free debate

July 25, 2011

Fossil or Not?

Identifying fossils is a more complicated task then you'd think. There are lots of different kinds of rocks, which can have their own unique patterns in them. Some of these patterns are the remains of ancient life; others are abiotic structures, chances of mineralization or later erosion that lead to pseudofossils. A reader emailed me a few weeks back with a question about some "fossils" he'd found.

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
While these aren't fossils, they're still cool. They form as water seeps through the sandstone. This water has dissolved minerals in it, including manganese or sulfides. As the water gets filtered through the pores in the rock, the minerals are left behind, precipitating into manganese oxide or pyrite (depending on the type of minerals dissolved in the water). The way these minerals crystallize, they form elegant branching dendrites along the bedding planes (lines within the rock, formed when the rock was deposited as parallel layers of sand) of the rock. In my own rock collection, I have quite a few dendrites, because they're one of the prettiest pseudofossils, and have such an interesting story behind their formation. 

Thanks to Claude A., for permission to use this example.