In other words, artifacts found in the upper layers of a site will have been deposited more recently than those found in the lower layers.
Cross-dating of sites, comparing geologic strata at one site with another location and extrapolating the relative ages in that manner, is still an important dating strategy used today, primarily when sites are far too old for absolute dates to have much meaning.
The scholar most associated with the rules of stratigraphy (or law of superposition) is probably the geologist Charles Lyell.
The basis for stratigraphy seems quite intuitive today, but its applications were no less than earth-shattering to archaeological theory.
Stratigraphy is the oldest of the relative dating methods that archaeologists use to date things.
Stratigraphy is based on the law of superposition--like a layer cake, the lowest layers must have been formed first.
Though still heavily used, relative dating is now augmented by several modern dating techniques.
Now with carbon-14 and other modern dating techniques we have a very good idea how old things are.
The following is a list of dating techniques used in archaeology and other sciences. Stratigraphy Stratigraphy is the most basic and intuitive dating technique and is therefore also the oldest of the relative dating techniques.
It is more or less in the order of discovery of each procedure. Based on the law of Superposition, stratigraphy states that lower layers should be older than layers closer to the surface, and in the world of archaeology this is generally the case, unless some natural or manmade event has literally mixed up the layers in some fashion.
For example, JJA Worsaae used this law to prove the Three Age System.
For more information on stratigraphy and how it is used in archaeology, see the Stratigraphy glossary entry.