The pioneers of radiocarbon dating used this method because carbon-14, the radioactive isotope of carbon, is very active, decaying with a half-life of just 5730 years.The first radiocarbon laboratories were built underground, using antique materials from before the 1940s era of radioactive contamination, with the aim of keeping background radiation low.For instance, the uranium-to-lead decay cascade is really two—uranium-235 decays to lead-207 and uranium-238 decays to lead-206, but the second process is nearly seven times slower.
A hundred years ago, our ideas about the ages of rocks and the age of the Earth were vague. Judging from the amount of rocks there are, plus the imperceptible rates of the processes forming them—erosion, burial, fossilization, uplift—the geologic record must represent untold millions of years of time.
It is that insight, first expressed in 1785, that made James Hutton the father of geology.
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.
Today, with the help of isotopic dating methods, we can determine the ages of rocks nearly as well as we map the rocks themselves.