Rock Layers & Relative Dating
Understand that rock layers (strata) form in sequence with the oldest at the bottom and the youngest at the top, and that fossils found in deeper layers are older — this is the principle of relative dating
What a learner can do afterwards
- Explain that sedimentary rock forms in layers with the oldest at the bottom
- Use a diagram of rock strata to determine which fossil is older based on its position
- Define relative dating as working out the age of something by comparing its position in rock layers
1 · Read
Mud, sand, and tiny bits of shell pile up in layers, like a stack of pancakes, and slowly turn to rock. Each new layer sits on top of the ones already there and can trap fossils inside.
A fossil hunter named Priya explores a canyon wall. She finds a fern fossil near the top and a trilobite fossil much lower down. Because the trilobite's layer formed earlier, Priya knows it is the older fossil, even without knowing its exact age in years. Scientists call comparing ages by place relative dating.
Relative dating does not give you an exact number of years. It only tells you the order things happened: which fossil came first and which came later, based on how deep its layer sits.
The deeper the rock layer, the older it is, and any fossil inside follows the same rule.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.