The hills of Greti, and the environs around Vinci up to the river, were the landscapes familiar to Leonardo during his youth. In these places he had the opportunity to observe those elements of nature that, out of mere curiosity, became for him the occasion for scientific reflection. In San Pantaleo, for example, where his mother lived, he had probably been able to observe the fossiliferous clays of the place called colle marino (marine hill). In several passages in the folios of the Codex Leicester, Leonardo elaborated his theory about the way the different geological levels of this area of the Valdarno formed: today we would refer to this as the "Geological Stratigraphy of the Middle Valdarno". Leonardo noted the diversity of the geological formations emerging near Capraia and Montelupo, composed mostly of sandstone pebbles (today visible in the Arnovecchio fossil river bend), by way of comparison with the primarily fossiliferous clays he observed in the so-called "Collegonzi cut". His theories about the ancient sea that once reached the gorge of La Gonfolina explained the origins of the "gravel", the "sand", and the "bluish mud", and above all, the reason for the presence of the famous nichi, the fossil shells he observed in the clayey deposits of the area around Greti. Collections of fossils from the places where Leonardo saw, collected, and drew his nichi are on display in the Museo di Storia Naturale dell’Università di Firenze, in the Museo di Paleontologia di Empoli, and in the Museo Ideale “Leonardo da Vinci” at Vinci.