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Granites and metamorphic rocks

The Crystalline Massif

Corno Stella, an anvil of gneiss (PNAM archive).

The Maritime Alps are bold, severe mountains, falling away abruptly into the valley bottoms with breath-taking drops in height. These features are linked primarily to the particular rock type: we find ourselves in the heart of what geologists call the Argentera Crystalline Massif, a huge group of compact rocks laid out in large steeply inclined banks, in some cases vertical, that occupy the whole of the upper part of the Valle Gesso and a part of the Valle Stura di Demonte. The peaks are made all the more severe and dominating by the deep, narrow valley bottoms.

The Argentera Crystalline Massif is composed of different rock types, their different characteristics show in the scenery. In the centre of the massif, in the Fremamorta, Valasco and Portette area there are outcrops of intrusive magmatic rocks, formed by extremely slowly cooling magma within the Earth's crust. In this long process, the crystals were able to form fairly regularly, creating a medium grained granite, which was later exposed by erosion. Today this granite forms elegant rock formations like the Caire di Prefouns and the Cresta Savoia.

Around the central granite mass we find rocks that have been subjected to less extreme temperatures and pressures, which have altered their mineralogical make-up and structure without melting. These are metamorphic rocks and in particular, forms of gneiss, which have different characteristics according to the original rock type and the geological processes they have undergone. Very similar to granite in compactness and mineralogical make-up are the granitoid gneiss, which form among others, the unmistakeable, imposing outline of Corno Stella so popular with climbers.

The highest peaks of the Argentera ridge and those of Matto are made up of banded gneiss, so named because they show a close succession of light and dark minerals.

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