The Skeleton of a Lost Mountain Range
The gleaming, layered rocks of the Cévennes are primarily mica schists, the metamorphic center of a mountain range far older than the Alps. These formations are relics of the Variscan Orogeny, a massive continental collision that assembled the supercontinent of Pangaea. This mountain-building event spanned from approximately 380 to 280 million years ago. The rocks seen today began as thick layers of mud and sand deposited in a marine basin during the early Palaeozoic, between 570 and 400 million years ago.
During the continental collision, these sedimentary layers were buried deep within the Earth's crust. They were subjected to intense pressures and temperatures, a process called metamorphism. An important metamorphic event in the Cévennes is dated to around 340 million years ago. The original clay minerals in the mudstone recrystallized into platy minerals like muscovite and biotite mica. This alignment of new minerals gives the rock its sheen and layered texture, known as schistosity. The word "schist" itself comes from the Greek word for "to split," a reference to how easily the rock breaks along these parallel mineral planes.
Reading the Folds and Intrusions
The dramatic, ribbon-like folds visible in the schist outcrops are a direct record of the immense compressional forces that crumpled the crust. These structures show how the rock behaved like a ductile, plastic material under extreme heat and pressure. The Cévennes schists are part of the Massif Central, a large block of this ancient Variscan basement rock. While the Variscan mountains have long since eroded, the much later Alpine orogeny—the event that raised the Alps beginning around 65 million years ago—uplifted the entire Massif Central block. This uplift brought these ancient mountain roots back to the surface, exposing them to erosion which carved the deep valleys seen today.
Around 315 to 300 million years ago, as the Variscan mountain-building wound down, the crust began to stretch and relax. This allowed magma to intrude into the newly formed metamorphic rocks. These intrusions cooled slowly to form the granite massifs of Mont Lozère and Mont Aigoual, which are chemically distinct from the surrounding schists. The heat from these granite plutons further altered the adjacent schist, creating what is known as a contact aureole.