

valleys hanging with englacement non-existent or reduced: In this case, the problem of the difference in digging does not arise. The step of junction is related to the absence of digging in glacial period of the hanging valley. In certain cases, it could there have a diffluence of the principal Glacier in the secondary valley. It was in particular the case of the Glacier of the Romanche in the Mathésine or of the Glacier of the Isere in the valley of Lans.
dissymmetrical hanging valleys: If there were Glacier S in the two valleys, it is possible to call upon the structural conditions and the orientation of the valleys.

In homogeneous material, the most powerful Glacier S are in Northern orientation and the hanging valleys are thus more numerous; it is in particular the case in the valley of Vénéon (Oisans). In heterogeneous material, the structural conditions take the advantage on the orientation. Thus, in the valley of the Romanche, the Glacier S located out of Right Bank have deeper valleys, dug in the sedimentary rocks. They were more numerous than out of left bank, dug in the crystalline rocks (Montjuvent, 1978).

steps of junction: The unevenness of the step of junction is, generally, dependant on the surface of the hanging valley: the weaker this surface is, the more the unevenness of the step of junction is large.

throats of connection: The connection between the two tributary rivers can be done by a throat of connection. This throat was dug at the same time by the subglacial river and the river, heir to the glaciated valley. When the rocks are particularly resistant, there is no throat of connection and the connection is done by a cascade. The hanging valleys were a long time of the sites favorable to the development of hydroelectric factories and Barrage S of reserve.

The problem of the encased troughs

In certain glaciated valleys, there exist several shoulders which are staged to the surface of the Glace and which appear thus encased. The origin of these forms is posed as follows: it is about a difference in intensity and of duration of the glacial processes with a thickness of Glace more limited on the edges or of a difference in structural order, the “shoulders” are cut in rocks where the density of the fractures is less. Thus, the current troughs of the Glacier of Argentière and the Mer of Ice are delimited by fractures. However, even if the Glace cannot transmit pressures higher than 20.000 Pa (2 bars), one can only wonder about this problem by considering the following remarks (Bozonnet, 1981):

the shoulders do not exist in the glacial storing reservoirs and appear gradually towards the downstream;
There exists a fitment of troughs on left bank of the Glacier of the Fog (valley Veny) which is explained with difficulty by structural reasons.

Glacial obturation

The process of obturation glacial is related to a moderate diffluence of a glacial language inside a not englacée valley. The forms and formations associated with this process are:

One or more frontal morainic arcs, with convexity turned upstream and not towards the downstream as in the “normal” valleys;
a fluvio-glacial or glacio-lake fill, related to the stopping consisted the Glacier. It forms allors a bench or an alluvial plain.

In the majority of the cases, this valley is drained by a ravine parallel with the Glacier and notching the materials juxtaglaciaires. In the case of a karstic area, it can evolve/move in a closed depression of Karst covered, i.e. in pseudo Poljé whose water is lost in the slopes Calcaire S.

Profile longitudinally: glacial bolts and umbilical points

The profile longitudinally of the glaciated valley is more characteristic than the profile transversely: it shows forms of overdeepening or umbilical points, i.e. forms of digging, corresponding topographically to lake basins, small basins, even of the small alluvial plains, limited to the downstream by a bump, a counterslope, a simple tightening of the valley or bolt, circumvented by the principal rivers. The first succession of modelled of this kind is the glacial Cirque, birthplace of the Glacier.

Definition and environment of the bolt

One longitudinally calls bolt a rock bar accidentant the profile of a glaciated valley and generally located at the downstream of an umbilical point.

bolts under the Ice: At the time of englacement, the Glacier crosses the bolts by an separation of the Glace to the upstream and the downstream. The change of incline is crossed by a series of cracks and seracs. Subglacial water is spread out with the upstream, working the umbilical point and digs throats of bolt at their sides.

