Relative dating of a fossil
The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative datin g' as observed in geology from the 17th century to the early 20th century.
The only disturbance that the layers experience is bioturbation, in which animals and/or plants move things in the layers.
The principle of inclusions and components explains that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them.
For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer.
Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).
The law of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it.