4.3 Notes
“How do fossils help us date rocks?”
Objective: Describe how the relative ages of fossils and rock layers
can be determined.
Sediments are deposited by water and wind in layers on Earth’s
surface. Layers are deposited on top of other layers. The bottom
layer is the oldest and the top layer is the youngest.
The law of superposition states that each undisturbed rock layer is
older than the layer above it.
Which rock layer is the youngest in the rock bed illustrated in
Figure 4-11? (p. 96)
Relative age is the age of an object compared to the age of
another object. Using the law of superposition, scientists can tell
the relative age of the rock layer. Relative age does NOT tell the
exact age.
What is the relative age of rock layer C in Figure 4-12? (p. 96)
Index fossils can be used to help determine the relative age of
rock layers. Index fossils are from species that only existed for a
short time. Index fossils are found in rock layers.
Trilobites and Graptolites are index fossils. Trilobites were small,
shelled animals that lived in the ocean.
Trilobites appeared about 590 million years ago and lived until
250 million years ago. If a sedimentary rock contains trilobite
fossils, it was formed between 590 and 250 million years ago,
when trilobites were still living.
Index fossils are used to date rock layers from different parts of
the world.
What are index fossils used for?