Student Sheet
Name___________________________________
Title: Underlying Assumptions
Introduction: Most of Earth’s history occurred long before people were around to
record what happened. However, scientists have theories about what happened
thousands and millions of years ago. They base these theories on two assumptions.
(an assumption is a statement that is accepted as true and from which a conclusion can
be drawn) One is that events in the past occur in the same way as events do now. For
example, if we have floods now that deposit sediments on valley floors, they did the
same thing in the past. Another assumption is that sedimentary rocks stack up on
Earth’s surface with older layers on the bottom. We observe this happening today and
assume it happened in the past. In this activity, you will look at three models of
sedimentary layering to help you understand it.
Part I
Imagine that you have a big toy box in your basement. Over the years, when you are
asked to clean your room, you take big armfuls of your toys and clothes down and dump
them in the toy box. Over time, the toy box fills up. In the box below, use pictures or
words to describe what is in the toy box and where it is in the toy box. Answer the
questions when you are done.
MY TOY BOX
Questions:
1. Where are the oldest toys?
2. What does the type of toys and size of the clothes show about you?
3. Which layer would contain toys and clothes most like the ones you are currently
using?
4. If you went downstairs and saw a very old toy on top of the pile in your toy box, what
would you assume?
5. Create a timeline for your toy box. Divide it into parts and name them by the
dominant toys. Scientists have also done this with time. You may be familiar with the
name “Jurassic”. The dominant organism during the Jurassic was the dinosaur.
toys
years
Part II
On Earth’s surface, erosion by wind and water carry sediments like sand and gravel to
new places. Often a flood or windstorm adds a new layer of sediment on top of an old
layer. Sometimes living things die and get fossilized in the layers. In this part of the
activity you will create a model of sedimentary rock layers as they form in a lake bottom.
Directions:
1. Take a beaker and fill it 1/2 full of water. This is the “lake”. Add a layer of one of the
colors of sand or gravel. To model a dead organism, drop one of the plastic pieces in it
next to the side of the beaker so you can see it. It is now a fossil.
2. A flood fills the lake with new sediments. Add another color of sand or gravel to the
“lake”. Add another plastic piece for the new fossil.
3. A wind storm carries a large amount of dust into the lake. Add another layer of
sediment and a fossil.
4. Draw your beaker below and label the fossils (give them a name of your choice)
Then answer the questions.
Questions:
5. Why are sedimentary rocks layered?
6. Which layer is oldest?
7. Which fossil would you expect to be most like
organisms of today?
8. What evidence shows how living things on Earth
have changed?
Beaker
Part III
Paleontologists (scientists that study the past) have constructed diagrams to show what
rock layers have formed in different places on Earth. One of the most famous is the
Grand Canyon in Arizona. On the next page is a picture of some of the rock layers in
the Grand Canyon. Use the diagram to answer the questions.
Questions
9. Which type of rock is youngest?
10 . What assumptions do scientists make to determine the relative ages of rock layers?
11. If it were discovered that the Coconino sandstone was younger than the Toroweap
formation, what would scientists assume?
12. Fossils found in the Kaibab limestone include brachiopods, coral, mollusks, and sea
lilies. How has this part of Earth changed since these fossils were formed?
Conclusion: How have these models helped you understand the assumptions that
scientists make about sedimentary rocks on Earth?