So we knew about "deep time," but exploring it was frustrating.
For more than a hundred years the best method of arranging its history was the use of fossils or biostratigraphy.
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
Each isotope is identified with what is called a ‘mass number’.
That only worked for sedimentary rocks, and only some of those.
Rocks of Precambrian age had only the rarest wisps of fossils.
The work of geologists is to tell the true story of Earth's history—more precisely, a story of Earth's history that is ever more true.
But, how can we determine how old a rock formation is, if it hasn’t previously been dated?
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.
Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves.
For that we can thank radioactivity, discovered at the turn of the last century.