Why is geologic time scale only a relative time scale and not an absolute time scale?

Question

Why is geologic time scale only a relative time scale and not an absolute time scale?

Answer 1

It's both a relative and an absolute time scale.  Relative dates come from studying the stratification and the fossils that may be found.  Absolute* dates sometimes come from suitable probes containing radiometric samples; eg. volcanic ash.  Relative dating tells us that the Purbeckian Limestone Group deposits in Dorset were laid down early in the Lower Cretaceous.  Aboslute dating information from elsewhere tells us that was the Early Cretaceous of about 140 million years ago.

The geologic time scale isn't only a relatie one.

* Absolute dating isn't meant absolutely absolutely.  The numbers are plus or minus a bit.