The Vein of Chalk extends along 25 kilometers between the provinces of Ravenna and Bologna and is interrupted transversely by the valleys between Brisighella, Riolo Terme and Borgo Tossignano. This area, which develops between 100 and 500 meters above sea level, is characterized by gullies of considerable visual impact. The origin of this spectacular formation dates back to 7 million years ago, when the Mediterranean remained isolated from the Atlantic Ocean due to a rise in the seabed, which caused the closure of the Strait of Gibraltar.
The Mediterranean thus became a closed sea poorly fed by rivers and rains. An almost total drainage occurred, caused also by the intense evaporation, which transformed the sea into a multitude of brackish pools. The saline solution concentrated to such an extent that the plaster began to crystallize and settle on the seabed. Following the reopening of the Strait of Gibraltar, a deep sea environment was formed where the sedimentation of materials rich in fossils resumed, covering the chalky formation. These clayey sediments, which the atmospheric agents then modelled, are the origin of the gullies. As a result of earthquakes, the seabed subsequently rose, forming the Apennines. This area is a great attraction for speleologists due to the presence of numerous underground cavities, such as the Tanaccia cave, the cave of King Tiberio and the Bandit cave.