EXPLORING FOSSILS
By Cathy Diez-Luckie
What is a fossil? A fossil is the impression or remains of an organism that has been preserved or formed in rock.
When an animal or plant dies, it is usually destroyed by scavengers or decay. Sometimes the remains of the
organism are buried before they are destroyed, and if the conditions are right, these remains are preserved as a
fossil. Only a very few of the organisms that once lived were fossilized.
Where are fossils found? Almost all fossils are preserved in sedimentary rocks that formed when layers of sediment
(mud, clay, silt) are compressed and hardened. Depending on the area where you live, you may be able to look for
fossils at bluffs, sea cliffs, and where roads or streams have cut through layers of earth. You may learn more about
fossils by looking at rock exhibits in local museums and parks. Most of us cannot dig up fossils in our own backyards, but we can explore what kind of fossils there are and how they were made.
Types of fossils.
A trace fossil is an impression left in the sediment that is preserved when the surrounding mud or dirt hardens
and keeps it shape.
The mold fossil is a type of trace fossil that is formed when an organism is pressed into soft mud, leaving its impression. An imprint fossil is a thin version of the mold fossil.
A cast fossil is formed when a mold fills up with mud, sand or volcanic ash, and hardens over time.
A permineralized fossil is created when minerals replace material in the cells of the organism.
Activity: Make Mold, Imprint, and Cast Fossils
Trilobites by Barrande Joachim, 1885
Materials:
Small cardboard box or paper cup
Shell, bone, peanut, or seed pod; leaf
Damp clay or mud
Vegetable or mineral oil
Plaster of Paris, stick, old milk carton, water
1. Mold Fossil
Place some damp clay into the bottom of the box. For best
results, make sure that the surface of the clay is as smooth as
possible. Press the shell, bone, peanut, or seed pod into the
clay, making a deep print. Carefully remove the object. The impression that is left is similar to the mold fossil left by animal
or plant remains.
2. Imprint Fossil
Form two flat thin layers of clay. Press a leaf between the two
layers. Carefully peel back one the clay layers and the imprint
of the leaf remains. The impression left duplicates the imprint
fossil.
3. Cast Fossil
Lightly coat the surface of the clay mold (from Activity 1 or 2)
with vegetable or mineral oil. Following the manufacturer’s directions, mix enough plaster of Paris to fill the clay
mold. After it is hard, carefully lift it out of the clay mold. This is similar to the cast fossil.
Next month we will learn about different kinds of fossils and where they are found.
Dinosaurs on the Move by Cathy Diez-Luckie, © Figures In Motion™