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Moldy Oldies

Windows to the Past

A fossil is a window to the past. It gives us clues about what life was like in Utah long, long ago.


Most fossils are found in sedimentary rocks. The plant or animal must be buried quickly, and remain undisturbed for a long period of time. Usually you will see fossils of organisms that have hard or bony parts. Soft parts usually decay before they can fossilize.

There are three kinds of fossils: preserved organisms, mineral replacement of organisms, and impressions or tracks of organisms. Remember, an organism is simply something that is or once was living. Plants and animals are organisms.


Preserved Organism

Once in awhile you find preserved, unchanged organisms. An animal that falls through the ice or into a tar pit is preserved this way. Have you seen insects trapped in amber? Amber is a yellowish stone from petrified tree sap. Bugs get caught in the sticky substance and covered. When the sap turns into stone the insect is perfectly preserved.


Replacement Fossils

Replacement fossils are made when water dissolves part of the dead plant or animal and washes it away. This leaves an empty imprint. Minerals fill in the imprint and harden to form a fossil. Sometimes the minerals fill in so perfectly, that you can see very detailed parts of the once living organism - like the rings in a tree trunk. Replacement fossils can be very colorful because of the minerals involved.


Impressions and Tracks

Impressions are whole organisms, footprints, or even tail drags. Dinosaur footprints are often found in the roofs of coal mines in Carbon County. Did the dinosaurs walk upside-down? No. The coal (formed mainly from plants) is deposited under where the dinosaur walked. You are looking up at the bottom of the footprints!

Choose one of the following activities to make your own fossil:
Replacement fossil option
Film trace option
Mold / Cast option

Animal option


  • Sponge you can cut up
  • Salt
  • Very warm water
  • Three or four paper cups
  • Food coloring or liquid tempera paint
  • Pie pan or plate


  1. Cut the sponge into the shape of a plant or animal part. Be creative. How about a raptor claw or a stegosaurus thigh bone?
  2. Place the sponge "organism" on the plate. 
  3. Make a "mineral" mixture by doing the following: 
    • Put three tablespoons of warm water into each paper cup. 
    • Pour salt into each cup and stir until the mixture is very cloudy.
    • Add different color food coloring or paint to each cup and mix to represent various minerals.
  4. Pour a little of each mineral mixture over different areas of the sponge organism. 
  5. Wait a few days for the fossil to harden.
  6. Observe your "replacement fossil" and record your observations.

Did the minerals fill in the holes in the organism? How did the organism stay the same? How did it change?

Plant option


  • Tree or plant leaves (Ferns and maples are great!)
  • Black tempera paint
  • Paint brush
  • Paper plate
  • Art paper cut like a tombstone
  • Paper towels


  1. Place the leaf on the paper plate. Paint the back side with black paint. 
  2. Shake off the extra paint.
  3. CAREFULLY place the leaf painted side down on the art paper. 
  4. Place a paper towel over the leaf and gently press down on all parts of the leaf. (Be sure the leaf doesn't move.)
  5. Carefully lift the leaf.
  6. You have now made a stamp of your leaf.

How is this procedure different from nature's method of "stamping?"


  • Two colors of clay (homemade or from the store)
  • Shells, bones, etc.
  • Cooking oil spray


  1. Flatten a small ball of clay and spray the top lightly with cooking spray.
  2. Press the bottom of the shell or bone into the clay and spray lightly again.
  3. Flatten another small ball of clay and press over the top of the shell.
  4. Carefully separate the two balls of clay and remove the shell.
  5. Allow the clay to harden for a few days. You have two "mold fossils."

Cast Fossil:

In order for a cast fossil to form, the molds must be filled in with minerals, mud or other sediments and allowed to harden.

  1. Roll a new color of clay into a small ball and push it gently into one side of the mold fossil.
  2. Put the other side of the mold on top and press gently together.
  3. Open the molds and gently remove the "cast fossil."
  4. Allow a day or two for it to harden.

Download the plug-ins: Get Adobe Acrobat Reader , and Get Quicktime Player. (The QuickTime plug-in is needed to play sounds and movies correctly.)

Want to share photos of you or your friends doing this activity? Send it in an e-mail with the following information:

  1. The title of the activity
  2. The URL (Internet address)
  3. Your name.

Remember that no pictures can be used that show student faces or student names on it. 

Teachers should view the Teacher Site Map to relate Sci-ber text and the USOE 4th grade science core.

Updated October 24, 2008 by: Glen Westbroek

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