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How is it that fossils are often found in sedimentary rock but not igneous or metamorphic rock? The answer lies in how the different types of rocks are formed. Igneous and metamorphic rocks undergo immense heat and pressure during geological processes, destroying any fossil remains within them.
Sedimentary rock often contains fossils because it is formed differently. It consists of sandstone, limestone, or shale that was once mud or sand in which the bodies of dead animals settled. The temperatures and pressures that create sedimentary rock are much lower and don’t destroy fossil remains.
For fossils to form, the parts of a dead animal or plant must have time to absorb the minerals that replace the bone, feathers, teeth, shells, leaves, stems, and other structures to become rock. Sedimentary rock is created by the gradual deposition of mud, sand, and silt around the body in layers, preserving the form buried within it.
Sedimentary Rock Formation and Its Impact on Fossils
The sedimentary rock usually starts as mud or sand, and it is closely associated with the ebb and flow of water. Rivers deposit minerals in layers along their banks as water currents churn up the riverbed.
Ancient sea beds also consist of sandy sediment deposits that are soft at first and remain undisturbed for thousands of years.
Shale is one type of sedimentary rock in which fossils are often found. It forms when large rocks disintegrate through erosion into the minute, often microscopic particles transported by wind and rain and deposited into bodies of water.
The particles settle in calm swamps, lakes, and seas, covering the bodies of creatures living there. Over time, the mud and clay layers combine with other minerals, hardening into shale.
Shale is made of layers of rock that can easily be split open to reveal fossils within.
Biological limestone, another type of sedimentary rock, forms when calcite in water crystallizes, and coral and shell fragments are cemented together by sand and mineral deposits.
This limestone usually forms in calm, warm, shallow seawater in which organisms, such as mussels, oysters, and corals, with shells or skeletons containing calcium carbonate, thrive.
When they die, their bodies and waste contribute to the sediment in the water. This biological material, consisting of mostly organic debris, hardens with time and pressure to eventually become limestone.
Limestone can also form due to chemical processes resulting in the direct precipitation of calcium carbonate from fresh or saltwater. In this case, it is a chemical sedimentary rock that scientists believe to be less abundant than biological limestone.
The formation of limestone through evaporation can often be seen in caves. Slowly dripping water creates stalactites, stalagmites, and other formations, called speleotherms.
As the water evaporates, the calcium carbonate it contains is deposited in these various formations. The limestone in which caves form is a chemical sedimentary rock known as travertine. Paleontologists have found human and animal fossils in caves.
Limestone that contains an abundance of fossils is called fossiliferous limestone. It usually includes the remains of marine invertebrates such as gastropods, mollusks, crinoids, and brachiopods.
Sandstone is another type of sedimentary rock that forms when grains of sand are cemented together. It is coarser than limestone and shale, and although it can contain fossils, they are not usually as detailed as those found in the other types of sedimentary rock.
However, the exciting thing about sandstone is that it can contain both land animals and sea creatures’ fossilized remains. If you are searching for dinosaurs or large land mammals like mastodons, sandstone may be much more promising than limestone or shale.
The quartz grains that mostly make up sandstone are deposited in seas, rivers, beaches, or deserts. Very hard and pure sandstone is called quartzite. It can also contain small quantities of other minerals such as iron oxide and calcium carbonate. Sandstone is very common and can be found all over the world.
TIP: Do you know what is difference between fossilization and petrification? Many people often cannot distinguish these two terms correctly. Find out the main differences in the article below:
How Are Fossils Formed In Sedimentary Rocks?
Fossils become part of sedimentary rocks when sediments such as mud, sand, or silt from the sea and river beds surround an organism and preserve its body over time. The hard parts of creatures are preserved as fossils when they consolidate with minerals and other sediment materials.
Fossils often found within shale include algae, plant parts, brachiopods, arthropods, and crustaceans that became trapped in hardened mud. The fine silt and clay particles allow the smallest details to be preserved as these organisms gradually turn to stone.
The remains of living creatures and plants become fossils if they are preserved for around ten thousand years. However, fossils can go much further back in time than this. Some of the oldest ever found are those of algae that lived more than three billion years ago.
Creatures naturally decompose when they die, leaving nothing behind, which means that fossils are created in rare and unusual circumstances.
The remains must effectively be preserved for long enough that minerals in the surrounding sediment can seep into them and eventually take on their shapes. The sediment must then be subjected to a hardening process from pressures within the deep sea or earth and high temperatures.
Fossils rarely form in igneous rock because it is created from magma deep within the earth. The temperatures in molten rock are too high to preserve the shapes of animal and plant remains.
Occasionally a fossil may form from a footprint, trail, or imprint made by an animal or plant. These are called trace fossils because they don’t reflect a plant or animal’s anatomy but rather evidence of its presence.
True form fossils are parts of plants or animals that have been converted into rock, while cast fossils form when the outside of an organism, like a shell, for instance, becomes filled with sand or mud.
Fossils are common in shale because excellent imprints can be formed in fine-grained sediments like mud. However, only some shales contain fossils because many ocean floor areas were not suitable for animal life.
In these areas, fossils only occur from a few dead organisms whose bodies drifted into them and were deposited in the mud.
Sandstone can contain fossilized crustaceans, bryozoans, trilobites, and brachiopods. Desert sandstone does not usually contain fossils unless the desert was once a sea.
Fossilized wood, plants, or vertebrates can be found in sandstone in old river beds and deltas. Sandstone formed on beaches contains fossils of arthropods, mollusks, crinoids, and other marine creatures.
TIP: A lot of fossils come from animals so you might be wondering if the rocks contain DNA. Find out the article to this question in the article below:
How To Locate Fossil Bearing Sedimentary Rocks?
Before going fossil hunting, you should explore the geological formations in your area to identify which ones consist of sedimentary rocks.
Keep your eyes open when you pass through the land in which roads have been cut, and take note of the exposed layers in the embankments. You can also use geological maps of the area you want to explore to see where sedimentary rock is located.
If you go on hikes in the wilderness or along beaches, lakes, or rivers, carefully study any exposed rock faces you encounter.
Also, be aware of any stretches of rock you find in old riverbeds and deltas. Fossils may be under your feet and not in rock faces. Be mindful of odd shapes in the rock that could be ancient footprints or tracks.
Try walking in streams while gazing at the ground, not looking for anything in particular but watching for flat, sharp-edged rock fragments that have recently broken off of rock faces. They may contain trace fossils. Particularly lumpy rocks could be fossilized feces called coprolites.
Sedimentary rocks often occur near water sources because of the erosion that takes place in these places. Just remember that rivers and lakes dry up, leaving only their channels and basins behind.
Old lake and river beds may contain sedimentary rock even though the water is long gone. One way to ascertain whether a rock is sedimentary is to see how hard it is and whether it is composed of small grains.
BTW: Do you want to know more about rocks and minerals identification? The books listed below are the best ones you can find on the internet (Amazon links):
- The Crystal Bible
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
You don’t have to be a geologist to find fossils, but it helps to be able to identify sedimentary rocks. Fossils occur mostly in these rocks because the geological processes that create them offer the most favorable fossil formation conditions.
TIP: When looking for fossils, you can easily get lost and have trouble finding your way home. That’s why it’s a good idea to always have a portable GPS with you. Check out the best portable GPSs for rockhounding in the article below: