Archaeologists use that assumption, called the law of superposition, to help determine a relative chronology for the site itself. Then, they use contextual clues and absolute dating techniques to help point to the age of the artifacts found in each layer.
Why should archaeologists date their materials?
The dating of remains is essential in archaeology, in order to place finds in correct relation to one another, and to understand what was present in the experience of any human being at a given time and place. Inscribed objects sometimes bear an explicit date, or preserve the name of a dated individual.
How do you date objects?
Perhaps the most famous absolute dating technique, radiocarbon dating was developed during the 1940s and relies on chemistry to determine the ages of objects. Used on organic matter, the technique measures the amount of radioactive carbon decay to determine an objects age.