A Foundation of Ancient Seabeds
The white cliffs of Bonifacio rise up to 70 meters above the Strait of Bonifacio. These are not granite, like much of Corsica and neighboring Sardinia, but a younger limestone. The rock, a type of calcarenite, formed from marine sediments deposited during the Miocene Epoch, roughly 16 to 21 million years ago. Geologists refer to this layer as the Bonifacio Formation.
This formation contains many fossilized remains of marine life from a temperate, shallow sea. The primary components are the skeletons of bryozoans, red algae, foraminifers, and echinids. These organisms formed extensive bioclastic sand dunes on the sea floor. Tectonic forces later uplifted this entire seabed, exposing the soft, porous limestone to the elements. The town of Bonifacio, established in the 9th century, is on this Miocene foundation, a vertical slice of a prehistoric marine ecosystem.
A Drowned Valley Undercut by Waves
The dramatic coastal geomorphology is the result of two distinct processes. First, the harbor itself is a ria, a drowned river valley. During past ice ages, when sea levels were lower, a river carved a steep canyon through the limestone. As the ice sheets melted and sea levels rose, the ocean flooded the canyon, creating the narrow, fjord-like inlet that shelters Bonifacio's port.
The second process is ongoing and relentless: wave erosion. The constant force of the sea carves away at the base of the porous limestone cliffs. This creates a deep notch or undercut, leaving the rock above unsupported. This process is especially pronounced along this stretch of coastline, which is considered highly sensitive to erosion. Eventually, the overhanging rock collapses in large sections. This slow, persistent undercutting is why the medieval buildings constructed on the very edge of the precipice appear to lean out over the water, with some at risk of eventually falling into the sea.