Which Rocks Contain Fossils? | Identify a Fossil Rock | Fossils in Rocks
Do you ever look around a beach and think it would be amazingly interesting to find a fossil, but never do? Or, are you wondering which rocks contain fossils? Well, so are we. It turns out, there are ways to identify a fossil rock so you’re not just turning over every single fragment of soil to see if there’s fossils in rocks.
The first thing you’ll need to understand is; the three types of rock. Only one of these rocks contains what you’re looking for and we need to learn how to identify them as well as where we can expect to be extracting them from.
There are igneous, metamorphic and sedimentary rocks. In short, igneous rocks form when molten lava begins to cool and solidifies. Metamorphic rock is the result of existing rock that has come into contact with extreme pressure, heat or reactive chemicals. Sedimentary rock, on the other hand, is developed through particles from the air or water settling to form a new component or layer.
In this case, the rocks you can expect to find fossils in are sedimentary. This is because life ends at the earth’s surface and sedimentary rock is formed on top. Not only this, but the sedimentary rock layers are the only layers that aren’t subject to extreme heat or pressure, making the conditions just right to preserve fossils.
What Are Fossils?
So, now we know which types of rocks we’re looking into, we should probably start answering the question; “which rocks contain fossils?”. And, in order to do that, we need to fully understand what a fossil might be and what to look for in the sedimentary bed.
According to the Jurassic Coast Trust, which is not a trust dedicated to making the next Jurassic World movie disaster, fossils are the remains of ancient life that have been carefully preserved through natural causes.
When palaeontologists identify a fossil rock, they need to understand that fossils accumulate anywhere an organism has died. But, only specific conditions will allow fossils in rocks to form rather than become scavenged or eroded by coarse sandy winds.
For example, fossils in rocks can form in an area that will have covered the organism in mud or sand. When submerged, minerals can begin to fill the spaces left over by soft materials that have rotted away (leaving hard materials such as bones). It is these exoskeletons or endoskeletons that provide the distinct shape of a fossil. However, it must also be noted that there are 6 ways in which these fossils can be formed.
What Are the 6 Types of Fossil?
While there are two major groups that palaeontologists will refer to at the initial excavation, there are many types of fossils within that. To know which rocks contain fossils, we’ll go through the two categories of fossils in rocks; body fossils and trace fossils.
While these might be self-explanatory, sediments in which rocks contain fossils of body of an organism or plant are body fossils. While, sediments in which rocks contain fossils of a mere foot-step or coprolite (ancient poop) are trace fossils.
Within these two categories, there’s 6 definitive types of fossils in rocks you can expect to find;
Permineralization
Also known as petrified fossils, the permineralization of fossils occur when an organism is exposed to minerals in water than can break-down the organic composition by invading the pores of the original structure.
It happens when a carcass has already been grounded into the sediment bed. Then the groundwater is soaked into the fossil-area. If the organism decays slowly over time, it can give a much more detailed fossil.
Recrystallisation
When a fossil has been subject to heat and pressure from the sediment bed, the chemical composition can often change. For example; a calcium-based skeleton such as us humans can see our bones recrystalise into calcite or aragonite.
While this process can cause the fossil to become more brittle and durable, the recrystallisation process can often reduce detailing of the fossil, meaning it is more difficult to identify after excavation.
Carbonisation
Carbonisation (A.K.A carbon films or compression) is a process where time and pressure squeezes the liquid and gases out of an organism to leave a flat surface of black carbon.
This process preserves the natural and fine details of the organism, leaving an imprint into the fossil of the animal itself. These are very commonly found with fish and plant materials because of the fine sediment produced with dense water.
Bioimmuration
In this rather different case of fossilisation, organic overgrowth will allow a deceased body or micro-organism to house a new encrusting organism. This is because the carcass can provide vital nutrients and minerals for the new organism to thrive. Over time, the original wil fade from consumption and the underside of the new organism will have an imprint of the original body.
