France

Italia

Schweiz

Documents

The geomorphological landscape

1_paysagegeomorphologique

The landscapes you discover along the pathos of Alptrekking are very varied and often simply imposing. The observant walker will be intrigued by this variety: why are some rocks very soft, and others less so? Why is there a crest in the middle of a valley? How was all this formed?
Here we wish to give some basic information to walkers in order to understand the formation of the principal geomorphological shapes they will come across on their way. We will first present the influence of the rocks and their formation; then the mechanisms of erosion and deposits and to conclude the tie between geomorphology and man.

Rocks and their formation
Mountains are made up of rocks of different textures and colours: each one has its own origin and chemical composition. That is the key to understanding the shapes of the landscape!
Certain rocks were formed very slowly, in the depths of the earth (magmatic rocks); they are very rich in quartz, a clear, very hard mineral much loved by crystal seekers, glass cutters and engravers. These rocks are highly resistant to erosion. You can find them in Mont Blanc and the Aiguilles-Rouges, at the top of the Matterhorn, the Dent-Blanche,Weisshorn, etc. (1).
Other rocks were formed by accumulation and compression of sediments (sedimentary rocks). In the region we are crossing, the sediments settled in an ocean which preceded the formation of the Alps. You can find them in the form of calcareous slabs like the slab of the Lé Blanche below Champex (2). Below the lake of Emosson, these slabs even reveal the ripples of sea beds and traces of dinosaurs!
The formation of the Alps began 96 million years ago with the collision of the European and African Tectonic plates. Imagine the pressure they produced! The magmatic and sedimentary rocks were thus transformed (metamorphic rocks). The schistous rocks reveal clearly the mechanical compression of layers of sediment. They are not very stable, they crumble easily, break up; you can find them amongst others in the coomb of the A (5).
The formation of rocks also influences their chemistry, their texture and thus their resistance to erosion. Let us see how erosion acts and creates the shapes.

Erosion and deposits
Erosion does not take place without deposits: material picked up in one place will be carried and left somewhere else. In the Alps, there is above all the action of the glaciers, freezing and melting and water eroding, transporting and depositing material, thus creating typical shapes.

The action of glaciers
All the landscapes you can see in the Alps have been more or less created by glaciers over a long period. Since the beginning of the Quaternary (1.65 million years ago), the Alps have undergone periods of extreme cold (glaciations). The valleys were then filled with ice up to around 2,000 metres. You can often see the U-shape of the large valleys at this altitude, proof of the glacier’s passage.
Why does a glacier plane the slope so effectively? Its mass exerts an enormous pressure on the rock below and it flows like a stream. In fact, the material it tears away from the soil scrapes, polishes and scores the soil. The milky colour of the glacial mountain streams comes from the dust generated by the pounding of these rocks. As the glacier retreats,“moutonnées” rocks appear, planed, rounded by the glacier. If you look at them closely you can even see the striation engraved by a particularly resistant pebble carried along by the movement of the ice. You can also see the hollowing of the rock, called “rock basins”; they are often formed just above a rupture of a slope; the ice, carried away by its weight, loosens some of the shelf leaving a place for the water under pressure to exercise its power of erosion. As the glacier retreats these rock basins are often filled by lakes. The lakes of Chéserys and Lake Blanc (6) are good examples.
The glacier doesn’t only transport and plane, it also deposits all the material. Once there is no more ice, the movement stops and the material is deposited. This is why there is an accumulation at the front and all along the glacier of material of all shapes and sizes: these are moraines. A frontal moraine has the shape of a large smile and a lateral moraine creates large ridges or often a path. These shapes can be observed around all the glaciers, but the one at Miage (7) is unusual: a lake has formed within the lateral moraine. The erratic blocks are rocks, sometimes enormous, transported by the glaciers and deposited far from their origins. At col des Montets (8) there is one, used as climbing block.

The action of freezing and melting
All along the path, jagged peaks contrast with slopes planed by glaciers. This is the action of freezing and melting. Water infiltrates a cleft and freezes; it thus takes up more space and also enlarges the cleft which ends up by splitting from end to end (frost wedging).This happens at all levels: from tiny pebble to a rock face. This is what has happened with the landslide of Randa (VS) in 1991. The melting of frozen soils (permafrost) poses another problem: the ice no longer plays a cementing role between the elements of the soil which move more or less rapidly (congeliflux).

The action of water
In the landscapes we are crossing, water in its liquid state also models much of the landscape. The stronger the current, the more the water can transport large material and thus “sweep” the plains and/or erode the terrain. The frontal moraines are thus often split and flood after flood can disappear (moraine of the glacier of Lex-Blanche (10)).
If the slope is gentle, the water does not know which path to take and flows here and there at random; the stream thus takes on the shape of a plait. The lake at Combal is at example (11). If the current is a little stronger, the water is immediately channelled and follows a single path which avoids all obstacles creating often large detours. These are called meanders. The current erodes the outside of the bend, so that it is weaker inside and heavier sediments are deposited. If the slope becomes steeper, the water takes the shortest route and erodes the valleys in a linear fashion which take on the typical V-shape (for example the Val d’Arolla (12)).
Water from the melting spring snows or from summer thunderstorms, can cause enormous damage: a large quantity of water arriving in a short space of time on a small area provokes muddy flows which carry away everything in their path: the soil saturated with water, can no longer contain it, so it is carried away. You can see this damage at the bottom of the coomb of the A and in the Val d’Arolla (13).

The tie between geomorphology and man
Place names reflect very well the tie which there has always been man and his environment: Nants and Dranses mean streams, Seigne, the marshy plain, Bagne, a depression where water accumulates. Lavachey evokes avalanches, Otemmas, the August pastures, Praz the meadows…
Man has exploited the different rocks found in the region; in the Valleys of Hérens and Anniviers serpentinite is sculpted to make large stoves (pot stone); tiles of slate schist are used for roofs and on the piling of the raccards in order to stop rodents entering hay lofts; gold from Macugnaga, copper and nickel from Turtmanntal have also been mined.
All the landscapes of these regions have also been used for scientific research (since the 18 th century with the work of De Saussure), mountain climbing (from the 19 th century with the English), and skiing (20 th century). It is also thanks to their steep slopes and abundance of water that, since the early 20 th century, the race for “white gold”, the production of hydro-electric energy began. On the other hand, dozens of dams and an immense underground network collecting glacial water have been built, so well that the flow from most of the streams today is quite low!
All these activities mark the landscape with diverse constructions (roads, buildings, ski tracks, pylons, dams, etc.) but traditional mountain agriculture has also done so. For example, in the Val d’Hérens and Anniviers, the slopes have been shaped, generation after generation, into small terraces for the cultivation of cereals; or again kilometres of canals have been created, to irrigate with water from the glaciers, the dry slopes of the valleys. Some of these bisses (canals) have been restored today and offer pleasant walks on the flat, for example the one at Trient (15). Alptrekking offers you the chance to discover a multitude of mountain landscapes where nature and man have always lived together.

Leave then with your eyes and heart wide open! Have a good trip!

Jeanne-Charlotte Bonnard, Pascal Tissières