The Sassello area, belonging to the so called Bacino Terziario Piemontese (tertiary Piedmont Basin), is of remarkable interest from a geology point of view, as it is characterized in many spots by fossil remnants of marine microorganisms dating back from the superior- inferior Oligocene, around 33-25 millions of years ago. In particular it is worth mentioning the resort of Maddalena-Ponte Prina, where a huge presence of colonial fossil corals, generally in life position, can be noticed.
The Bacino Terziario Piemontese is characterised by an imposing succession of sedimentary rock deposited in the ancient Mare di Ranzano (Ranzano sea), that covered the lands which now emerge, towards the buttresses of the Alpine range. The geographic set up in the Oligocene was therefore completely different from the present one, practically the opposite, with sea where the Pianura Padana is today, a transition belt, featuring alluvial plains covered in lush vegetation and finally the emerged lands in place of the Ligurian sea.
In the town of Maddalena-Ponte Prina, thanks to some rocks and the fossils that they contain, the first stages of this transition can be reconstructed, consisting of the formation of a shallow sea environment, where small coral bioconstructions developed. Maddalena- Ponte Prina, one of the few sites where high quality Oligocene corals can be easily observed, is one of the most remarkable paleontological highlights of the Beigua Geopark.