Radioactive decay and dating rocks
The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation.
The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created.
By matching similar fossils in different regions throughout the world, correlations were built up over many years.
Only when radioactive isotopes were developed in the early 1900s did stratigraphic correlations become less important as igneous and metamorphic rocks could be dated for the first time.
On the other hand, the concentration of carbon-14 falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.
However, a more powerful tool was the fossilised remains of ancient animals and plants within the rock strata.
Fossil assemblages 'fingerprint' formations, even though some species may range through several different formations.