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After Charles Darwin's publication Origin of Species (Darwin himself was also a geologist) in 1859, geologists realised that particular fossils were restricted to particular layers of rock.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate.For example, decay of the parent isotope Rb-87 (Rubidium) produces a stable daughter isotope, Sr-87 (Strontium), while releasing a beta particle (an electron from the nucleus).("87" is the atomic mass number = protons neutrons.He noted that different formations contained different fossils and he could map one formation from another by the differences in the fossils.
As he mapped across southern England, he drew up a stratigraphic succession of rocks although they appeared in different places at different levels.
Divisions in the geological time scales still use fossil evidence and mark major changes in the dominance of particular life forms.