Congratulations, you have just figured out how geologists assigned ages in years to the Geologic Time Scale. The fossil bearing sedimentary rocks must be bracketed by isotopic ages from igneous or metamorphic rocks. Through countless hours of work and literally thousands of isotopic dates on underlying, overlying, included, and cross-cutting igneous and metamorphic rocks from all over the Earth, geologists have managed to calibrate (in years) the ages of periods in the Geologic Time Scale. And this work continues. Some ages will be refined as additional isotopic dates are obtained.