The atoms of some chemical elements have different forms, called isotopes.

The unstable nature of carbon 14 (with a precise half-life that makes it easy to measure) means it is ideal as an absolute dating method.Geologists often need to know the age of material that they find.They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in number of years.In many sedimentary rocks, particularly those of a continental or freshwater origin, fossil spores can be quite widespread, abundant and may be highly diverse and evolved over time.This makes them ideal zone fossils for biostratigraphy and dating of sedimentary rocks (for some examples of spores, see the section on the Rhynie flora).