Why are there fossils of leaves and fresh fish if fossils form slowly?
Delicate materials don’t usually fossilize. In fact, usually nothing at all is preserved because decomposition and predation/scavenging is so ubiquitous. This is true for large animals and plants and especially for delicate materials, but let me give you one of the coolest examples in the world and then briefly explain how the fossils there were preserved.
Florissant Fossil Beds National Monument is in Florissant, Colorado, just a short drive from Colorado Springs. This place is spectacular in terms of fossils. The lower quality photos below were taken by my wife in the visitor center there, the higher quality ones were lifted from the internet.
There are not only leaves…
and fish…
but fruit!…
As well as insects and even spiders!
In fact, Florissant has some of the largest petrified tree stumps in the world:
Preserving insects in stone is incredibly rare, so how exactly did it occur? Well, Florissant was a pretty unique environment.
About 35 million years ago, Florissant was a stream valley near- at that time- active Guffey volcano. That stream was dammed by a volcanic mudflow, as happens sometimes, which turned it into a lake. The cement of that mudflow deposit, by the way, is what preserved the massive petrified stumps.
The lake flourished, but the nearby volcano remained active, dropping ash periodically into the lake which fed massive diatomaceous algal blooms. It basically turned the lake into a green algae soup from time to time. When leaves and insects would fall into it, they would sink to the bottom, as always happens.
Now, decomposing algae sucks all the oxygen out of water… it’s a major problem in many parts of the world. Oxygen, by the way, is required for decomposition. So with the algal blooms periodically depleting the lake’s oxygen, when a leaf or insect or fish reached the bottom of the lake, it did not decay quickly and was instead soon buried under a layer of diatoms, ash and clay. Once buried, the organic materials inside were effectively preserved, leaving behind either impressions or carbon imprints within what is now called paper shale for its delicate layering.
Not only can you visit Florissant whenever you want- and you absolutely should- but you can also go next door to where a private business operates at the edge of those old Florissant lake deposits just beyond the protected boundaries of the national monument. There you can buy a bucket of shale collected from the property and sift through it carefully for fossils of your own. It is wonderful fun for any amateur geology enthusiast, child or adult. I brought my freshman geology field camp there one year and while I found a whole lotta nothin, others succeeded in finding leaf and insect fossils… nothing museum quality, but still very cool.
Anyway, that’s one way to preserve delicate fossils and pretty much the only way to preserve the most delicate ones.
