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A gap in the sequence of rock layers is called an unconformity. Hutton saw that the lower rock layers are very old. There are no layers in between the ancient and recent layers.Hutton thought that the intermediate rock layers eroded away before the more recent rock layers were deposited.To create the geologic time scale, geologists correlated rock layers. Steno's laws were used to determine the relative ages of rocks. Erosion may have worn away some of the rock, but layers on either side of eroded areas will still “match up.” Look at the Grand Canyon in Figure below. You can clearly see the same rock layers on opposite sides of the canyon.The matching rock layers were deposited at the same time, so they are the same age.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?

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This made him realize that Earth must be much older than people thought. It meant there was enough time for life to evolve gradually. [Insert a link to a video or animation about Hutton’s unconformity.] When rock layers are in the same place, it’s easy to give them relative ages. For example, the famous White Cliffs of Dover are on the coast of southeastern England.

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.

Index fossils are commonly used to match rock layers in different places. If two rock layers have the same index fossils, then they’re probably about the same age.

You can watch a video about index fossils and how they are used at his link:[Insert a link to a video about using index fossils to match rock layers in different places.] Using Index Fossils to Match Rock Layers.

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