Aspect and genesis of the bolt-bars: In the thawed zones, one can notice that the bolt-bars are variable sizes, but their characteristic is to be on a surrounding relief scale. But, they can correspond only to one simple tightening of the valley associated with a change of incline like the throats with Poya in the valley with Chamonix or tightenings between City-Old woman and Needles or Aiguilles and Abriès in Queyras). It is then necessary to imagine the umbilical points much more deeply dug under the Glacier than they are it with Holocene after trapping various glacial, glacio-lake, fluvio-glacial, lake and fluviatile Clastic rocks. When they monolithic i.e. consist of only one rock, the origin of the dissymmetrical profile of the bolts arises clearly, because of double action of abrasion to the upstream and the quarrying to the downstream:

the aspect of moutonnées rocks that one notes with the upstream of the bolt is related to the abrasion caused by the compression of the Glace: the rock then is smoothed and striated, i.e. moutonnée.
the aspect emaciated with the downstream of the bolt - profile in “stairs” - is due to the quarrying , process to be put in connection with alternation freezing thaw under the Glacier separated of the bolt.
Certains thawed bolts can be partly ennoyés and form islands in the sea - with broad of the fjords, for example - or in a lake as the islands Borromées on the Lake Maggiore with broad of Stresa (Italy North). In other cases, they form almost islands with the image of the rock the Expensive ones on the Lac of Annecy.

bolt-steps: They are discontinuities in a bed or old glacial bed without counterslope with the upstream and overdeepening of the umbilical point. One generally finds them with the upstream of the glaciated valleys, between the glacial Cirque S and amphitheaters or with the downstream of the hanging valleys then which can be qualified steps of junction.

Umbilical points

The umbilical point indicates a topographic basin generally located at the upstream of a bolt within a glaciated valley. The umbilical points are dependant on the spreading out of subglacial water and a less speed of the Glacier upstream of the bolt. These two phenomena are accentuated by the possible stagnation of a base of Glace died for the periods of cast iron of the Glacier.

The relationship between the bolts and the umbilical points

In the very large majority of the cases, the bolts are preceded by an umbilical point. The bolts are worked if not in hard stones, at least in rocks more resistant than the umbilical points. This is due to the increase in the hydrostatic pressure to the upstream of an obstacle which allows the lowering of the freezing point and allows fusion. The solid materials present at the base of the Glacier will be able to penetrate inside the fact of the overload in Glace.

In the heterogeneous sedimentary rocks: When lithology is heterogeneous, in fact the most rebellious layers with the glacial erosion are highlighted (hard sandstones, Calcaire S) and the tender layers which are excavated.

When lithology appears homogeneous: When lithology appears homogeneous, i.e. the rocks are the same ones in the content of the umbilical point and on the bolt, differential erosion was based on other factors:

tectonic conditions making alternate zones of crushed rocks and solid rock zones);
direction of the glacial flow compared to the dip of the layers, even with the foliation of the rocks.

It is admitted that a bolt is not obligatorily preceded by an umbilical point. Thus, the bolt of Séchilienne or Doors of Oisans (valley of the Romanche), is preceded by no umbilical point. It corresponds to broad band of Amphibolite S very resistant. The umbilical point of Vizille has a considerable cashing (135 m upstream of the city). It comprises two basins (Vizille and Séchilienne), separated by the contracting of Bathie. It is dug with the contact between the crystalline solid mass of Belledonne and the subalpine furrow, in position monoclinale.

Genesis of the bolts and the umbilical points

The safeguarding of the bolts and the deepening of the umbilical points are thus an established fact, but it is necessary to reconsider the conditions of their existence and their façonnement.

In the high glaciated valleys with homogeneous structure and lithology: With the image of the high valley of Vénéon, there is neither umbilical point neither bolt, nor widening of the profile to the many confluences. There is thus no relationship between the genesis of these forms and the power of old the Glacier S.

