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Geologists can learn a lot about Earth’s history by studying sedimentary rock layers.

But in some places, there’s a gap in time when no rock layers are present. The unconformity was discovered by James Hutton in the 1700s.

Older rocks are at the bottom and younger rocks are at the top.

The early geologic time scale could only show the order of events.

For example, the Cretaceous ended when the dinosaurs went extinct. Which period of geologic time was the last in which dinosaurs lived? Why are sedimentary rocks more useful than metamorphic or igneous rocks in establishing the relative ages of rock?