The geological time scale is used by geologists and paleontologists to measure the history of the Earth and life. It is based on the fossils found in rocks of different ages and on radiometric dating of the rocks.
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.
The volcanic ash layer is dated as 507 million years old. The fossil species below the ash must be slightly older than 507 million years, and the species above the ash must be slightly younger.
If rocks in different places contain the same fossil species, they must be similar in age. Tracing of rocks and fossils from one place to another is called correlation. We cannot be sure if the rock layers with the same fossils are identical in age. We can say that the rocks formed during the time in which the fossil species lived.
If a fossil has been dated radiometrically in one place, correlation allows use to work out the age in other places. In the above example, we know that the fossil in Nevada is slightly older than the ash layer dated to 507 million years old. We can say that same species in Oklahoma must also be about 507 million years old, even though there is no volcanic ash layer present at this site.