La Gonfolina - a narrow stretch of the River Arno downstream from Lastra a Signa - conventionally defines the demarcation between the Upper and Lower Valdarno. The Gonfolina, where today one observes only a great rock, is all that remains of the old natural barrier along the shore of the ancient lake which was situated in the plain between Florence and Pistoia. The Montalbano elevations were thus joined to those which on the left side of the valley marked the limit of the Pesa Valley to the south of Montelupo; in a remote era, probably by erosion, a fracture occurred on that side with the consequent creation of the bed of the River Arno and the gradual draining of the lake.
Leonardo understood the different nature of the lands upstream and downstream from the Gonfolina. The gentle hills of the Lower Valdarno originate in fact in great part from the sea bed in the Pliocene age and are thus rich in fossils of shells and marine life. As Leonardo wrote:
“Where the valleys do not receive salt water from the sea there are no shells to be seen as is clearly seen in the great valley of the Arno above the Gonfolina, the rock united in ancient times with Mount Albano in the form of a very high bank which held back that river before it flowed to the sea.” (Codex Leicester c. 9r).