Consider this analogy: a barbecue grill full of burning charcoal.The charcoal burns at a known rate, and if you measure how much charcoal is left and how much ash has formed, you can tell how long ago the grill was lit.This process happens at a uniform rate, as steady as a clock, unaffected by ordinary temperatures or ordinary chemistry.
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.
A hundred years ago, we had little idea of the story's length—we had no good yardstick for time.
Bertram Boltwood in 1907 used lead, the end-product of uranium decay, as a method to assess the age of the mineral uraninite in some ancient rocks. The rocks appeared to be astonishingly old, ranging in age from 400 million to more than 2 billion years. Once isotopes were explicated, during the 1910s, it became clear that radiometric dating methods were not ready for prime time.
(For an introduction to atoms and isotopes see this article by About Chemistry Guide Anne Marie Helmenstine.)With the discovery of isotopes, the dating problem went back to square one.Soon after radioactivity was discovered, experimenters published some trial dates of rocks.Realizing that the decay of uranium produces helium, Ernest Rutherford in 1905 determined an age for a piece of uranium ore by measuring the amount of helium trapped in it.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.So we knew about "deep time," but exploring it was frustrating.