Relative geologic dating and numerical geologic dating
When rocks are formed, small amounts of radioactive elements usually get included.
A few isotopes do decay at a, slow enough, rate to be able to date things into the millions of years range, and can be effectively used as geological clocks.
Smith noticed that specific forms of life were fossilized in particular layers of rock, giving a time line story indicating when, in time, events occurred.
This progression of or evolution of life occurred in a vertical fashion up through the layers.
Since there is no set rate of deposit for these layers, the actual length of time represented by these layers is a good guess.
Stratigraphy: is the science or study of layers or strata of rocks.