The history of life on our planet and the many changes it has been through are fascinating. Because of this, fossils are a great addition to any rockhound’s collection.
But, where do you find these fossils, and what kind of rocks can I find them inside?
Today we will go over the types of rocks that contain fossils and where you can find these rocks.
4 Rocks That Contain Fossils
Mudstone is a fine-grade sedimentary rock created from hardened silt and clay.
It is usually soft and lacks any layering.
It is also smooth to the touch and varies in color.
Fossils are common in mudstone.
A mudstone’s fine grade actually protects the fossils from erosion, which is why fossils are so common.
Mudstone can be found in dried-up lakes or sea beds.
One of these places is Ravenscar, North Yorkshire in the United Kingdom.
This deposit is from the Jurassic period, about 180 million years ago.
They contain fossils of shells of ammonites, a relative of squids and cutter-fish.
They are a dark, almost black color.
Ravenscar is considered a Site of Special Scientific Interest, so you can’t hammer into the rock without permission.
If you don’t want to journey all the way to England, you can purchase these fossils online.
Shale is another fine-grade sedimentary rock found at the bottom of ancient seas and lakes.
The difference between it and mudstone is that it is fissile.
Fissile means that it tends to split into flat layers.
This splitting is due to being laminated or made up of many layers.
Organic materials, like shells, commonly get stuck between the layers and fossilize.
It can typically be found near oil deposits or contains oil in them.
One place you can find shale fossils is at the U-dig fossils site in Utah.
It is located 52 miles west of Delta, Utah.
For $33, you can dig there for fossils for two hours. Longer stays cost more, but hounding tools are provided.
The fossils you are most likely to find are trilobites.
Trilobites are hard-shelled invertebrates that lived 550 million years ago.
Again, you can purchase the fossils on their website if you don’t want to go to Utah.
As the name implies, sandstone is condensed sand. It is (as you might suspect) sand-colored and coarser than shale or mudstone.
For this reason, the rock is more porous than the other stones on this list and less likely to contain fossils.
A more porous rock is more likely to have water seeping in and erode any fossil.
It is a very prevalent type of stone, so it isn’t too uncommon to come across some containing fossils.
Some of the best places to find fossils are inside outcrops.
Outcrops are areas of exposed rock that ” crop out” of the ground.
This rock was forced out by geological upheaval and often takes otherwise buried fossils with it.
Artificial geological upheaval, like quarrying, can also reveal hidden fossils.
Cliffs and dried river beds are also places to find fossils—any place where natural or unnatural forces have cut into the land might yield a fossil.
Limestone is made mainly of calcium carbonate and dolomite.
In some cases, limestone is a fossil.
Formed by coral and seashells where waves broke down and sunk to the ocean floor, limestone could be considered fossilized.
Due to this organic origin, fossils are so commonly found in it.
Limestone has many industrious uses, including being used in making cement. It is also commonly quarried for building.
Fossiliferous limestone is a term for limestone that contains abundant or obvious fossils.
Fossil hunters find them in Germany, particularly in the quarries around Nuremberg and Stuttgart.
They include ammonites, coral, brachiopods, and crinoids.
Fossiliferous limestone is used in the chemical industry to produce ammonia and calcium carbonate.
Why Are Fossils Found in Sedimentary Rocks?
Of the three different types of rocks (sedimentary, igneous, and metamorphic), only sedimentary commonly holds fossils.
Why is that? What about other kinds of stones?
Let’s start with how sedimentary rocks form.
Sedimentary rocks are created by the accumulation of minerals on top of each other.
Metamorphic rocks are formed when other rocks end up under the Earth and are exposed to great heat and pressure.
Igneous rocks are formed from magma or lava cooling.
Sedimentary rock formation usually lacks the heat and pressure that is involved in making a metamorphic rock or the hot magma or lava that forms igneous rocks.
This heat and pressure that is required to form metamorphic rocks would also destroy any organic material before it fossilizes.
It would also destroy any fossils that may have already formed in it.
Why Are Fossils Not Found in Igneous Rocks?
It is particularly rare, if not unheard of, for fossils to be found in igneous rocks.
This is because igneous rocks form straight from molten rock, either from magma or lava.
The temperatures that can melt rocks destroy any organic materials and melt any fossils that may have already formed in the rock.
Volcanic places where igneous rocks form are also hazardous to life.
It is unlikely that any complex life needed to make fossils would have lived in a volcano or on top of a lava flow.
This point is not to say a fossil can’t form in igneous rock in theory.
It is just so unlikely that one has yet to be discovered, as far as we can tell.
Limiting your fossil hunting to sedimentary rocks is probably best, or you risk wasting your time.
Sedimentary rock is more common than igneous rocks anyway.
So, What Have We Learned?
We have learned that a variety of rocks contain fossils and that these include mudstone, shale, sandstone, and limestone.
We learned how these rocks form and what kind of fossils can be found in them.
Then, we learned where you could find these rocks with fossils in them.
And lastly, we learned why only sedimentary rocks have fossils in them. We wish you luck on your fossil hunting and happy digging.
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