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Prerequisite:
This lesson builds upon ISM Geology Online Lesson Plan 12.3:
How Old is Illinois?
Grade Level: 9 – 12
Purpose:
The purpose of this activity is to draw the rock layers under the surface
of Illinois between Rock Island and Chicago using the ages determined
in Lesson 12.3.
Suggested Goals:
The goal of drawing the rock layers will be met by teaching your students
what the law of superposition is and how it can be used to predict the
arrangement of the strata beneath the surface. Students will practice
drawing layers and then create a three dimensional drawing complete with
the appropriate symbols for the rocks.
Targeted Objectives:
Illinois is covered with thick layers of sedimentary rocks. In many places
they are piled layer upon layer until they reach thicknesses of thousands
of feet. In this activity students will use the rock layers that are visible
at the surface to predict how the layers are arranged far underground
where they cannot see. They will use the data from Lesson
12.3 to make a cross section of Illinois between Rock Island and Chicago.
- Students will learn the law of superposition.
- Students will learn how sedimentary rocks are arranged.
- Students will learn how to predict the arrangement and inclination
of rock layers far underground using clues at the surface.
- Students will learn the common geologic symbols for sedimentary rocks.
- Students will make a cross section using the common symbols for sedimentary
rocks to label them.
- Students will learn how the rock layers of northern Illinois are arranged.
Background:
Material called sediment spreads out evenly over the ocean floor
so that a layer of sand or mud covers miles and miles of the sea bottom
in roughly horizontal layers. If tectonic forces (mountain building) raise
the continent the flow rate will increase and larger particles will reach
farther out into the ocean covering the original sediment so that a new
bed of coarser sediment might form on top of the first. For hundreds of
millions of years, layer after layer settle on top of each other. The
great thicknesses of sediment are compressed by their own weight and calcium
carbonate, which is abundant in the sea, and change the layers to sedimentary
rocks.
Since new sediments are deposited on top of those that are already there,
we know that the bottom layers are older. This principle, called the Law
of Superposition states that the oldest rocks are on the bottom.
It is one of the most important principles in the field of geology. Another
principle called the Law of Horizontality states that originally
the sedimentary rocks were laid down in flat horizontal layers. These
two principles will be used in this activity to determine how the rock
layers are arranged under Illinois.
Materials:
- Copies of the handouts labeled, "Rules," "Stratigraphy
Worksheet," "A Block of Illinois," and "Symbols";
all are available in the PDF download version
of this lesson
- Colored pencils
- Ruler
Preparation and Procedure:
To begin this lesson, there should be a discussion of the Law of Superposition.
Compare the rock layers to someone building a brick house. Which layer
did the bricklayer lay first? How do we know? It is the same way with
the rock layers. The bottom layer of brick and rock had to have been there
before the others.
Remind the students that sedimentary rocks are laid down horizontally
but that forces in the earth often fold them. Tell them that Illinois
is covered with thousands of feet of rock from times when there the area
was covered by ocean or the delta of a great river.
Begin the stratigraphy activity by telling the students that now they
are going to learn how to tell how the rock layers are situated thousands
of feet underground without leaving the surface. Tell them that they did
the hard part already when they did Lesson
12.3. Pass out copies of the pages labeled “Rules.” And
go over each rule with your students. Draw the following on the board
and ask if any student can draw the layers:
The correct answer will be Devonian on top of the Silurian:
Continue putting examples on the board. I usually let a student attempt
to do all of a puzzle until they make a mistake and then I let the student
who caught the error come up and try to finish the layers.
At first it may be hard to get volunteers, but in a short time your students
will be begging you to let them go to the board.
At the conclusion of the practice session, I put a sequence from one
end of the board to the other with about twenty time periods on it. Just
remember that it is easiest for you to make the sequences if you do not
skip any periods. If you start with Ordovician then the next period should
be either Cambrian or Silurian. In nature periods of time are often missing
but for ease in the practice time it is best not to do that at least not
at first.
When you feel your students are adept at figuring out the rock layers,
pass out the sheet labeled “Stratigraphy Worksheet” and have
them try to finish it. I usually assign one at a time and then walk around
to see who has the correct answer first. You might want to reward the
first person to get each correct.
