Leonardo was quite familiar with the Golfolina (“a boulder in antiquity united with Mount Albano, now forming part of the most recent river bank”), an interesting rock formation situated along a narrow stretch of the Arno halfway between Vinci and Florence. In particular, he mentions on a folio from the Codex Atlanticus (785b-r) how this strait could slow the course of the Arno, causing it to overflow its banks when the river was high and flood the surrounding plain. He notes on two folios in the Codex Leicester that the Golfolina offered a useful observation point for hydrogeological studies of the zone of separation between the freshwater of the lake and the saline waters of the sea just a few kilometres downstream.
On folio 8B-8v Leonardo refutes the theory that the marine fossils found on mountaintops were the shells of already dead molluscs that had been swept inland by waves from the sea. He believed geological explanations could be found for this – from a rise in the level of the seabed to the deposition of gravel and other material by sediment – filled river waters or the current flowing from the Mediterranean through the Strait of Gibraltar to the Atlantic. He cites among the rivers that emptied into the sea from a great height the Arno River at Golfolina near Montelupo, where gravel and different types of rock tended to conglomerate, and describes the curious volcanic tuff near Castelfiorentino which resembled “congealed sand”..