Why is it difficult to assign a numerical age to a sedimentary rock? A radiometric date taken from a grain within a detrital sedimentary rock only gives the age of that one grain, which likely came from a source different from the other grains within the rock.

Why is radioactive dating not useful for determining the ages of sedimentary rocks?

Radioactive atoms are inherently unstable; over time, radioactive parent atoms decay into stable daughter atoms. 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.