Leonardo sees the presence of marine animal fossils on emerged land and on mountain tops as the irrefutable proof of the cyclical transformations of the terracqueous globe. He demonstrates the absurdity of the traditional view that those creatures had been carried to mountain tops by the Biblical Flood. And he mocks the authors who had attributed those singular presences to celestial influxes. The only plausible explanation is that the emerged lands had, long ago, been on the ocean floor. At a certain point, cataclysms caused the lands to emerge, and the marine animals inhabiting them left the imprint of their skeletons in the dried mud.
Above the plains of Italy where flocks of birds are flying today, fishes were once moving in large shoals.
Codex Leicester, f. 10v