The crystalline massif of Argentera
Granite and gneiss
The Maritime Alps are bold and severe mountains, falling sharply away to the bottom of the valley. This is due above all to the particular type of the rock: we are in fact in the heart of what geologists call the Argentera crystalline massif , a gigantic mass of compact rocks arranged in large, steeply inclined or even vertical banks, which occupies the entire upper Gesso Valley and part of the Stura di Demonte Valley. Furthermore the peaks are made even more severe and imposing by the narrow, deep valleys.
An alysis of the relationship between the characteristics of the mountains and the nature of the rocks, makes it necessary to differentiate several cases within the Argentera Massif. At the centre of the massif, in the area of Fremamorta, Valasco and Portette, a mass of intrusive magmatic rocks emerges, that originated by the very slow cooling of the magma inside the earth's crust. During this long process, the crystals developed fairly evenly, until they gave rise to a medium-grain granite, which was later exposed by erosion. Today this granite forms elegant rock structures like the Caire di Prefouns and the Cresta Savoia.
Around the central granite mass there are rocks that have undergone somewhat lower temperatures and pressures that have altered their mineralogical composition and structure without causing their fusion. In other words, they are metamorphic rocks and in particular gneiss , which have different characteristics depending on the original rock and the geological events it went through. Very similar to granite for its compactness and mineralogical composition are the granitoid gneisses , which form, among other features, the unmistakable shape of Corno Stella, particularly coveted and frequented by mountaineers.
The highest peaks of the Serra dell'Argentera and those of Mt. Matto are instead made up of banded gneisses , so named because of their alternating layers of light and dark minerals.