Fossils and
The Law of Superposition
By L. Badino, Truman Middle School
Adapted from Liz LaRosa 5th Grade Science http://www.middleschoolscience.com 2009
This PPT was created with the information from the FOSREC Activity “Who’s on First?” and “Fossil Inferences” by UEN.
Fossils and Superposition

What is a fossil?
The trace or remains of an organism
that lived long ago, most commonly
preserved in sedimentary rock

What is the Law of Superposition?
Younger rocks lie above older rocks if
the layers have not been disturbed
Relative Age Dating and Index Fossils

What is relative age dating?
◦ Determining whether an event or object is older
or younger than other events or objects.

What is an index fossil?
◦ A fossil that is used to establish the age of the
rock layers.
◦ It is found in the rock layers of only one geologic
age.
◦ It is found in rock layers
around the world.
ex Trilobites
Activity # 1




Obtain Set A materials: 8 index cards or slips
of paper with letters on them.
Each slip of paper represents a rock layer.
Each letter represents a kind of fossil.
Your task is to determine what the correct
sequence of the letters are, from oldest to
youngest.

Method:
1. The card with the letters “T” and “C” is on
the
bottom, or the oldest layer.
2. Look for a card that has either a “T” or “C”
written on it for the second layer. Since this
card has a common letter with the first card, it
must go on top of the first card. This second
layer is younger than the TC layer.
3. Sequence the remaining cards using
the same process.
Note – Sometimes organisms disappear
or become extinct. Extinction is forever once an organism disappears from the
sequence it cannot reappear later
Answer
This is the sequence of life
on the planet
Questions for Activity 1:
1.
2.
3.
4.
5.
6.
7.
Neatly list the layers in order, with the oldest
on the bottom and the youngest at the top.
Label as such on the side of your listing.
Explain why “M” is the same age as “D” in
the layer “DM”.
Explain why “D” in rock layer “OXD” is
older than the “D” in rock layer “DM”.
Which letter(s) showed up the most?
Which letter(s) showed up only once?
What could have happened to the organisms
represented by the letters that disappeared?
Which letter(s) could be index fossils?
Explain why you made this choice.
Activity # 2





Obtain Set B materials: 8 index cards or
slips of paper with fossils on them.
Each slip of paper represents a rock layer.
Your task is to determine the correct
sequence of the letters, from oldest to
youngest.
The oldest layer has the letter “M” in it.
Use the same method that you used in
Activity # 1.
M
S
A
G
I
N
R
O
Questions for Activity #2:
1.
Neatly list the layers in order, with the oldest
on the bottom and the youngest at the top.
Label as such on the side of your listing.
2.
Explain why a pelicypod and an ammonite are
the same relative age.
3.
Which is older – a crinoid or a horn coral?
Why?
4.
Which fossils could be used as index fossils?
Explain why - how is an index fossil is
different than just any fossil?
Questions for Activity 2:
5. What kind of rocks would you find fossils in?
6.
Explain how this activity illustrates the Law of
Superposition and Relative Age Dating?
7.
Compare these fossil images to the real
fossils provided to you. Describe.
What Did You Learn?
1.
2.
3.
How does the Law of Superposition
help us determine the relative age of
rock layers?
How do fossils help us determine the
relative age of rock layers?
What two characteristics must a fossil
have to be considered an index fossil?