This activity was adapted from "What are Fossils?" found in Science Activities for Middle School Students. 2nd Ed.

by George C. Lorbeer. McGraw Hill, NY 2000. p. 177.

Background: Shells (or other hard exteriors) are buried in sediment. After time, if they dissolve, they leave an open cavity in the form of the original shell. The cavity left behind in the sedimentary rock represents the external form of the shell (an external mold). Sometimes, the soft parts of the organism inside of the shell decay before the shell dissolves. Hard water can enter leaving behind minerals that will eventually harden and form an internal mold of the shell. If the cavity is filled with sediment after the shell has dissolved, then a cast is formed from the mold.

Objectives: To replicate a process of fossilization in the classroom in order to understand it better.

Materials Needed: box, plaster of Paris, marine animal shells, small plastic toy animals, clay, salt, examples of casts and molds

Introduction: Show examples of casts and molds. Question: How are they formed?

Lesson Body:

Split students up into small groups. Have each group prepare the materials before they make their own casts and molds.

Preparation: Mix a little salt with the plaster-of-Paris so that it does not harden too quickly. Put a 1 1/2 - 2 inch layer of clay on the bottom of the box.

Activity: Press the shell or plastic animal into the clay. Remove object. Fill the impression with plaster-of-Paris. When this has hardened, remove plaster-of-Paris mold.

Questions: Relate this activity to the process of making casts and molds

1. What does the plaster-of-Paris represent? What does the clay impression represent?

2. Describe someone else's organism based on his/her cast or mold.

3. Go back to the real examples of casts and molds. Take a closer look and describe what the original organism must have looked like. Draw your predictions.

Wrap Up: What characteristics must an organism possess in order to be fossilized in this particular way? Brainstorm some organisms that would not form casts or molds.


National Standards Addressed:

This hands-on activity addresses the Earth Science Standard. Specifically, the students learn of fossilization processes and can see how the fossils represent once living organisms.

Link to Fossilization Page