The deposit thus occurring forms layers depending on the nature of the material brought in by the people inhabiting the area.
According to this method, the upper deposits are younger and the lower deposits are older.
Love-hungry teenagers and archaeologists agree: dating is hard.
But while the difficulties of single life may be intractable, the challenge of determining the age of prehistoric artifacts and fossils is greatly aided by measuring certain radioactive isotopes.
There are two techniques for dating in archaeological sites: relative and absolute dating.
After death the amount of carbon-14 in the organic specimen decreases very regularly as the molecules decay.
Until this century, relative dating was the only technique for identifying the age of a truly ancient object.
By examining the object's relation to layers of deposits in the area, and by comparing the object to others found at the site, archaeologists can estimate when the object arrived at the site.
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.