In other glaciated valleys, smaller, with heterogeneous structure: As in the valleys of the Water of Olle or Good (Oisans), one finds umbilical points (Large' House, Valbonnais) on tender outcrops of rocks, closed by bolts (Maupas, Pas of the Priest). With Gap, the bolt is in not very resistant rock (Marno-limestone S) but which constitutes a relative mole of resistance in a depression dug in the black marl S callovo-oxfordiennes around the dome of Rémollon. However, the Glacier S surcreusent at the places where them flow is slow and only erode very little where the flow is fast, because the Glace is a very viscous fluid, according to the general physical laws which govern this matter state. The undertaken experiments show thus that a fluid circulating in a conduit with a presumedly constant flow exerts on its walls a pressure inversely proportional at its speed. There is thus overpressure in the broad section compared to the narrow section:

With the upstream of the bolts, the Glace is subjected to strong pressures and tends to melt. These alternations engel/thaw thus allow façonnement umbilical points;
With the downstream of the bolts, the Glace is subjected to lower pressures and the water released with the upstream tends to freeze again, which allows the wrenching of blocks (quarrying) the bolts.

It is thus enough to a transverse contracting, inherited modelled preglacial, so that the overdeepening in consequence of the dynamic differences in pressure starts upstream that the Glacier exerts on its bed. Thus, there is no possible overdeepening without contracting with the downstream, as in the Dombes, in the North-East of Lyon, however made up of tender formations - “Yellow Alluvia” or lake formation of the lower Pliocène -, invaded by the Glacier S at the time of the “Riss”.

Synthesis

The longitudinal profile of a glaciated valley thus seems the result of a double work of erosion and accumulation:

evolution of the valleys in glacial period: The digging of the funds of valleys as of the lower edges of the slopes is carried out mainly in glacial period. It is accompanied by a morainic fill of variable width, made up of basic tills then of tills of ablation at the time of the Déglaciation. During the periods of Déglaciation this modelled characteristic allows the breakdown of the icestrœm: the bolts are generally thawed more precociously than the umbilical points.

evolution of the valleys in interglacial period: The postglacial fill of the funds of valleys is carried out in particular by fluviatile and lake processes. The processes affecting the slopes are very variable, according to the sites: certain sites tend to widen, by collapse of sides of slopes, while others tend to be regularized, by the periglacial actions or the streaming.

The umbilical points are often embanked by morainic deposits (basic tills), surmounted by alluvia fluvio-glacial finish-glacial, then lake and/or fluviatile postglacial. These materials come from the upstream of the valley or the small valleys and the slopes perpendicular to the umbilical point, whose frequency and intensity of the contributions are related to the climatic fluctuations. The control of sedimentation inside the umbilical point is ensured by various elements:

control-downstream, represented by the temporary stoppings, subjected to processes of embâcles and routs, these last being able to be brutal or not;
the lithography-structural control which regulates the presence and the height of the bolt as well as the depth of the umbilical point.

Moreover, this control of the filling of sedimentation at the local level conditions the basic level, the slope of the umbilical point like indirectly, the geometry of the formations contained, because of the possible ruptures of stoppings which allow the staging or the superposition of the deposits.

The problem of the overdeepening of the umbilical points

The overdeepening can be defined as the deep digging of a portion of valley limited to the downstream by a counterslope, result feature of the glacial shares. The basins left by large the Glacier S quaternary are often of impressive size and depth. For example, Léman is limited to the downstream by a threshold of 310 m of which 255 m are related to the overdeepening and 55 m with the frontal morainic stopping of Laconnex-Bardonnex, with the downstream of Geneva (Charollais & Al, 1990).

Theoretical localization of the surcreusées zones

The localization of the surcreusées zones depends on two factors: the position compared to the lithography-structural inlansdsis and influences.

the position compared to the icecap or in the ice cap: Under the glacial languages and especially under the caps and the ice caps, the overdeepenings are deep. These glacial organizations had several types of impacts on the regional geomorphological evolution:
They protected the relief from the central areas, having had only Glacier S not very mobile and thus not very suitable for erosion;
the icestrœm diverging starting from the central cap planed, surcreusé the circumference of the central areas;
They deposited various types of morainic and fluvio-glacial materials along the icestrœm and on the piémonts.

