How fossils form
Identify evidence from patterns in rock formations and fossils in rock layers to support an explanation for changes in a landscape over time
What a learner can do afterwards
- Explain that rock layers (strata) form over time, with the oldest at the bottom
- Describe how fossils in different layers provide evidence of organisms that lived at different times
- Use patterns in rock formations to explain how a landscape has changed over millions of years
The lesson
Rock layers build up over long stretches of time. Sand, mud, and ash settle down and harden into rock, one layer at a time. Scientists call these layers strata. Since new layers always settle on top of old ones, the layer at the bottom of a cliff or canyon wall is normally the oldest, and the layer at the top is the youngest.
In a real canyon wall, scientists once found trilobite fossils only in the bottom layers. Trilobites died out hundreds of millions of years ago, so finding them low down told scientists that layer formed a very long time ago. Higher up, they found shell fossils from animals still alive today, showing those layers are much younger.
Look for layers that are tilted, folded, or cracked instead of lying flat. That is a clue too: forces pushed, bent, or broke the land sometime after the layers first formed, and it can help you explain how a landscape changed over millions of years.
Oldest rock layers sit at the bottom, and both the fossils inside them and any tilting or folding of the layers reveal how a landscape changed over millions of years.
Watch it
Where it sits
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Nothing builds on it yet.
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.