From Seabed to Summit
The sandstone cliffs of the Monti della Laga in central Italy are not what they seem. Today they form peaks reaching over 2,400 meters, but they began their existence on a deep ocean floor. These rocks are part of the Laga Formation, a thick sequence of turbidites deposited during the Messinian age of the late Miocene, between 5 and 7 million years ago. A turbidite is the geologic term for the deposit left by a turbidity current—a catastrophic underwater avalanche of sand, silt and clay that flows down a submarine slope.
The setting was the Laga Basin, a deep foreland basin that formed as the Apennine mountains began to rise. Material eroded from the emerging Alps and Apennines was washed into the sea, accumulated on the continental shelf, and periodically collapsed. These slurries raced for tens of kilometers across the basin floor, depositing their sediment in vast lobes. The evidence for these events is preserved in the rock as graded bedding. Each distinct layer shows coarser, heavier grains at the bottom and progressively finer grains toward the top, a record of particles settling from a submarine landslide. Some individual layers are meters thick, showing the immense scale of these ancient events. The total thickness of the Laga Formation reaches up to 3,000 meters in some areas.
An Orogeny in Action
The transformation of the Laga Basin from a deep sea to a high mountain range is continental collision. The Apennine mountain-building event, or orogeny, is driven by the ongoing collision between the African and Eurasian tectonic plates. As Africa pushes north, the crust is compressed, folded, and thrust upwards. The sediments that had accumulated in the Laga Basin were caught in this tectonic vise, crumpled, and lifted more than two kilometers into the air. This process of uplift began in the late Miocene and continues today, with the central Apennines rising at rates of around 0.2–0.3 mm per year.
The deep marine origin of these rocks is confirmed by their structure. The fine-grained layers between the sandstone beds contain microscopic fossils of foraminifera, single-celled organisms that lived in the open ocean. Their presence shows that these rocks were once at the bottom of a deep sea. The entire mountain range is a fossilized deep-sea basin, exhumed and raised to become part of the Apennine backbone. The sandstone peaks are a direct and visible result of deep-time geological forces that shape the planet.