Figure 33 The walls of the Grand Canyon consist of many different layers of rock.
Interpreting Diagrams Which of the sedimentary layers shown here is older, the Bright Angel shale or the Tapeats sandstone? Explain your answer.
ddThe Law of Superposition
Sedimentary rocks form as horizontal layers. Geologists have used this fact to establish a principle for determining the relative ages of rocks. The law of superposition states that if rock layers are undisturbed, younger rocks lie above older rocks, and the oldest rocks are at the bottom. 
Geologists use the law of superposition to determine the relative ages of sedimentary rocks from the sequence of rock layers and the fossils within each layer. Rock layers often extend over large regions. Geologists have examined sedimentary rocks from locations around the world to develop a relative time scale for many rock layers.
Index Fossils and Relative Dating
Geologists can also determine the relative ages of sedimentary rocks by examining the fossils that are found in them. Most types of organisms preserved as fossils are now extinct. An extinct type of organism is one that no longer exists.
Fossils of organisms that are easily identified, occurred over a large area, and lived during a well-defined period of time are called index fossils. With index fossils, geologists can determine the relative ages of rocks. If a rock contains examples of an index fossil, then the rock must have formed during the time that that organism lived.
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