Relative geologic dating and numerical geologic dating
In the late 18 century, James Hutton developed the idea that the layers found in rock outcrops were representations of the story of time on the planet.
He concluded, the bottom most layers were deposited first, thus making them the oldest and subsequent layers were progressively younger in age.
Where do scientists begin the story of Earths history?
One place to start is by looking at the age of the materials that make up the Earth itself.
Thus, the older a rock is, the larger the number of daughter elements and the smaller the number of parent elements is found in the rock.
Radiometric dating is another term used to talk about Absolute Time dating.
As time passes, the "parent" radioactive elements change at a regular rate into non‑radioactive "daughter" elements.
The following chart shows this concept with a brief description of each age represented.
The age of a rock in years is called its absolute age.
Sedimentary rocks build up layers from the oldest deposits, at the bottom with the newest at the uppermost layers. Biostratigraphy is a specialized form of Stratigraphy, which uses plant and animal fossils to establish the different layers.
There are four basic principals of Stratigraphy, which help us understand Earths time line. Non-horizontal layers have been folded or tilted after the original sedimentation.