Pass out the sheet labeled “A Block of Illinois” and tell
your students that this sheet has the rock ages on it that they found
when they did their fossil assemblage in Lesson
12.3. It shows the ages of the rock layers between Rock Island and
Chicago. Tell them that now they are going to predict what the rocks are
like under their state. Have the students do the same thing on the front
of the block as they did in the preceding exercise. Have them draw the
layers lightly, in case they make a mistake. If they draw the layers correctly
hey will create a sequence of layers sitting on top of Cambrian rocks.
When they have completed the layers have them draw the sides as shown
on the top of their sheet. Draw the layers back parallel to the top and
bottom lines on the side to create a 3-D view.
Pass out the sheet that has the rock symbols on it and briefly go over
them. Then explain how to draw the layers of limestone, dolomite, and
shale so that they are parallel to the bedding plain (bottom of the layer
above). It is fortunate for Illinois that it has so much limestone and
dolomite but it would have been better for this assignment if the Silurian
rock would have been something else since there is a sequence of limestone
dolomite limestone which all have pretty much the same symbol on the Illinois
Block. This problem is reduced when the students lightly color the different
periods of rock creating a very professional looking product.
Discussion:
- Do the rock layers in northern Illinois appear to be folded upward
or downward? [Upward]
- What other information about the land would be helpful that the model
does not have since it is flat on top? [It would be good to know
the contour of the surface.]
- There is a fault on the east side of the Cambrian rock, which side
rose the east side or the west side? [The west rose because it brought
older rock to the surface.]
- Oil is often found in rock that forms an anticline (upward fold) if
there is a layer of sandstone between two layers of shale. This model
does show an anticline. Is there any place on the cross section that
would likely have oil and why? [No because there is no place where
sandstone is between two layers of shale. Oil is usually found in rock
layers much younger than the ones in northern Illinois.]
- The Silurian and Ordovician rock appear to be rising on the left side
of the map and then reappear on the right side. If they were once joined,
why is there a break in the layers? [Weathering and erosion have
carried away the top of the anticline since it was highest up and thus
more vulnerable to erosion by streams and wind.]
- Why is there no coal deposits in the eastern half of Illinois shown
by the cross section? [There is no Pennsylvanian or Mississippian
rock in the western part. They are the periods when coal was formed.]
- Dinosaur fossils are found in Mesozoic age rocks which includes the
Triassic, Jurassic, and Cretaceous. Why will there never be a dinosaur
found in northern Illinois? [There is no Mesozoic age rock in northern
Illinois.]
Assessment:
- One assessment would be the number of students that raise their hand
to go to the board to try their hand at drawing the layers.
- Observe the students’ work as you walk around the class while
they are working on the two sheets.
- Collect the pages of student work, “Stratigraphy Worksheet”
and “Block of Illinois.”
- Give a quiz with several of the stratigraphy puzzles on it. Begin
with some short easy ones and work toward much more difficult ones.
Correct them to see how well the students do on them.
- Students’ responses to the discussion questions would be a good
indicator of student understanding of the concept.
Extensions:
- Have your students create a cross section from Cairo to Rockford.
The bottom of Illinois is a basin syncline (basin) appropriately named
the Illinois Basin.
- Visit a rock outcrop or a park where layers are visible. If this is
not possible, show your students pictures of rock strata such as the
Grand Canyon.
- Make an actual 3-D model of the rock layers of Northern Illinois.
The Illinois State Geological Survey sells such a model of Illinois
that would be perfect to show after your students have completed this
lesson.
- Look at an Illinois Geologic Map to learn where the various age rocks
can be found.
Web Resources:
Lesson Specifics:
- Students should have completed ISM Geology Online Lesson
12.3.
- Duration: two days
- It is best for each student to create a drawing of their own, but
students can work together while doing their individual pictures.
Illinois State Board of Education Goals and Standards:
- 11A: Know and apply the concepts, principles and processes
of scientific inquiry.
- 12B: Know and apply concepts that describe how living things
interact with each other and with their environment.
- 12E: Know and apply concepts that describe the features and
processes of the Earth and its resources.
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