Relative age dating and cross cutting relationships
These distinctive rocks are matched by similar white cliffs in France, Belgium, Holland, Germany, and Denmark (see Figure below).
It is important that this chalk layer goes across the English Channel.
Sediments were deposited in ancient seas in horizontal, or flat, 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.
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.
Rock layers with the same index fossils must have formed at about the same time.
The presence of more than one type of index fossil provides stronger evidence that rock layers are the same age. Geologists divide this time span into smaller periods.
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?