However, it will be noted that the zurcreusées zones do not correspond obligatorily to the zones of strong accumulations of masses of Glace: this phenomenon, even if it is currently definitely visible, more appears as a consequence of the overdeepening that of a true cause. Indeed, “the variations of pressure are not proportional to the variation thickness of Glace which overcomes the points of measurements. The Glace cannot transmit any constraint exceeding 200 Pa, that is to say 2 bars” (Vivian, 1975).

lithography-structural influences: The data lithostructurales influence not only by the nature of the rock, but still by the orientation of the stratigraphic joints, the fractures, the Failles, the diaclases and the zones of crushing. The Glacier S tend to surcreuser in particular sectors where the following conditions are noticed:
the flow of the Glace is slow;
the flow of the Glace is spread out over a broad portion of the bed;
subglacial water is largely spread out;
With these three phenomena is associated differential erosion in more fragile rocks.

These factors allow façonnement umbilical points by the multiplication of the cycles of consecutive alternations freezing/thaw the variations of pressure because to the upstream of the obstacles, the Glace is subjected to strong pressures and tends to melt. On the contrary, when speed is fast, erosion is limited.

Thus, the basin of bottom Grésivaudan it is particularly surcreusée, since one passes from a broad valley to a narrower valley - the cluse of Grenoble -, where speed had to be accelerated, whereas the flow was increased by the contributions of the Romanche (Monjuvent, 1978). High the Maurienne upstream of the bolt of Esseillon is an interesting exemble with alternation of bolts and umbilical points related to the lithography-structural conditions:

the umbilical point of Bramans-Termignon, 10 km length, dug in the sole gypsifère of the tablecloth of the Schist S glossed. It in is copied more on a major accident consisted the fault of Modane-Brook of Chavrière;
the umbilical point of Lanslebourg, 6 km length, which belong to the same unit morphostructurale;
Lastly, the umbilical point of Bessans, 6 km length also, is surcreusé in the tablecloth of Schiste S glossed itself.

Dynamic interpretation of the overdeepening

One can distinguish two great types of overdeepening: subglacial overdeepening and proglacial overdeepening.

subglacial overdeepening: It is the cutting up of blocks to the downstream of the protuberances of the bed, related to regelation and with the cavitation which occur there and does not depend on the appearance of a strong slip by coalescence of the cavities. Subglacial erosion can thus continue even if the bed grows hollow and if, correlatively, the thickness of the Glace growing, the speed of the slip decreases. Polishing, on the other hand, is function the speed of the slip: it is maximum above the bolts, where the Glacier is thin and the speed is very high.

In Antarctide, the surveys and the radioones show that the studied icestroem are located in zones in hollows of the substratum which are broad widened valleys in cradle alternating with deep and narrow umbilical points. On the surface, the Glace is presented with a more or less concave longitudinal profile, the clearest concavity being marked by the convergence of flows of Glace (Godard & André, 1999).

proglacial overdeepening: When a Glacier is in period of retreat, the bedrock appears initially on the bolts and the Glacier ends up splitting up, isolating from the bases of dead Glace which stagnate in the umbilical points. In these intermediate bearings, there is no more in advance nor of retreat of the face, for lack of food coming from the upstream, which causes an alternation of frequent periods of gelivation, associated at periods of evacuation of the remains previously torn off.

The typology of the glaciated valleys

The role of the structure in the development of the glaciated valleys is obvious: the glaciated valleys open in varied geological formations modelled slopes of slope and regularity variables. Thus, in the Queyras, the valley of the Guil opposes two fields clearly:

With the upstream, a valley in “V”, very widened, in the Schist S glossed Piedmontese. The requests of the structure are read in the dissymmetry of the perpendicular valleys;
With the downstream, a very narrow valley - throats of Guil - in the Calcareous S of the tablecloths briançonnaises.

In the resistant rocks

The three great types of resistant rocks react differently to the tectonic constraints which will have an effect on the later glacial erosion.

