Rock layers as evidence of change
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
1 · Read
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.
Layers settle flat. So a layer that is tilted, folded, or cracked is a clue on its own: forces pushed, bent, or broke that land sometime after the layer had already hardened. Put the clues together and you can explain how a whole 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.
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