Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
How Old Is Illinois?
Grade Level:
9 – 12
Purpose: The purpose of this activity is for students to use the fossils found in rock
layers to determine the age of the strata between Rock Island and Chicago.
Suggested Goals: Students will determine the age of each rock layer by using the
fossils found in them. They will use a sheet to identify the fossils and then use the
periods tin which they existed to determine the age of the layers.
Targeted Objectives: Everyone wonders how scientists are able to determine the age
of rock layers and most people are fascinated with fossils. This activity makes use of
this fascination to show HOW scientists use fossils to learn the ages of rocks. Students
will learn how fossils, called index fossils, can be used to find the age of strata. They
will learn what qualities make a good index fossil. They will then discover that it is even
possible to determine the age of rock layers even when no good index fossils are
available. Students will become amateur geologists and use the fossils to determine
the age of the rock layers found at the surface of Illinois.
Students learn what a fossil is.
Students learn about the Geologic Time Scale
Students learn what an index fossil is and what qualities it should possess.
Students will learn what a fossil assemblage is.
Students will use a fossil assemblage to determine the ages of the bedrock
between the Mississippi River and Chicago.
Background: Since life appeared on earth, trillions of organisms have lived and died.
Most of these are soon returned to the soil and eventually no trace of them remains, but
somehow a few are preserved. These are called fossils.
A fossil is any evidence of prehistoric life. It can be footprints, bone, a shell, feathers,
skin or sometimes even an entire frozen mammoth or an insect caught in tree sap
called amber.
Fossils can be used to determine the time period when a sedimentary rock layer was
deposited.
The time since the earth formed is called geologic time. It is divided into huge blocks
that have been given names. Geologic time is similar to the way a school day is divided
into periods and classes. Someone might say that something happened in third period
or maybe in science class without giving the exact minute the event occurred. It is much
simpler to say that fish were the dominant organism in the Devonian than it is to say that
fish were dominant between 410,000,000 and 360,000,000 years ago. The earliest time
in earth history is called the Precambrian but no rocks from that ancient time are found
at the surface in Illinois.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Figure 1. Geologic Time
ERA
Period
Period began
Millions of years
ago
Cenozoic
Quaternary
Tertiary
1.6
66
Mesozoic
Cretaceous
Jurassic
Triassic
145
206
250
Paleozoic
Permian
Pennsylvanian
Mississippian
Devonian
Silurian
Ordovician
Cambrian
290
320
360
410
440
490
570
Precambrian
4600
Fossils can often be used to determine the time period when a layer of rock was
formed. They have been found in all of the periods on figure 1. A few have even been
found in the Precambrian. If fossils exist in a rock layer, it is usually possible to use
them to determine the time period when the layer was deposited millions of years ago.
Some fossils are more helpful at determining the age of a layer than others. Such a
fossil is called an Index Fossil. If an index fossil is found, a geologist can immediately
tell the age of a layer because the organism lived in only one period.
A good index fossil has four traits:
1. It should have lived in only one period or even in just one part of a period. If
an organism lived in several periods, it is useless for dating rocks because it lived
in more than one.
2. It should be easy to identify. Sometimes several fossils look so much alike
that it is not easy to tell them apart without a perfect specimen.
3. It should be abundant in rocks of its age. If a fossil lived in only one period but
is almost never found, then it would not be a good index fossil. A fossil that is
almost always present in a certain age of rock would be much better.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
4. It should be widespread. A good index fossil would not just be common in
Cambrian rocks of the United States but would be present in rocks of that age all
over the globe.
The trilobite, Phacops, is an example of a good index fossil. It is easy
to identify and is common in rocks of the Devonian period
Sometimes a good index fossil is not found or the fossils that are found are in such a
poor state of preservation that none of the fossils can narrow the age down to a single
period. At these times it is necessary to create a fossil assemblage.
A fossil assemblage is a collection of fossils from a single layer of strata that as a
group can be used to determine the age of the layer.
To determine the age of a rock layer using a fossil assemblage a geologist would
determine the time period when all of the fossils found in the layer existed together.
