Ginkgo gardneri fossil © Natural History Museum, London
The Ginkgo leaves were preserved when they fell into water and were quickly covered by fine sediment. This stopped bacteria from eating the leaves and they were gradually squashed as more sediment piled up on top of them.
The sediment eventually turned into rock and was pushed up towards the surface by tectonic activity, allowing palaeontologists to access the fossils.
The leaves extracted from the Palaeocene rocks on Mull are so well preserved that fine detail such as veins and stomata (breathing pores) can be seen.