Difference between relative absolute dating
The same rock formation also contains a type of trilobite that was known to live 415 to 425 million years ago.
Since the rock formation contains both types of fossils the ago of the rock formation must be in the overlapping date range of 415 to 420 million years.
But, how can we determine how old a rock formation is, if it hasn’t previously been dated?
This uses radioactive minerals that occur in rocks and fossils almost like a geological clock.