Natural History of Oregon Country
The purpose of this activity is to learn some of the ways that fossils can be preserved.
• Shell(s) or fossil shells
• Plaster of Paris (available in hobby shops, craft stores, home improvement stores)
• Dixie cup (large enough so that you can place your fossil or shell in it.)
• Food coloring
1. Lightly coat your shell or fossil with Vaseline to keep it from sticking to the plaster.
2. Mix about ½ cup of the Plaster of Paris according to the directions for each fossil you
want to make. Be sure that it is thick enough that your shell won’t sink into it.
3. Pour enough plaster into your cup to fill it half way.
4. Gently place your shell in the cup. Be sure that it doesn’t sink so far that you can’t pull it
out when the plaster hardens.
5. Tap the cup on the table a few times to settle the shell and to eliminate bubbles.
6. Wait until the plaster has hardened.
7. Pull your shell out – what do you see? In the rock record, this type of fossil is called a
8. Lightly coat all the exposed plaster inside your cup. You will be adding more plaster and
you don’t want it to stick.
9. Now mix more of the plaster, this time adding a couple drops of food coloring and mixing
10. Pour the new plaster into the cup, filling the cup.
11. Once this has hardened, peel off the cup and try to pull the two halves of plaster apart.
12. You will now have two different renditions of the same shell, the colored side is called a
cast and the white side is the mold.
1. Which of the two fossils looks more like the original shell? Why do you think so?
2. Which of the two fossils are you more likely to find in the fossil record?
3. How do you think casts and molds compare to fossils formed from original remains?
4. What types of natural conditions are necessary for this type of preservation to occur?
5. After watching the video ‘Wallowa Mountain Fossils’, how did most of these fossils appear
to be preserved?
6. Do you think it is likely that any individual organism is likely to be preserved in the fossil
record? Why or why not?