For those who chase fossils, hunting grounds are almost invariably in areas comprised of sedimentary stones like limestone and sandstone. It can be a bit mystifying if you’re not familiar with some basic geological principles.
So, let’s dig in and we’ll teach you why fossils are only found in sedimentary stones.
Types of Stones
There are three main types of stone that emerge from the earth, each with its own unique qualities.
Sedimentary stones are deposited material that’s been compressed into a single mass over time. These stones are “young” and are often soft. Limestone, comprised of ancient crushed shells, is a classic example.
Farther along the scale we have metamorphic rocks. Metamorphic rocks are farther along the scale, having been around for longer. Metamorphic stones occur only under extreme conditions underneath the surface of the Earth and they’re part of the “cycle.” The most famous is marble, which is actually metamorphosed limestone!
Lastly, we have igneous stones. These are formed directly from volcanic activity, whether a partial melt under the surface or from a volcano. Igneous stones are the beginning of a stone formation’s lifecycle, weathering on the surface reduces them to sand which will eventually be compacted into sedimentary stones.
How Sedimentary Stones Form
Sedimentary stones are formed from crushed material placed under heat and pressure. The process usually requires moderate heat.
Much of the material that forms these stones come from weathered igneous rocks. Some, like limestone, come from areas rich in calcium from shells and other organic sources.
What’s important to understand for the fossil hunter is this: sedimentary stones are the first place in a stone’s lifecycle that flora and fauna can leave an impression.
The crushed matter gets hot as it gets pressed underneath the weight of the erosion on top. Sedimentary stones are… sediment and sometimes dead things leave an impression. These range from the dinosaur footprints that can be found in some places to large bits of dinosaurs and other megafauna.
The majority of the fossils found in sedimentary rock are imprints rather than original matter. There are exceptions, where bones and teeth have been preserved for millions of years, but you’ll usually find a “cast.”
Here’s the takeaway: sedimentary stones are formed from eroded igneous stones and past fossils, which makes them ideal for having “caught” organic matter to leave an impression.
When we talk about “young” and “old” in geology, we’re discussing incredibly long timescales. The entire history of humanity’s rise is insignificant to time scales measured in billions of years.
The oldest known limestone formation is roughly 2.7 billion years old, and even the youngest limestone is almost 200 million years old.
Heat, Time, and Pressure Play a Big Role
In addition to the way they form, it’s important to understand that sedimentary rocks form under less extreme conditions than metamorphic stones. This increases the chance of a fossil surviving over time.
As the stone goes “down” the pressure and heat increase, which is how we end up with metamorphic rocks. These conditions destroy any trace of the original creature as time goes on.
Naturally, as the stone gets heated and compressed the risk to fossils becomes enormous. Remember that most of them don’t “survive” even as a sedimentary stone is formed in the first place, when conditions get even more extreme you’re looking at the metamorphic process.
Time also plays a role, just because the conditions have emerged for longer.
So, How Do Fossils Form in Sedimentary Stone?
As noted above, the majority of fossils are simply impressions left behind.
When organic matter falls into a deep sedimentary layer, it often ends up decomposing in a different way than you’d expect. The majority of bacteria that feast on dead things are aerobic, so they require oxygen which isn’t present in deep layers of mud or silt.
Anaerobic bacteria still operate, but more slowly and they often leave behind more of the creature or plant. Given time, this slow rot leaves behind the impressions that we know as fossils.
On rarer occasions, a hard bit of organic matter like a dinosaur’s bones is slowly replaced with more durable minerals during the fossilization process. They’re usually considered “rock” by the time they’re unearthed.
In recent years, there’s been some debate as to just how much of the original matter remains.
In any case, the specimens that most collectors are seeking in the field are smaller imprints. Vertebrate fossils, including fish, are usually considered state property but invertebrates are usually fine to bring home.
In certain places, you can find more than imprints. Shells, for instance, rarely seem to dissolve in limestone since they’re made of the same material. These leave long-lasting, heavily fossilized pieces, one of the most famous being ammonites.
Fossils all tell us stories, and each is unique. The journey is always similar fossils are formed through mineral replacement of organic structures or imprints left by undecayed matter.
Exceptions to the Rule
Now that we’ve established why you’re looking at all fossils being in sedimentary stone… we’re about to talk to the exceptions to the rule.
As a child, I was once fascinated by a slab of marble with a shell contained in its surface used as a table. The cross-sectional piece showed the internal structure of the shell, but the surface was indistinguishable by touch from the stone around it.
I thought I must have been mistaken as I learned about geology until I discovered that ammonites have been found in marble. While rare, they’re still common enough to find a place in home decor use. Commercially these bits of marble are usually known as Fossil Marble.
That said, fossil specimens in metamorphic rock are vanishingly rare and notable in even museum-grade collections. These are usually the result of some freak one-in-a-billion occurrence.
For the most part, however, it can be assumed that 99.999% of fossils on the planet are contained in sedimentary stones. It’s younger, softer, and subject to less heat and pressure which creates the perfect environment for fossil preservation.