You'll have to point me to the leaves, but the shells on mountain tops are easy: The rocks they fossilized on were once under water.
We've dated many of these shells to be hundreds of millions, if not billions, of years old. And we can also measure the rate that tectonic plates move and the resulting creation of mountains. So once you know that the fossils are older than the mountains, it's easy to see that they died there and that the mountains later formed. This is only a mystery if you believe that both the mountains and the sea both always existed in their present forms, which we know they did not do.
RE: The End Of Reason: A Glimpse Behind The Curve