Scientists discovered layers of sedimentary rocks in a place that was once a seabed. They also discovered several fossils in the layers. Which fossils do you think will be older - those found in the top layers or those found in the bottom layers? Why?
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Answer:
Sedimentary rocks (made from mud, sand, gravel or fossil shells) and volcanic lava flows are laid down in layers or beds. They build up over time so that that the layers at the bottom of the pile are older than the ones at the top. Geologists call this simple observation the Principle of Superposition, and it is most important way of working out the order of rocks in time. Ordering of rocks (and the fossils that they contain) in time from oldest to youngest is called relative age dating.
Once the rocks are placed in order from oldest to youngest, we also know the relative ages of the fossils that we collect from them.
Relative age dating tells us which fossils are older and which fossils are younger. It does not tell us the age of the fossils. To get an age in years, we use radiometric dating of the rocks. Not every rock can be dated this way, but volcanic ash deposits are among those that can be dated. The position of the fossils above or below a dated ash layer allows us to work out their ages.