Dating & Correlating Rock Layers
Name:
Background:
Throughout the long history of the earth, the surface has undergone many changes. From time to
time parts of the continents have been covered by shallow seas. Sediments deposited in these seas
later formed sandstone, shale, and limestone. At times when the seas no longer covered the land,
weathering and erosion wore away and removed layers of rock material that had formed beneath the
water. Lava and ashes from volcanoes sometimes covered these layers of rock. Today, scientists
find out what parts of the land had undergone such changes by comparing and correlating rock
layers from different places.
In order to correlate rock layers from different places, scientists must first find out the ages of the
different layers. Layers of igneous rock can be dated from the radioactive matter in the rock. But
sedimentary rock cannot be dated in this way. In order to date sedimentary rock, scientists look for
certain kinds of fossils in the rock. These fossils are of living things that were present on the earth
for only a short period of geological time, and are called index fossils.
Part 1: Dating rock layers.
Procedures:
1. Study the diagrams of index fossils. Note that each fossil shown lived during a different
period of geologic time. The names of the different periods and the approximate dates of
each period in millions of years ago are given for each fossil.
2. Now study the diagram that shows the different rock layers formed in an imaginary canyon.
Notice that each kind of rock is represented by a different symbol. Also notice that fossils are
shown for some layers but not for others.
3. Each rock layer in the diagram has a number and a line beside it. Find a rock layer that has
an index fossil on it. Find that index fossil in the key. Record the date for that index fossil
on the line beside the rock layer in which that index fossil was found.
4. Do the same for every rock layer in the diagram that has an index fossil on it.
5. Notice that some rock layers do not have index fossils on them. Try to determine the ages of
these rock layers. Write the ages you determine on the lines beside each of these layers.
Questions:
1. What are the numbers of the rock layers having index fossils?
2. Which of these layers is the oldest? Which is the youngest?
3. What kind or kinds of rock contained index fossils?
4. What kind or kinds of rock did not have any index fossils?
5. How are layers that do not have any index fossils dated?
Part 2: Correlating rock layers from two different places
Procedures:
1. Study the two diagrams. Notice that the rock layers of diagram 1 are numbered and that the
rock layers of diagram 2 are lettered. Assume that the diagrams are rock layers found in the
same canyon, but some distance apart.
2. Locate a rock layer in diagram 1 that has the same kind of rock and the same index fossil as a
rock layer in diagram 2. Using a ruler, draw a line from the top of the layer in diagram 1 to
the top of the matching layer in diagram 2. Now draw a line connecting the bottoms of the
two layers.
3. Draw similar lines connecting the other layers in the two diagrams that are made up of the
same kind of rock and have the same index fossil.
4. Study the diagrams again. Some layers in each diagram do not have index fossils but match
layers in the other diagram in kind of rock and in position of the layer. Draw lines to connect
layers of the same kind of rock in the same positions in the two diagrams.
Diagram 1
Diagram 2
Questions:
1. Which layers in diagram 1 matched layers in diagram 2 in kinds of rock and in index fossils?
2. Which layers or layers in diagram 1 could be matched with a layer in diagram 2, even though
no index fossils were present?
3. Which layer(s) in diagram 1 did not have a matching layer in diagram 2?
4. Which layer(s) in diagram 2 did not have a matching layer in diagram 1?
5. Why do you think scientists look for index fossils as well as the same kind of rock in
correlating rock layers?