Geology

Jean-Luc Epard, Institute of Earth Sciences, FGSE

The canton of Valais is privileged to feature a wide variety of geological formations that are characteristic of the Alps. The Valais Alps are also distinguished by the diversity of the rocks that compose them. The region hosts both minimally metamorphosed sedimentary rocks at the forefront of the Alps and deeply transformed rocks formed during the creation of the Alps, as seen in areas like Simplon, Nufenen, and Zermatt. The Valais Alps have been the subject of geological studies since the 19th century and continue to be a topic of interest for both Swiss and international researchers.

The paleogeographic domains in the Valais Alps begin in the north, where molasse outcrops appear in a few locations near Bouveret. The left bank of the Rhône River between Monthey and Le Bouveret is made up of tectonic units from the Pre-Alps, which have been transported over long distances from the Penninic units on the southern side of the Rhône. The areas located on the left bank of the Rhône between Monthey and Martigny, as well as most of the land to the north of the Rhône between Martigny and Brig, belong to the Helvetic domain and form the "High Calcareous Alps." The massifs of the Aiguilles-Rouges, Mont-Blanc, and Aar represent the foundation (basement) of these units and are primarily composed of Paleozoic or older gneisses.

South of the Rhône, in the Val d'Entremont upstream of Orsières, and in the Saastal, outcrop Penninic units made up of Mesozoic sedimentary rocks, often associated with their Paleozoic gneissic basement. Higher up, in the upper sections of these valleys, now occupied by large reservoir lakes (Mauvoisin, Dix, Moiry), and in the Zermatt area, one can observe the rocks that once formed the ocean, the Tethys, whose closure was responsible for the formation of the Alps. The rocks that once constituted the continent to the south of this ocean now form the high peaks such as Mont Collon, Dent Blanche, Weisshorn, and the Matterhorn.

Valais is not only an ideal site to study the formation of the Alps, but it also offers the opportunity to observe the phenomena related to their erosion. It is worth noting that it was in the Val de Bagnes that crucial observations were made, which contributed to the glacial theory. This theory concluded that climate warming had led to the disappearance of glaciers that once filled the Alpine valleys in ancient times.