So, we have a “clock” which starts ticking the moment something dies.
Obviously, this works only for things which were once living.
For example, a geologist may examine a cutting where the rocks appear as shown in Figure 1.
Here he can see that some curved sedimentary rocks have been cut vertically by a sheet of volcanic rock called a dyke.
It makes no sense at all if man appeared at the end of billions of years.
Since the 1950s, geologists have used radioactive elements as natural "clocks" for determining numerical ages of certain types of rocks. "Forms" means the moment an igneous rock solidifies from magma, a sedimentary rock layer is deposited, or a rock heated by metamorphism cools off.
It's this resetting process that gives us the ability to date rocks that formed at different times in earth history.
Using relative dating the fossil is compared to something for which an age is already known.
For example if you have a fossil trilobite and it was found in the Wheeler Formation.