If there were three fossils in a layer of strata and one lived during both the Cambrian
and the Ordovician periods, another lived from the Cambrian to the Devonian, and a
third lived from the Ordovician to the Silurian, then the age of the rock layer would have
to be Ordovician. That is the only period in which all three fossils existed. It age could
not be the Cambrian because the fossil did not exist in that period and it could not be
either the Devonian or the Silurian because the first fossil was extinct by then.
In the following activity students will use a lab sheet that shows the various rock layers
that a person would encounter if they drove across Illinois from Rock Island to Chicago.
On the lab sheet students will view examples of fossils that might be found in each
layer. They will try to identify the fossils by matching them with pictures that show when
the organisms existed. They will then draw lines on a chart of the geologic time scale to
show the time that each organism lived. When all of the organisms in a layer are drawn
on the graph, the student will identify the one period when all were in existence together
and label the rock layer with that time period.
Materials:
A collection of several fossils native to Illinois
Fossil sheet
Lab sheet Rocks Across Illinois
Chart of the Geologic Time Scale
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
A fossil identification book
Phacops
Preparation: Begin the lesson by asking students what a fossil is. Ask them to give
some examples of fossils. Share the information about fossils that is found in the
background section. Divide the class into small groups and pass out the sheet with the
fossil names and pictures. Give each group of students a collection of some of the
fossils that are pictured on the sheet and allow time for them to identify as many as they
can. When they have finished, have them trade collections with other groups and try to
identify the fossils in those collections as well.
Ask the students how it is possible to determine the age of rocks. Write the student
responses on the board. Explain that in many cases rocks can be dated using
radioactive elements found in them but the technique is not usually feasible if a
geologist is in the field. A much simper method is to use fossils.
Introduce the geologic time scale. Compare it to the way a students school day is
divided into periods. Show a chart of the time scale that shows what events happened
during each geologic period. Explain that often the organisms that lived in one period
are very different from those that lived in the very next period. Show a fossil
identification book and tell your students that geologists have tried to identify the period
or periods that each organism lived. If a fossil is found we can often determine the age
of the rock strata because the organism may have only lived in one period.
Explain what an index fossil is and list the qualities that make a good index fossil on the
board. Discuss why each is an important attribute. Show a specimen of Phacops and
explain that whenever Phacops is found in a rock layer we know that the layer is from
the Devonian since that is the only period that Phacops lived.
Ask the students if it would be possible to determine the age of a rock layer if all of the
fossils that you find in it lived in more than one period. Explain what a fossil assemblage
is. If the life span of each fossil is drawn on a chart it is likely that the period and usually
the part of a period such as the upper Devonian can be determined.
Ask the students what the age of a rock layer would be that had fossils of Leptaena,
Platystrophia, and Billingsella. Show them how to draw the life spans of each on a chart
of the geologic time scale.
Billingsella
Platystrophia
Leptaena
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
TRIASSIC
PERMIAN
PENNSYLVANIAN
MISSISSIPPIAN
DEVONIAN
SILURIAN
ORDOVICIAN
CAMBRIAN
PRECAMBRIAN
The age of the strata would have to be Ordovician since that is the only period that all
three organisms were alive at the same time.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
Procedure: You are about to become a geologist. Your job is to determine the age of
all of the rock layers between the Mississippi River and Lake Michigan. You begin your
work in Rock Island and continue all the way across the state to Chicago. As you cross
the state you collect all of the fossils you can find in each layer. You will use them to
determine the geologic periods in which all seven layers were deposited. Some of the
same fossils are found in different layers but most of the time the fossils seem quite a
bit different in each new bed of rock.
The sheet labeled Rocks Across Illinois shows which fossils you found in each of the
layers. To determine the age of layer one, draw the life span of the three fossils on the
chart which has the geologic time periods listed. Identify the fossils in the layer by
matching them up with the pictures on the sheet labeled Fossils. The top fossil is a
specimen of Platyceras which lived from Silurian to Permian. Draw a line on the chart
from the bottom of the Silurian to the top of the Permian. Do the same thing with the
other two fossils. When you have finished you will see that there is only one period of
time when all three fossils lived. That is the age of layer one. Write the name of that
geologic period on the line under the pictures of the fossils where it says “Age Is” and
begin the second layer which has five fossils in it.
