While out hiking, he had found "some designs on rock that looked like fern frond fossils" or "like something bacteria have left." From just that description, I had some ideas about what these "fossils" could be, but asked for a photograph anyway. This is what he sent me:
|"Fossil" from Nevada|
This is actually a really, really common thing to find, so I recognized it right away. While these pretty, leaf-like designs look like ferns, they're actually not fossils at all. Instead, they are a type of pseudofossil, a manganese-oxide mineral stain. These stains almost always form this type of dendritic (tree- or fern-like pattern) in the bedding planes of the rock. They often occur in sandstone, as it does in the example above. For comparison, here is another examples of dendritic mineral stains.
|From the Arkansas Geological Survey|