Wikipedia radiocarbon dating uranium lead dating
The earliest geological time scales simply used the order of rocks laid down in a sedimentary rock sequence (stratum) with the oldest at the bottom.However, a more powerful tool was the fossilised remains of ancient animals and plants within the rock strata.Divisions in the geological time scales still use fossil evidence and mark major changes in the dominance of particular life forms.For example, the Devonian Period is known as the 'Age of Fishes', as fish began to flourish at this stage.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.
This feature allowed William Smith (an engineer and surveyor who worked in the coal mines of England in the late 1700s) to order the fossils he started to collect in south-eastern England in 1793.
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.
principle sources: Australian Museum https://au/the-geological-time-scale Wikipedia https://en.wikipedia.org/wiki/Radiometric_dating Carleton University numerical (or "absolute") age is a specific number of years, like 150 million years ago.
A relative age simply states whether one rock formation is older or younger than another formation.