Until the advent or discovery of Radiometric dating, a type of Absolute time dating, there was no way to verify the accuracy of the relative ages of materials found in the various layers of sedimentary rocks.
He concluded, the bottom most layers were deposited first, thus making them the oldest and subsequent layers were progressively younger in age.
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.
When scientists are able to determine the age of rocks and fossils, they can then tell a more accurate story of the Earths history.
When Geologists and other scientists refer to the age of the Earth, they are using one of two methods to determine this.
Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter).