Areas of Science Geology
Time Required Short (2-5 days)
Prerequisites None.
Material Availability Readily available.
Cost Low ($20 - $50)
Safety This project uses plaster of Paris. Read and follow the safety instructions on the packaging, including wearing gloves and avoiding breathing in the dust. Adult supervision is required.


Have you ever stood close to a fossilized T. rex skull and counted its razor-sharp teeth? Or, have you seen a fossilized stegosaurus skeleton and thought about how it defended itself with tail spikes and armored plates? A trip to a natural history museum lets you imagine what dinosaurs looked like and wonder what life with them on Earth was like. Fossils give us information about animals and plants that lived long ago. Certain places around the world contain more fossils than others. Why? Would the type of soil play a role? In this geology project, you will make fossils and see if certain kinds of soil produce better fossils.


To make cast fossils using a seashell and three types of soils, and determine if the quality of the cast fossil depends on the kind of soil in which it was created.

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Michelle Maranowski, PhD, Science Buddies

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Science Buddies Staff. "Fantastic Fossilization! Discover the Conditions For Creating the Best Cast Fossils." Science Buddies, 23 June 2020, Accessed 13 May 2021.

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Last edit date: 2020-06-23


Teeth, jaws, skulls, claws! Are we talking about a scary movie? Nope, we are talking about fossils. When you look at a fossil, you see something that used to exist in prehistoric times, before recorded human history. A dinosaur fossil can tell us about what that animal used to eat, how big it was, and how it used to move. But, fossils are not just dinosaur bones; they can also include fossilized plants, leaves, insects, footprints, and many, many other things from the past.

How are fossils made? In the case of a skeleton, after the animal died, the body was buried in sand and sediment, which protected it from predators. The skin and flesh slowly decayed until all that was left were the bones. Meanwhile, layers of sand and sediment continued to cover the bones. The bones remained buried under layers of sediment hundreds of feet deep, slowly turning from bone to stone.

There are four different kinds of fossils: original remains, trace fossils, mold fossils, and cast fossils. Table 1 describes each type of fossil in detail.

Type of Fossil What Is It? Example
Original Remains These fossils are the remains of the actual animal or an animal part. Shown here is a fossil of a sloth skull from about 11,000 years ago. A large sloth skull fossil
Trace Fossils These fossils are signs or evidence left by an animal describing its behavior or activities. Trace fossils include footprints, burrows, and nests. Shown here are tracks of a mammal-like reptile from about 275 million years ago. Fossilized reptile tracks in rock
Mold Fossils These are impressions of an organism (like a shell or a leaf) made in rock. They look as if the shell or leaf were pressed into clay and then the clay hardened. Shown here is a mold of a mollusk about 345 million years old. Imprint of a seashell on a large brown rock
Cast Fossils These fossils form when a mold fills with sediment and sand and then hardens. Shown here is a paw print of a cat that is between 2 and 5 million years old. A casting of a large paw print
Table 1. The four types of fossils, with the description of the fossil type and an example of each pictured.

Fossils are found all over the world, but some areas contain more fossils than others. Why? Could it be because the soil is different from place to place? Some areas may have more sand and others more sediment or soil. Does it matter? In this experiment you will use different soils, a seashell (or other object with ridges), and plaster of Paris to make cast fossils. The soils you will use are sand, moist topsoil, and a mixture of sand and topsoil. You will press the shell into each type of soil, remove it, and then make a cast in each spot. Will a certain type of soil make a better cast fossil than the others? Can you make a hypothesis about which soil creates a better fossil? In a good cast fossil, you can see all of the details in the cast that are part of the thing that is fossilized. Time to start fossilizing and find the answer!

Terms and Concepts

  • Fossil
  • Prehistoric times
  • Sediment
  • Predator
  • Decayed
  • Original remains fossil
  • Trace fossil
  • Mold fossil
  • Cast fossil
  • Depth
  • Data
  • Bar graph


  • What is a fossil?
  • What can you learn from studying fossils?
  • What is the difference between a mold fossil and a cast fossil?
  • What is amber and how is it related to fossilization?
  • Scientists have found and studied fossilized wooly mammoths. How are wooly mammoths fossilized?


  • Oxford University Museum of Natural History. (n.d.). How do fossils form? The Learning Zone. Retrieved April 8, 2013.
  • Shepherd, R. (n.d.). What is a fossil? Discovering Fossils. Retrieved April 8, 2013.
  • BBC Nature. (n.d.). Fossils. Retrieved April 8, 2013.

For help creating graphs, try this website:

  • National Center for Education Statistics, (n.d.). Create a Graph. Retrieved June 25, 2020.

