Dating method using volcanic ash
A clastic sedimentary rock is made up of older rock and mineral fragments, and when the rock forms it is almost certain that all of the fragments already have daughter isotopes in them.
Furthermore, in almost all cases, the fragments have come from a range of source rocks that all formed at different times.
Sedimentary rocks can be dated using radioactive carbon, but because carbon decays relatively quickly, this only works for rocks younger than about 50 thousand years.
The universe is full of naturally occurring radioactive elements.If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock.