Name
Period
Date
CHAPTER 4 ADDITIONAL INVESTIGATION B
Fossils
MATERIALS
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modeling clay
plaster of Paris
petroleum jelly or cooking oil
small paper cups (6 oz)
objects such as leaves, dead insects, shells, or twigs, to make imprints
water
container to mix plaster
paper towels
toothpicks
markers
hand lens
TIME
45 minutes
OVERVIEW AND PURPOSE
As you have read, fossils are the preserved traces or remains of once-living organisms. In
this investigation you will use what you have learned about fossils to
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make model fossils
distinguish between a mold and cast fossil
describe the differences between mold and cast fossils
PROCEDURE
1. Cut the top off of two paper cups, leaving approximately 3 cm to the bottom. Write
your name on the bottoms of the cups.
2. Press clay into the bottom of the cup, leaving about 1 cm of space at the top of the
clay. Use your fingers to smooth out the clay as much as possible.
3. Select 2 objects to use to make your model fossils. Use one object for each cup.
4. Lightly coat the objects in petroleum jelly or cooking oil.
5. Carefully press the objects into the clay and then remove the objects. If the image is
unclear, you may smooth the clay and try again.
6. Make the plaster of Paris. You need only a small amount for your cups. Your teacher
may make enough for the whole class.
7. Pour the plaster into the paper cups, on top of the clay with impressed images. Your
teacher may come around and pour the plaster for the whole class.
8. Let plaster set for 2-3 minutes until it is stiff enough to hold the shape of an object.
9. Once the plaster is stiff, carefully press the object into the plaster and let sit for a
minute or two.
10. Carefully remove the object.
11. Let all models sit undisturbed for at least 30 min.
12. When the plaster is dry, peel the paper cup from the plaster and clay. Carefully
remove the clay from the plaster. Write your name on your model fossils.
OBSERVE AND ANALYZE
1. Infer Which of your model fossils represent cast fossils, and which represent molds?
Draw a picture of each and label the model fossils cast or mold.
2. Compare Make two lists of observations, one for a model cast fossil and another for a
model mold fossil. Use your list to compare and contrast molds and casts.
3. Explain Draw a sketch of the model you made. How does your model represent the
way fossils form?
4. Explain How does the model you made differ from the way fossils actually form?
CONCLUDE
1. Apply Examine one of your model mold fossils. If it was a real fossil, how might it
have formed?
2. Apply Examine one of your model cast fossils. If it was a real fossil, how might it
have formed?
3. Infer Suppose that another rock layer formed on top of the plaster. What kind of fossil
might form?
4. Apply Exchange fossils with one of your classmates. Try to find someone who you did
not see make the model fossil, so you do not know what objects the person used. Imagine
that you are a scientist who found these fossils. Make sketches and record detailed
observations of the fossils. What can you tell about the object that the fossil represents?
Return the fossil to your classmate when you are done.
ANSWER KEY
Observe and Analyze
1. The model fossil that was made by pressing the object into the plaster (and into the
clay) is a cast. The model fossil that was made when the plaster filled the cast is a mold.
2. A cast shows a raised image of an object. A cast looks most like the actual object. A
mold shows a depressed image of an object. Both show about the same detail.
3. The clay and the plaster represent rock layers. The model has two rock layers. An
object was pressed into the bottom layer to represent an organism. Then plaster was
poured over the bottom layer to represent sediment covering the first layer. The plaster
hardened like sediment turns into rock.
4. Sample answer: In this investigation, the object was removed to make the mold and
cast fossils. When a mold or cast fossil actually forms, the object decomposes. This
investigation took only about an hour, while real fossilization takes much longer.
Conclude
1. The object may have fallen into soft sediment and then become covered with more
sediment. As the sediment turned to rock and the object decomposed, it could have left a
mold fossil in the rock. Students may describe something specific to the object they used.
2. Sediment may have filled a mold fossil. As the sediment turned to rock, it might have
formed a cast fossil. Students may describe something specific to the object they used.
3. There is a cast fossil on the top of the plaster, so it could fill with sediment and form a
mold fossil.
4. Students should make a sketch and record observations. Students should describe the
object they think the fossil represents.
TEACHER NOTES
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Purchase the plaster of Paris from a craft store. Ask for volunteers to bring in
small (~ 1 cm in diameter) clam or snail shells.
Make sure students don't place too many objects into the plaster of Paris.