Sedimentary rocks are generally hard to date because common cements like silica don't have datable radioisotopes, and minerals like glauconite that are common in sedimentary rocks are very prone to resetting.
If only there were long-lived isotopes of silicon, calcium, and magnesium!
If the ratio is constant, we can be pretty sure there's been no gain or loss.
But there are some questions that come to mind: Calculus students typically meet this problem somewhere in the second semester.
It is one of the simplest examples of a differential equation.
A minimum age is the youngest the object can possibly be.
A maximum age is the oldest the object can possibly be. But they obviously have to have been made first, so 1920 is the maximum age of the burial.