What is stratigraphy dating
The rock is eroded into sediment -- sand or dust or gravel -- and settles over a broad area.
Most of the earth is covered in layers of sediment, as new sediment settles on top of old sediment.
Animals, partizan those in the seas, are found over vastly wider areas than the sediment from one particular rock seam.
Those are generally too rare to be of much stratigraphic use.This was enough for the earliest stratigraphers to make considerable progress in mapping the geological history of the West of England.However, when the exposures are separated by hundreds of miles, this system will eventually fail.We know that layer A is older than layer B, even if we have no idea how long ago A and B were deposited.In some cases, things get scrambled and even reversed, but geologists learned to diagnose these aberrant cases early on, as these processes leave obvious clues.