Radioactive dating of rock samples is a method of
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.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.
As he mapped across southern England, he drew up a stratigraphic succession of rocks although they appeared in different places at different levels.
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.
This built up the first generalised geological time scale.
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.
Over hundreds to thousands of millions of years, continents, oceans and mountain ranges have moved vast distances both vertically and horizontally.