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Materials and Equipment

  • Disposable aluminum pans, 7-15/16 x 5-7/8 x 1-13/16 inches (9)
  • Permanent ink pen
  • Dry measure cup, 1 cup
  • Dry measure cup, ½ cup
  • Play sand (14 cups)
  • Organic, moist topsoil (14 cups)
  • Seashell with ridges like a cockle, or other item with clear ridges or dimples. The seashell should have a diameter no smaller than a ping-pong ball and no larger than a softball.
  • Straw, plastic
  • Ruler, metric
  • Tweezers
  • Paper towels (1 roll)
  • Plaster of Paris (3 cups); small shells may require less plaster of Paris, large shells may require more.
  • Disposable bowls (9); bowls should be large enough to comfortably stir a 1-1/2 cup mixture in.
  • Plastic spoons (3)
  • Cold water
  • Lab notebook

Experimental Procedure

Preparing the Experiment

  1. Prepare three disposable aluminum pans. On the side of the first pan, write "sand" using the permanent ink marker. On the side of the second pan, write "topsoil." On the side of the third pan, write "sand topsoil mix."
  2. Using the 1 cup measure, place 3 cups of sand in the pan marked "sand." Smooth and level the surface of the sand.
  3. Again using the 1 cup measure, place 3 cups of topsoil in the pan marked "topsoil." Smooth and level the surface of the topsoil.
  4. Using the 1 cup measure and the ½ cup measure, place 1½ cups of sand in the pan marked "sand topsoil mix." Then, place 1½ cups of topsoil in the same pan. Mix the two soils together completely. Smooth and level the surface of the mixture.
  5. With the marker and ruler, mark a ring around the straw 2 centimeters (cm) from the bottom. You will use the straw as a measure to press the shell into the soils to the same depth as marked on the straw.

Making the Cast Fossils

  1. Gently place the shell in the sand pan. Use the bottom of the straw to press the shell down. Press until the ring mark on the straw is even with the surface of the sand.
  2. Carefully lift the shell with the tweezers so that the impression in the sand doesn't shift or change. Use the straw to help lift up the shell. Wash the shell and dry it completely with a paper towel. Set the shell aside.
  3. Repeat steps 1 and 2 with the pan of topsoil and again with the pan of sand-topsoil mix.
  4. Once you have created the shell impression in all three types of soil, it is time to mix the plaster of Paris. Follow the directions that come with the plaster of Paris, making sure to mix two parts of plaster to one part cold water. Make enough (at least 1 cup of plaster of Paris and cup of water) to fill all three impressions.
    1. Wear rubber gloves and try not to breathe in the dust from the plaster of Paris as you mix it.
    2. The plaster of Paris will look like pudding when it is thoroughly mixed.
  5. Slowly pour the plaster of Paris into the impression in each pan. Make sure that you have filled each impression completely. Try not to touch the plaster after filling the impression with it. Let the plaster harden overnight.

Repeating the Experiment

  1. It is important to repeat the experiment to make sure that the information you get from your experiment, the data, is similar.
  2. Repeat the first two sections of the Procedure, Preparing the Experiment and Making the Cast Fossils, two more times for a total of three trials. You can choose to do all three trials on the same day, or on different days.

Examine Your Cast Fossils

  1. After waiting overnight, carefully remove the cast fossils and place them in their pans so that you know which fossil goes with which soil. Use a paper towel to clean each fossil carefully. Always make sure to keep each fossil with its pan.
  2. Compare the cast fossils from each trial to the shell that you used to make them. Can you see any ridges or dimples in the cast fossil? Does a particular type of soil produce a better cast fossil? The perfect cast fossil would have all the same ridges and dimples as the shell you used to make the cast, and a poor cast would show no ridges or dimples. Write your observations down in your lab notebook.
  3. You can use a camera to take pictures of your casts to go on your project display board.
  4. If you would like to quantify your data (that is, show your results using numbers and graphs), try counting the number of ridges and the number of dimples in the real shell and in each impression. You can turn your data into a bar graph — ask an adult for help if you do not know how to make a bar graph.
    1. Make a bar graph showing the number of ridges and dimples for the real shell and each impression.
    2. There should be ten bars (one for the shell and nine for the cast fossils) in your graph.
    3. Label the shell and types of soil for each impression on the x-axis. Mark the y-axis with the total number of dimples and impressions for each shell /cast fossil.
    4. You can make your bar graph by hand or use the free online tool, Create a Graph, to make it on the computer.
  5. Was your hypothesis wrong or correct? Either is fine as long as you can explain why.

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  • How does water in the soil affect the quality of the cast fossil? Mix in 4 tablespoons (tbsp.) of water for each cup of soil in the pan. Redo the experiment and investigate the impressions of the cast fossils. Were they better or worse at showing the same ridges and dimples as on the original shell?
  • What kinds of items make good cast fossils? Try experimenting with shells, leaves, and feathers.

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