Les roches du montagnard
Pour une escalade
toute en finesse
Pour une escalade
plus athlétique
Pour se faire peur
Le grès
Est constitué de grains de sable liés entre eux par un ciment. Il est, de par sa nature même, friable. Mais s’il est recouvert d’une patine qui le met à l’abri de la désagrégation on a alors une roche compacte et solide. La surface peut en être relativement lisse avec des grattons allongés (réglettes), des trous et des excavations, là où l’érosion éolienne s’est exercée. On le trouve rarement en haute montagne. En France, la forêt de Fontainebleau est connue pour ses blocs de grès. Annot, au coeur des Alpes de Haute-Provence, offre de magnifiques falaises de grès.
Les calcaires
Forment une vaste famille dont les membres se rencontrent partout. Des Calanques à l’Annapurna, les calcaires s’entassent sur 8 kilomètres d’épaisseur et servent d’armature à la plupart de nos montagnes. En Europe, on le trouve en général à moins de 3000 mètres, sauf dans l’Oberland Bernois, les Dolomites et la Vanoise. C’est une roche à base de carbonate de chaux.
Certains calcaires contiennent des débris organiques d’autres, comme les coraux, ont étés construit par des organismes vivants, c’est le cas du calcaire urgonien du Vercors ou de la Chartreuse. Certains contiennent de la silice, d’autres du carbonate de magnésium (dolomie).
Le caractère primordial du calcaire est sa solubilité. Selon sa composition, le calcaire engendre des formes très diverses. Dans le cas de la dolomie, par exemple, le carbonate de chaux est soluble tandis que le carbonate de magnésium ne l’est pas d’où les formes ruiniformes, très travaillées du relief dolomitique. C’est une roche qui est également sensible au gel. Le résultat, ce sont les chutes de pierres incessantes de l’Eiger et de la Grande Casse.
Au total le calcaire n’est pas sans défauts, mais son abondance et sa variété en font la roche reine du varappeur. Il engendre un type d’escalade aérienne, tout en finesse.
C’est la roche pédagogique par excellence: « tout dans la tête et les pieds ».
Les granits
Sont la roche mère de l’écorce terrestre. Ils n’apparaissent pas souvent à la surface dans les massifs jeunes. Les granits sont relativement importants dans les Pyrénées et en Corse. Dans les Alpes, on les trouve surtout dans la zone axiale des Alpes occidentales: massif des Ecrins, chaîne du Mont-Blanc, en Suisse centrale (Salbitschin), et dans les Grisons (Piz Badile).
C’est une roche à structure purement cristalline qui évolue par altération chimique. Cela donne une surface extrêmement rugueuse, très adhérente.
La Protogine est un granit dont les cristaux ont été déformés par compression et dont le mica a été chloritisé. Il est très solide et donne de superbes monolithes tels que les Drus, le Capucin et la Dibona. Ce granit présente des diaclases qui favorisent l’érosion mécanique. La roche tend à s’écailler en fragments de grande taille, c’est la desquamation. Peu plastiques, ces granits cassent selon des lignes nettes et engendrent de belles failles.
Ces structures déterminent un style d’escalade particulier avec prépondérance des mouvements d’opposition et des coincements.
Le gneiss
A la même composition minérale que le granit (quartz, mica, feldspath), mais avec une disposition stratifiée des éléments. C’est une roche multicolore avec des effets de gaufrage.
Il y a différents types de gneiss. Le gneiss piémontais est comparable au meilleur des granits avec toutefois une rugosité moindre, une grande fréquence de petites écailles et une forte sculpture superficielle des dalles (Argentera, Grand Paradis, Aiguille du Pouce). Le gneiss de l’Oisans lui, chevauche les granits dans presque tous les grands sommets du massif homonyme. Il se caractérise par une grande raideur, par une structure imbriquée riche en prises mais souvent difficile à pitonner. Les gneiss helvétiques et tyroliens se retrouvent dans la plupart des massifs des Alpes centrales (Cervin, Oberland). Il se signale par des constructions pyramidales grandioses. Il est souvent fracturé et pas toujours solide. On pourrait le situer quelque part entre le gneiss piémontais et les schistes cristallins. Il donne de belles arêtes.
Pour gravir le granit et le gneiss « En plus de la tête et des pieds… il faut avoir des bras ».
Les schistes
Il présentent une structure très feuilletée. Les pentes sont rarement fortes. Ce sont des roches à gravir … par les couloirs de glace. Les schistes cristallins se présentent sous forme de plaques solides et compactes aux rebords friables. Ils donnent de belles dalles aux arêtes tranchantes avoisinant de lamentables éboulis.
Pour se faire peur.
Mountaineer’s rocks
For climbing
with finesse
For a more
athletic climbing
To scare yourself
Sandstone
Made up of grains of sand bound together with cement. By its very nature, it is friable. But if it is covered with a patina that protects it from disintegration, the result is a compact, solid rock.
The surface may be relatively smooth, with elongated scrapes, holes and excavations where wind erosion has taken place. It is rarely found in the high mountains.
In France, the Fontainebleau forest is famous for its sandstone boulders. Annot, in the heart of the Alpes de Haute-Provence, boasts magnificent sandstone cliffs.
Limestone
They form a vast family whose members can be found everywhere. From the Calanques to Annapurna, limestone is 8 kilometres thick and forms the framework of most of our mountains. In Europe, it is generally found below 3,000 metres, except in the Bernese Oberland, the Dolomites and the Vanoise. It is a carbonate of lime-based rock.
Some limestones contain organic debris, while others, such as coral, were built by living organisms, as in the case of the Urgonian limestone of the Vercors or Chartreuse. Some contain silica, others magnesium carbonate (dolomite).
The most important characteristic of limestone is its solubility. Depending on its composition, limestone produces very different forms. In the case of dolomite, for example, carbonate of lime is soluble while magnesium carbonate is not, hence the ruiniform, highly worked forms of the dolomitic relief. This rock is also sensitive to frost. The result is the incessant rockfalls on the Eiger and Grande Casse.
All in all, limestone is not without its faults, but its abundance and variety make it the rock of choice for rock climbers. It gives rise to a type of aerial climbing that’s all about finesse. It is the teaching rock par excellence: « everything in your head and feet ».
Granites
Are the parent rock of the earth’s crust. They do not often appear at the surface in young mountain ranges. Granites are relatively important in the Pyrenees and Corsica. In the Alps, they are found mainly in the central zone of the western Alps: the Ecrins massif, the Mont Blanc range, in central Switzerland (Salbitschin) and in Graubünden (Piz Badile).
It is a rock with a purely crystalline structure that evolves through chemical alteration. The result is an extremely rough, highly adhesive surface.
Protogine is a granite whose crystals have been deformed by compression and whose mica has been chloritised. It is very solid and produces superb monoliths such as the Drus, Capucin and Dibona. This granite has diaclases that encourage mechanical erosion. The rock tends to flake into large fragments, a process known as desquamation. Not very plastic, these granites break along clean lines and create beautiful faults.
These structures determine a particular style of climbing with a preponderance of oppositional movements and jamming.
Gneisses
It has the same mineral composition as granite (quartz, mica, feldspar), but with a layered arrangement of elements. It is a multicoloured rock with embossed effects.
There are different types of gneiss. Piedmont gneiss is comparable to the best granites, although it is less rough, has a high frequency of small flakes and is heavily sculpted on the surface of the slabs (Argentera, Grand Paradis, Aiguille du Pouce). The Oisans gneiss overlaps the granites on almost all the major summits of the Oisans massif. It is characterised by great steepness and an interlocking structure rich in holds, but often difficult to piton.
Helvetian and Tyrolean gneiss are found in most of the massifs of the central Alps (Matterhorn, Oberland). It is characterised by its grandiose pyramidal structures. It is often fractured and not always solid. It lies somewhere between Piedmont gneiss and crystalline schist. It produces some beautiful ridges.
To climb granite and gneiss « As well as your head and feet… you need arms ».
Shales
They have a very flaky structure. Slopes are rarely steep. These rocks are best climbed… through ice corridors. Crystalline schists come in the form of solid, compact slabs with crumbly edges. They produce beautiful slabs with sharp edges, next to pitiful scree.
To scare yourself.
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