Rock Record: What are Fossils?

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Author: Janice VanCleave


What are fossils?


  • 1-inch (2.5-cm) -thick piece of modeling clay with a surface larger than the shell petroleum jelly
  • Seashell (or small shell-shaped soap)


  1. Cover the top surface of the clay with a thin layer of petroleum jelly.
  2. Press the outside of the shell firmly into the clay until most of the shell is surrounded by the clay.
  3. Gently lift the shell out of the clay.

Rock Record


The shell leaves an impression in the clay.


A fossil is any record of past life, such as a shell or a bone preserved in rock. Fossils can also be prints from animals or plants that were made in soft sediment that gradually hardened into solid rock. Fossils were and can be made from the remains of an organism buried in sediment. The remains rot away completely as the sediment hardens into rock, leaving in the rock a cavity the size and shape of the organism. This impression of an organism within a rock cavity is called a fossil mold.

This experiment represents the formation of a fossil mold. The shell represents the remains of the organism, and the clay represents the soft sediment that will harden into rock. The print left by the shell is a model of a fossil mold.

Let's Explore

  1. The cavity in which an object can be shaped is called a mold. A fossil mold can be used as a mold to create a reproduction of the surface texture of the organism. Repeat the experiment, making another impression of the shell. In a paper cup, mix 2 tablespoons (30 ml) of plaster of paris with 1 tablespoon (15 ml) of tap water, then stir with a craft stick. Fill this mold with the wet plaster. Allow the plaster to dry (about 20 to 30 minutes), then remove the clay. The dried plaster retains the actual surface texture of the original object, the shell. NOTE: Do not wash the paper cup or the craft stick in the sink, because the plaster can clog the drain.
  2. Under the right conditions in nature, mud that fills fossil molds will harden and retain the shape of the mold. When a mold is filled with a substance such as mud or plaster that hardens, the result is a solid reproduction of an organism, called a cast Casts have the same outer shape as the organism. Mud casts that change into rock form fossil prints of the organism. Make a mud cast by repeating the previous experiment, but replace the plaster with garden soil. Science Fair Hint: Use the mold and cast of the shell as part of a science fair display.

Rock Record

Show Time!

  1. Another way to show how fossil molds form in sedimentary rock is described in Experiment 26, "Imprints." Follow the steps described in the procedure to make an imprint of your hand. Display photographs of the procedure and the results to represent the formation of a mold.
  2. How does metamorphism (the process by which rock changes from one type to another due to pressure and heat) affect fossils? Use the fossil mold model made in the previous experiment to represent how pressure changes sedimentary rock into metamorphic rock. Roll a rolling pin back and forth across the hardened mold. Take photographs of the mold before and after applying pressure, and use them to prepare a poster representing the effects of metamorphism. The title of the poster might be "Fossils versus Metamorphism."

Check it Out!

  1. Fossils are more commonly found in limestone and shale than in sandstone. Find out more about fossils and the rocks in which they are found. Why are fossils found in sedimentary rock but not in igneous or metamorphic rock? What is fossiliferous limestone?
  2. Petrified wood forms when silica minerals in groundwater replace wood fibers and/or fill pores in buried wood. Find out more about this process. How long does it take for complete petrification? Where is petrified wood of gem quality found?

Rock Record

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