The archaeologists record and analyze the changes in types and styles of human-made items from different levels according to the principle explained below.
Sediments are usually laid down in horizontal beds.
Relative-dating techniques are nearly always applicable but are not precise and require calibration.
Correlation techniques are locally useful and depend on recognition of an event whose age is known, such as a volcanic eruption or a paleomagnetic reversal.
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 more information on stratigraphy and how it is used in archaeology, see the Stratigraphy glossary entry.Though still heavily used, relative dating is now augmented by several modern dating techniques.Radiocarbon dating involves determining the age of an ancient fossil or specimen by measuring its carbon-14 content.Geologic assessment of active tectonism depends on two key measures: the age and the amount of deformation of a given stratigraphic unit.The amount of deformation can normally be measured with greater accuracy than the age.