An ocean floor in the mountains
The peaks of the Carnic Alps, along the border of Italy and Austria, hold a geological contradiction. High on mountains like Creta di Collinetta and in the stone walls of the Volaia Pass, you can find the remains of a tropical sea floor. These limestone rocks are dense with fossils dating back to the Silurian Period, between 444 and 419 million years ago. At that time, this part of the world was not a mountain range but a shallow, warm sea on the northern margin of the supercontinent Gondwana, located near the equator.
The rock layers here are so well-preserved and continuous that geologists consider the area a global reference point. The Cellon section, for instance, is a well-known site for studying the Silurian. Its layers of limestone and shale record millions of years of Earth's history with exceptional clarity. The entire Silurian sequence in the Carnic Alps is relatively thin, not exceeding 60 meters, yet it shows a history of changing sea levels. Deeper basins formed dark, graptolite-rich shales, while shallower waters produced gray limestones with corals.
The movement from an equatorial sea to an alpine peak was a long one. This piece of crust, part of the African tectonic plate, drifted northwards for hundreds of millions of years. It eventually collided with the European plate, a slow-motion impact that crumpled the crust and uplifted the ancient sea beds. This mountain-building event, the Alpine Orogeny, raised the fossilized reefs thousands of meters into the air, creating the alpine terrain seen today.
Life in the Silurian sea
The fossils of the Carnic Alps provide a detailed record of a Paleozoic marine ecosystem. The warm, shallow waters supported a diverse community of organisms. The limestone layers, called "Orthoceras limestone," are packed with the straight, conical shells of orthoconic nautiloids, which were dominant predators. Alongside them are found abundant trilobites, bivalves, and microscopic conodonts. Less common, but still present, are the remains of crinoids (sea lilies), gastropods, ostracods, and brachiopods.
The state of preservation is often remarkable. In some deposits, an early precipitation of iron oxides and hydroxides coated the organisms' skeletons, creating detailed ferruginous replicas that survived even when the original calcareous shell dissolved. This process preserved fine details of the shells of trilobites, bivalves, and echinoderms.
Specific formations within the Silurian sequence tell different parts of the story. The Kok Formation consists of brownish, iron-rich limestones filled with nautiloids. The Cardiola Formation is characterized by layers dense with the bivalve Cardiola docens. These distinct fossil assemblages allow paleontologists to reconstruct the specific environments and time periods with great precision, turning the mountain slopes into a read-out of deep time.