At first glance this seems to contradict what RAZD just posted about Leonardo's refuting the idea that the fossils in mountains had grown there
No contradiction there. He refuted the ideas that they had been deposited by any flood or had grown
*IN* the rocks. "In" means "inside existing rocks", which was one theory of fossils at the time. He realized that they had grown on an exposed sea floor and later been covered and leater been uplifted.
If local floods could have deposited them and not broken them to bits, why couldn't the worldwide Flood?
Local floods could not have deposited them without breaking them to bits.
Why is there such certainty about what the Flood would have done as long as it contradicts what the Bible says?
Because we know a lot of how the physical world works.
So the Flood laid down all the sediments along with the shells and other fossils, and after the Flood the mountains were raised: that's the tectonic activity that occurred with the splitting of the continents. See? It all works out just fine.
Back to magic. The fludde was an incomprehensible raging torrent when you need it, it was a millpond when you need that. Leonardo was smart enough to realize that it doesn't work out just fine, and all the evidence and knowledge we have gained since then supports that conclusion. If the fludde had laid down the sediments and fossils THEY WOULD BE BROKEN UP AND WOULD NOT SHOW OBVIOUS SIGNS OF HAVING GROWN WHERE THEY WERE FOUND.
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