Mountain peak containing 440-million-year-old corals. Fossil preservation shows they lived in tropical reefs that drifted north on a tectonic plate movement from the equator.
Mænsard vokser, CC BY-SA 4.0
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.
💡Fun Facts
The Cellon section in the Carnic Alps was where the world's first Silurian conodont zonation—a method of dating rocks using tiny tooth-like fossils—was developed in the 1960s.
Some of the most abundant fossils are orthoconic nautiloids, ancient cephalopods whose cone-shaped shells could reach considerable lengths.
The entire Paleozoic sequence in the Carnic Alps is one of the most complete in the world, documenting nearly 300 million years of Earth's history.
The rock layers are often referred to by facies, or rock types, that indicate the water depth, such as the shallow-water "Wolayer Facies" and the deeper "Plöcken Facies".
The outdoor fossil sites and hiking trails are accessible 24/7, weather permitting. The best season is when the trails are free of snow, typically from June to October.
Admission
Access to the geological trails is free. The Geological Museum of Carnia in Ampezzo offers free admission, though donations are welcome.
Accessibility
Accessing the fossil sites requires hiking on mountain trails which can be steep, rocky, and uneven. Sturdy footwear is necessary. The sites themselves are not wheelchair accessible. The Geological Museum of Carnia is accessible.
