Lesson 2: Principles of Stratigraphy

Start with an introduction activity, where students look at an outcrop of the Grand Canyon and write down their observations in the handout.  Ask them what patterns they see, what rocks they think formed first, etc. Ask how we can tell what came first. Use this activity to introduce that we want to understand relationships between rock layers, because then we can understand how the environment changed over time.

Explore

The teacher will give out kits containing at least three colors of playdough and toothpicks. With each concept, explain what it means, then have the students create a model and write down the time order of the model layers. Think of this like making a sandwich, laying first a

“bread” layer, and then other layers sequentially on top (if there isn’t enough time to make playdough, a quick easy alternative can be done with different slices of bread- white, wheat, rye, etc.). Then, to check their understanding, go through the real outcrop examples and have the students identify the order of events.

1. Law of superposition. Have students make flat layers of the three colors, then stack them on top of each other. Ask students to label the time order in which it happened in their handout (layer 1, layer 2, layer 3). Show an image of the example outcrop, and go through step by step to identify the order of events with them.
2. Original horizontality. Have students put down the layer in the first color, then stack the second color. Then have them tilt the two layers and stack the third color on top. Ask them to write down in the handout the time order sequence of the model (layer 1, layer 2, tilting event, layer 3). Show image of outcrop with tilted rocks and discuss time order sequence of the outcrop.
3. Cross-cutting relationships. Have students make flat layers of the three colors, then stack them on top of each other. Then, have them put a toothpick through the stack. Ask them to label the order in which things happened (layer 1, layer 2, layer 3, toothpick). Show an image of an outcrop and point out this occurring in rocks – when something cuts through rocks, you know that it happened after those rocks were deposited. Examples of this include faults, volcanic dikes, and veins.
4. Unconformities. For the next example, have students put down layer 1 and 2, then ask what happens if layer 2 is eroded. Have them scrape off the top of layer 2, then put layer 3 on top. Ask them if erosion counts as event (yes, it does!) In the other scenarios, no erosion happened and it is implied that all of these things happened in order. Now, that erosion event is part of the time order. Have them label the time order sequence of the model (layer 1, layer 2, erosion, layer 3). Explain that unconformities show that either a pause in deposition (hiatus) or erosional event occurred. Show outcrop image with labeled unconformity and walk through the time order sequence of the outcrop. Note: point out that the wavy line is used to denote an unconformity!

For the final exercise, students will use their knowledge to determine which rock layer of a cross-section likely has dinosaur fossils. Give students the cross-section with identified rock units. First, they will write the geologic history listing the relative timing of layers and events. Then, based on the additional information on rock types contained in the handout, they will determine which rock layers are the correct age and type to contain dinosaur fossils.

For this exercise, students will need to use their knowledge to discover which layer likely contains dinosaur fossils. Based on the information given, they should be looking for rock layers that formed on land (continental environments, not marine), and are younger than 66 million years old. There is one non-sedimentary rock (rock G, the volcanic intrusion), which has been chemically dated. Any rock layers the intrusion cuts through are older than that date.

Based on the stratigraphic principles, students should figure out that Layers A, H, and E are too old, while layers I, B, and F are too young. Layers C and D are the correct age to potentially contain dinosaur fossils. Based on the rock information from the handouts, Layer D is most likely to contain fossils because it is a sandstone, and fossils are usually found in sedimentary rocks with sand sized grains or smaller. Layer C is a conglomerate, so the layer probably does not contain fossils, even though it is the correct age.

Explain

Concepts:

Stratigraphic principles lay out clear ways to identify relationships between rock units. This is one way to determine the relative ages of rocks (x is older than y, but younger than w).  Geologists use these ideas to investigate geologic history.

Vocabulary: 

• Stratigraphy – the study of rock layers and their relationships

• Outcrop – an exposed rock unit

• Unconformity – a surface of contact between two rock units that are not continuous

• Fault – a crack that displaces layers of rock units

Evaluate

Formative Assessment: 

Throughout the lesson, the teacher will ask the students to explain the time ordered events for each outcrop. This will allow the teacher to check in and emphasize the concept of each principle.

Summative Assessment:

The final exercise will have the students use their knowledge to discover which layer in an outcrop likely contains dinosaur fossils by creating a written geologic history of a cross-section. The final question on the handout asks them which layer they would target and asks them to explain why.