Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type. There are two main methods determining a fossils age, relative dating and absolute dating.The carbon-14 it contained at the time of death decays over a long period of time.By measuring the amount of carbon-14 left in dead organic material the approximate time since it died can be worked out.
But this sediment doesn't typically include the necessary isotopes in measurable amounts.
For example, in 1991, two hikers discovered a mummified man, preserved for centuries in the ice on an alpine mountain.
Later called Ötzi the Iceman, small samples from his body were carbon dated by scientists.
His Ph D thesis was on isotope ratios in meteorites, including surface exposure dating.
He was employed at Caltech's Division of Geological & Planetary Sciences at the time of writing the first edition.