The process of bioimmuration is most common with marine life such as corals and other sessile species.
Mold and Cast Fossils
Sometimes, soft-bodied organisms can be buried in rock. But, due to its soft-composition, groundwater can completely wash away the body. This leaves behind a mold (or, cavity) in the exact shape of the outer-organism.
Then, if this cavity is later filled with another material, such as sediment or mineral matter, it simply becomes a cast fossil.
Preserved Remains
Even though were looking at which rocks contain fossils, rocks are not the only place to look. Some of the most famous cases of preserved remains include mammoths found in ice, bugs found in amber and Egyptian mummies found within their intricate pyramid burial chambers. Which is probably why these aren’t normally fossils in rocks and often referred to as mumification fossils.
Interestingly, there’s a pit called the La Brea Tar Pit in Los Angeles which has seen the discovery of perfectly preserved sabretooth tigers and woolly mammoths. All they’re missing is a Sloth named Sid and you have the story of Ice Age.
What Are the Most Commonly Found Fossils?
To have a definitive example of which rocks contain fossils, so we find them easily, we should probably turn towards the most commonly found fossils and which rocks house them.
According to the National Geographic, a palaeontologist team digging in the Bavarian region of Germany found a perfectly preserved pterodactyl fossil in limestone. Which means that limestone is a good housing condition for fossils to form.
Limestone is one of the common rocks where fossils can be found because it is one of the three main sedimentary rocks that palaeontologists base their findings on. The other two would be sandstone and shale (hardened clay). Such sedimentary rocks, have thin layers that have formed over time, looking like a cross-section of a stack of crepes often with varied colours.
It’s important to remember that when thinking about which rocks contain fossils, you’re not only thinking about trying to find large dinosaur bones in boulders. The most common fossils are small marine life found in small rocks on a coastline.
Here’s five of the most commonly found fossils in rocks:
- Ammonoids
- Brachiopods
- Corals
- Crinoids
- Trilobites
Why would you not expect to find Fossils in Metamorphic and Igneous rocks?
Fossils are typically not found in metamorphic and igneous rocks because these rocks are formed through processes that do not involve the preservation of biological remains. In the case of metamorphic rocks, they are formed when existing rocks are subjected to high heat and pressure, which can alter the original rock and destroy any fossils that may have been present.
Igneous rocks, on the other hand, are formed when molten rock cools and solidifies, and this process does not typically preserve fossils either.
Which type of rock could contain fossils of Woolly Mammoths?
Fossils of woolly mammoths are typically found in sedimentary rocks. Sedimentary rocks are formed through the process of sedimentation, in which layers of sand, silt, and other materials are deposited over time. When organic materials, such as the remains of plants and animals, are buried in these sediments, they can become fossilized.
Because sedimentary rocks are formed through the accumulation of sediments, they are the most likely type of rock to contain fossils.
Which rock layer would you not expect to find fossils?
You would not typically expect to find fossils in igneous rocks. Igneous rocks are formed through the solidification of molten rock, and this process does not typically preserve fossils. In addition, the high heat and pressure that is present during the formation of igneous rocks can destroy any fossils that may have been present in the original rock. Because of this, it is unlikely that you would find fossils in igneous rocks.
Do all rocks contain fossils?
Not all rocks contain fossils. Fossils are the preserved remains or traces of organisms from the past, and they are most commonly found in sedimentary rocks. This is because sedimentary rocks are formed through the process of sedimentation, in which layers of sand, silt, and other materials are deposited over time.
When organic materials, such as the remains of plants and animals, are buried in these sediments, they can become fossilized.
So, there you have it. A guide to the rocks you can expect to find fossils in and what they’ll look like.
As a final piece to this puzzle, you’ll probably want us to point you in the right direction of the sedimentary rocks. Well, Dorset’s famous Jurassic Coast has seen the findings of everything from woolly mammoth parts to shark teeth and from flowstone to plant leaf fossils. So, perhaps try your luck on a windy Dorset beach stay-cation.
