Fossils are collected along with rocks that occur from the same strata.
These samples are carefully cataloged and analyzed with a mass spectrometer.
For a century, the radioactive decay of unstable elements into more stable ones has been used as a natural clock to estimate the age of earth materials.
As the isotopes decay, they give off particles from their nucleus and become a different isotope.
The parent isotope is the original unstable isotope, and daughter isotopes are the stable product of the decay. In the first 5,730 years, the organism will lose half of its C-14 isotopes.
To determine the relative age of different rocks, geologists start with the assumption that unless something has happened, in a sequence of sedimentary rock layers, the newer rock layers will be on top of older ones. This rule is common sense, but it serves as a powerful reference point.
Geologists draw on it and other basic principles ( to determine the relative ages of rocks or features such as faults.