Caldecott Tunnel - Lesson Plan
- There are 2 pieces of information that are useful to review with students in the classroom before going on this field trip.
- Review the types of rocks students will encounter. This is a good opportunity to take out samples of the rocks and the summary tables created in the History of Rock lesson. Ask students to identify the rocks and describe the depositional environments that created them
- Review Steno’s 3 laws. In particular, review how to tell which layer is the oldest using the law of superposition, even when the layers have been tilted.
- Describe the observations students will be expected to make at the tunnel and set up the lab notebooks so that students know what they need to do.
- Go to the Caldecott Tunnel!
At the Tunnel
- Park in the gravel loop within the onramp to the freeway. Gather the students and ask them to look at the north face of the cliff. Have them describe what they notice. The 3 key observations are:
- There are distinct rock layers
- The dark brown rock layers on the top right look different from the light grey-green rock layers on the bottom left
- The layers are NOT flat but tilted
- Have students look across the freeway to the south side of the road cut. There, the contact between the dark brown basalt and the grey-green sedimentary rocks is much more obvious. The thickness of the basalt layer is also more distinct. Relate these observations to the rock layers created in the classroom in the Layers Upon Layers lesson. How are they the same? How are they different?
- Lead the students across the street, past the base of the cliff, and up a small trail on the left that climbs up to a ledge a little higher up on the cliff-side. Come back along the ledge towards the contact. Here, have kids make additional observations of the rocks layers they see. From this distance, students should be able to observe clear layers within the sedimentary rocks of the Orinda Formation.
- Have the students draw a full page picture of the cliff face, paying particular attention to the rock layers they can observe. If the students can, label each rock layer with the name of the type of rock (conglomerate, sandstone, mudstone, basalt, etc.).
- On the next page, write a short paragraph describing what you observe about the big picture in words (basically, the 3 key observations from step 1).
- As the students finish their drawings and big picture observations, assign each student a rock layer to study in detail. The first student to finish can work on the basalt in front of them. The next can examine the red, baked mudstone. The next can examine the wide conglomerate layer. And so on. If you have more than 20 students, in the interest of time when students are presenting their observations and theories back in the classroom, you may want to have students work on a rock layer in pairs rather than individually.
- Each student should draw a detailed picture of their rock layer and the layers that sandwich their layer. They should then make the following observations
- Identify the type of rock
- Measure the size of the grains in your rock in cm
- Measure the width of the entire layer in cm
- Identify the type of rock on either side of your layer - be sure to indicate which one is “above” your layer and which one is “below”
- Propose a theory describing what the area might have looked like before your rock layer existed, what the area looked like during the formation of your rock layer, and what the area looked like after the formation of your rock layer.
- A good way to have students confirm their observations and rock identifications is to have them share their observations and theories with the students studying the neighboring layers. They can confirm that they identified the rocks correctly and see if their theory matches that of their neighbors.
- Students that finish early can write down their theory of the sequence of events that led to the formation of these rock layers.
- Return to school.
Organizing the Data and Drawing Conclusions
- Have students open their lab notebooks. Ask them to share their observations and instruct you on how to recreate their big picture drawings on the white board in as much detail as possible. Get them to give you as many details as they can reassemble – the number of rock layers, the type of rock in each, the relative width, etc. Focus on accurately representing the students’ measurable observations. Drawing conclusions about the depositional environment and sequence of geologic events will come later.
- When a pretty detailed drawing to work from, begin to reassemble the geologic timeline as the kids see it, starting with the oldest layer that was studied in detail by a student. Of course, that means the students have to decide which is the oldest layer, the sedimentary layers or the igneous layers. Most likely, and correctly, they will select the bottom-most sedimentary layer. Discuss whether these layers were originally laid down at a tilt or if they were laid down flat and then tilted and how they know.
- Once you know which is the bottom most layer, each student should have a chance to explain their theory about the environment present at the time their rock layer formed and what evidence they have to support that theory. Keep track of the events on a line drawn across the board with markers for each of the rock layers. As more students present their findings, the presentations will become shorter, leaving time to speculate on how a fast-flowing river might change into a lake or a delta or a sand bank. You could also speculate on the length of time it would take to accumulate each layer of sediments – is it hundreds or thousands of years?
- When all the students have gone, there is the remaining problem of how the layers become tilted. Have students propose their theories of what happened to tilt the layers. Most likely, someone will mention earthquakes and propose that the earthquakes folded the rock layers and pushed one end up and the other end down, causing the tilt. The key is to let the students come up with the theory, don’t tell them.
- Finally, with the remaining time, I asked each student to create an index card drawing of what they thought that spot would have looked like at the time the rock layer they studied was forming. Imagine standing there thousands of years ago. Would it have looked like a river? Would you have been standing next to a giant volcano spewing hot lava?
- Optional: Since there isn’t a “rock layer” that shows the tilting, you might want to draw an index card, or series of cards, showing the hills being pushed up and the tunnel being carved.
- Arrange the drawings along the masking tape timeline. You can come back to this timeline, adding additional information (such as the periods and epochs - Miocene epoch 5-24 mya, Tertiary period, Cenozoic era - and the types of plants and animals that might have roamed the ancient river valleys and volcanoes) as students learn about geologic time in the upcoming lessons.