Example of how to draw the life spans
TRIASSIC
PERMIAN
PENNSYLVANIAN
MISSISSIPPIAN
DEVONIAN
SILURIAN
ORDOVICIAN
CAMBRIAN
PRECAMBRIAN
HINT…Some of the layers will be from the same periods.
Life Span of
Platyceras
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
ISM Geology Online Pictures:
Coal Sandstone…The first layer is made of sandstone with coal. The coal is a good
piece of information when dating this layer.
St. Peter Sandstone…Layer five contains a very important layer called the St. Peter
Sandstone. It was deposited in an ancient sea and is made of very pure quartz.
Results: The students should end up with the following answers:
Layer 1 Pennsylvanian
Layer 2 Devonian
Layer 3 Silurian
Layer 4 Ordovician
Layer 5 Cambrian
Layer 6 Ordovician
Layer 7 Silurian
Discussion:
1. Ask what is the oldest variety of rock in the region between Rock Island and
Chicago? Cambrian
2. Why were some of the fossils of very little help in determining the age of a rock layer?
They lived in several periods
3. Were any of the same organisms found in more than one age rock? Yes
4. Of all the fossils found, which was the best index fossil and why? Olenoides because
it only lived in the Cambrian
5. How is it possible to date a rock layer if there are no good index fossils? You can use
a collection of fossils from the rock layer and determine when they lived together
6. When several fossils are used to date a rock layer, the group of fossils is called? A
fossil assemblage
Assessment:
1. Have your students write a paragraph answering each of the following as a pre-test
and a post-test.
1. What is the geologic time scale?
2. What is an index fossil?
3. What qualities make a good index fossil?
4. What is a fossil assemblage and how can it be used to determine the age of a
rock layer?
2. Have your students determine the age of a rock layer using several of the fossil
pictures or some actual fossils that would pinpoint a period.
Illinois State Museum Geology Online – http://geologyonline.museum.state.il.us
3. Correct the lab sheets paying close attention to see that the lines are drawn through
the correct layers.
4. Student response to the discussion questions will be a measure of their
understanding.
Extensions:
Place actual fossils on tables to represent fossils found in rock layers. Have students
determine what age rock each table would represent.
Visit a quarry or a rock outcrop and have your students collect fossils and then try to
determine the age of the layer using a fossil handbook.
Have students glue fossils to a geologic map of Illinois that would show which fossils
might be found in each section of the state.
Do the same activity as was done in this lesson but use a line between two different
Illinois cities such as from Cairo to Rockford.
Print Resources
The poster Fossils of Illinois by Robert Vaiden is ideal for this activity. It has a geologic
map of Illinois and shows which fossils might be found in each age of rock. It may be
purchased from the Illinois State Geological Survey for $7.95 plus shipping.
The book National Audubon Society Field Guide to North American Fossils has pictures
and descriptions of hundreds of fossils and may be purchased at major bookstores.
Web Resources
http://geologyonline.museum.state.il.us/geogallery Illinois State Museum Geologyonline
Geogallery
http://geology.tqn.com/cs/geomapssusstates This Website has geologic maps of all fifty
states
http://www.isgs.uiuc.edu/fossils/mainpage,htm This is the Website for the Illinois State
Geological Survey. It includes sections on such things as “What are fossils,” Common
Fossils of Illinois,” Where to Look for Fossils,” and “The Geologic Timeline.”
Lesson Specifics:
The activity requires one or two days.
It is best done individually rather than in groups.
Illinois State Board of Education Goals and Standards:
11.A: Know and apply the concepts, principles and processes of scientific inquiry.
11.B: Know and apply the concepts, principles and processes of technological design.
12.A: Know and apply concepts that explain how living things function, adapt and change.