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 dating' as observed in geology from the 17th century to the early 20th century.
The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith.
This principle allows us to piece together the succession of fossil life that defines much of the geologic time scale. And we know of many more causes, from tectonics to intrusions, that can tilt and fold rocks.
"Materials forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way."This principle allowed Steno to link identical rocks on opposite sides of a river valley and deduce the history of events (mostly erosion) that separated them.
Stratigraphy is a branch of geology that studies rock strata with an emphasis on distribution, deposition, age and evidence of past life.
Nicolas Steno, William Smith, Georges Cuvier, Alexandre Brongniart, and James Hutton developed the basic rules for the science of stratigraphy.
Steno formalized the laws of superposition, original horizontality, original continuity and inclusions in his publication entitled states that any inclusion is older than the rock that contains it.