As a result, rocks that record its earliest history have not been found and probably no longer exist.
His Ph D thesis was on isotope ratios in meteorites, including surface exposure dating.
By 1907 study of the decay products of uranium (lead and intermediate radioactive elements that decay to lead) demonstrated to B. Boltwood that the lead/uranium ratio in uranium minerals increased with geologic age and might provide a geological dating tool.
As radioactive Parent atoms decay to stable daughter atoms (as uranium decays to lead) each disintegration results in one more atom of the daughter than was initially present and one less atom of the parent.
These radioactive elements constitute independent clocks that allow geologists to determine the age of the rocks in which they occur.
Applications are in the areas of Late Quaternary geochronology, sedimentology and biochronology.