Calcareous S solid masses: The Calcaire S solid masses have directions of obvious fractures and thus often give well gauged glaciated valleys. Lauterbrunnental (Bernese Oberland), dug in the Calcareous S resistant of the tablecloth of Wildhorn, shows shoulders (Wengen, Mürren), corresponding to the roof of the Calcaire S, the cascades (Staubach and Trümmelbach) and of the fossil avalanche corridors on the sides. The glacio-karstic small valleys correspond to old glaciated valleys dug in the Calcaire S and more or less surcreusées upstream of the bolts. The Lake Flaine (massive of Giffre), located 84 m under the collar of Neck, is a glacio-karstic good example of lake, having evolved/moved in polje inside a valley of this type. Its water is lost in the floor hauterivien.

homogeneous crystalline rocks: In these types of rocks, the network of fractures is very dense and generally, the very localized fractures (fault lines) are expressed by stiff walls or subverticales (valley of Vénéon). However, the genuine troughs are rather rare.

rocks in plan of schistosity very marked: In the Gneiss or the Schist S crystalline lenses, the glaciated valleys generally show the shapes of valleys more widened often, except when foliation is vertical or subverticale (valley of Chamonix). In the case of the glaciated valleys dug in resistant rocks, dug primarily by abrasion, the postglacial evolution of modelled slopes is carried out under the double lithological constraint (resistant rocks) and topographic (extreme stiffness of the walls):
the postglacial decompression of the allowed slopes the existence of collapses of all sizes;
the torrentiality is expressed really only in very fractured zones or in contact with two lithological units (basin of the Borough-D' Oisans);
the avalanche corridors represent modelled the most characteristic. The cones of avalanches are located thus at relatively low altitudes in the crystalline solid masses of the Alps (Oisans, Mont Blanc).

In the relatively movable rocks

In these types of rocks (flyschs, marl S, Schist S, Mica schist S), the valleys broad, are widened and the interfluves are soft reliefs with the very blunted forms, where the bottom is hardly dug better than in the resistant rocks. When the general dip is perpendicular to the axis of the valley, the two slopes are definitely dissymmetrical, with the image of the valley of the Acute Ewe-lamb (Queyras schistous). The wide range of the forms and formations which one can connect to the postglacial evolution of modelled slopes is only the reflection of the variety of the lithography-structural conditions.

In the sedimentary rocks with contrasted facies

The slopes record these variations of hardness in the forms of escarpments and structural projecting ledges, released more or less well at the time of the passage of the Glacier. The Western slope of the Grésivaudan, in the oriental party of the Large-Chartreuse watch thus a structural projecting ledge very Net on the level of the plate of the Small Rocks (between 900 and 1050 m of altitude), corresponding to the Calcareous outcrop of the S tithonic. As in the preceding case, the postglacial evolution of modelled slopes is varied, because of lithography-structural mosaic.

The interpretation of the glaciated valleys

The glaciated valleys correspond to old erosion valleys whose running water had made profitable the structural weaknesses. The majority of these old valleys could be dug as of the Oligocène (Monjuvent, 1978).

Old erosion valleys

The glaciated valleys were rectified or refitted by the passage of the Glacier S and often fuller, rectilinear and are smoothed. Thus, the valley of Chamonix is a “structural depression guided by erosion”, where the sedimentary grounds of the liasic Synclinal, gripped between the two crystalline scales of the Mont Blanc and the Red Aiguilles easily could be cleared (Veyret, 1959).

The role of the zones of structural weakness

The glaciated valleys are localized on structural zones of weakness. It is in particular the case in the majority of the valleys of the Massif of Mont Blanc or the Norwegian Fjords, which are only glaciated valleys invaded by the sea. The conditions Tectonique S also intervene in the layout of the tributary valleys, one saw it, like in the layout of the principal valleys. They explain the rectilinear layouts and the elbows, with the image of the valley the top the Rhone of Gletsch to the Léman, or bends it Martigny.

Appendices

See too

glacial Erosion
Fjord
natural Circus

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