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You Don't Look a Day Over 2000 Years!

How are fossils formed? What is a fossil? A fossil is the mark or the remains of an ancient animal or plant. Most fossils are preserved in rock. Rocks that contain fossils lie beneath the surface of Earth for millions of years. Finally, through weathering and erosion, the fossil-bearing rocks are exposed on the surface.

Image courtesy National Park Service, Museum Management Program and Dinosaur National Monument

Fossils are broken down into four main categories:
  • Bones, shells or other hard remains.
  • Impressions, molds or casts.
  • Black layer of carbon in the shape of the organism.
  • Nests, tracks, or trails left behind.

There are many fossils that have been found yet there are many that were destroyed before they could be fossilized or get destroyed by weathering and erosion. If an organism has hard body parts (bones, shell, etc.) it has a better chance of becoming a fossil.

A fossil forms by an organism dying or an impression gets left behind, buried by sediment. The remains get filled in and replaced with minerals that have been dissolved in water. The fossils we see are actually mineral deposits, not the real body parts.

Here are some examples of fossils found and collected in Utah

Trilobite - Middle Cambrian : Asaphiscus wheeleri

Specimen found at Antelope Springs, Utah.

Trilobites are quite common in Utah.


Pentacrinoids - This fossil is intesting. It is a five sided star shape crinoid fossil.

Specimen found at Thistle, UT.

In real life these are about 4 mm in diameter. These star shaped fossils are very interesting to look at with a microscope. Drag your cursor over the picture and a magnified view ~80X will appear. It shows much greater detail.


Whenever you find a fossil, you will usually find them in a sedimentary rock. So to get a fossil, you have to start with sediments and an organism. Usually the hard part of the organism, such as bone or shells is fossilized because it does not decompose very fast. The photographs below should help you understand the process that is involved with the formation of many common fossils. The blue clay represents sediments, while the seashell represents an organism.

When an organism dies it will lay on top of sediments.

Later sediments will bury it.

Over time even bone or shell will disappear leaving only its impression.

Over time sediments will fill in the impression, mainly chemical sediments. This process is called mineral replacement or petrification. This means that the organism is replaced by minerals, turning the organism into rock.

Last of all the fossil becomes uncovered by weathering and erosion so we can find them.



  • Plaster of Paris
    • Water
    • Paper cup
    • Shell
    • Petroleum jelly
Safety concerns: Be sure to keep all chemical safety rules. As with all science lab activities, the most important safety rule is to follow all teacher directions.


  1. Mix water with the plaster of Paris until you have a partially solid mixture.
  2. Pour the mixture into the paper cup.
  3. Rub the surface of the shell with petroleum jelly.
  4. Carefully press the shell into the plaster of Paris mixture.
    • Allow to dry over a period of twelve to twenty four hours.
  5. Carefully remove the shell from the plaster of Paris.
  6. You should now see the impression of the shell hardened in the plaster of Paris.
  7. You can change the impression into a cast of the shell by doing the following:
  8. Carefully cover the surface with more petroleum jelly.
  9. Mix up a small amount of plaster of Paris with water (making sure it is more liquid than solid.)
    • Pour the plaster of Paris into the mold you created.
    • Allow the plaster of Paris to dry.
    • Carefully lift your cast out of the mold.


  1. Did either your cast, or mold, turn out exactly the same as the original shell?
  2. How are imperfections in your model similar to real fossils that are found?
  3. Why would it be hard for fossils to form in igneous rocks?
  4. What would the pressure of metamorphic rocks do to fossils that are inside them?

Review Science safetey rules here.

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Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 8th grade science core.

Updated October 24, 2008 by: Glen Westbroek

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