There are three major divisions when dating material: chronometric or absolute dating, relative dating, and experimental.
Relative dating does not give the actual age of a site or artifact, but it does help in identifying what things are older or younger. The primary techniques utilized for relative dating include: superposition, cross dating, fluorine, and seriation.
Superposition is what is commonly used in fieldwork when excavating a site. The Law of Superposition states that something located above something else is not as old, since the lower item had to have existed before the material above was deposited on it. In essence, the deeper you go, the further back in time you travel. Obviously, there are many exceptions to this rule with distrubances. Human disturbance, rodent disturbance, and environmental impact can all cause a disturbance in superposition.
Cross Dating is simply comparing finds to what we know about other sites. For example, coins, pottery, or other items that are well dated and documented can give a relative date for when it had been deposited.
Fluorine dating measures the differences in fluorine content in materials. Fluorine is a common substance in water, but it varies with location. Bone absorbs fluorine, and the amount absorbed can be measured. If two bones are deposited at the same location at relatively the same time, the fluorine content should be nearly the same. If bone C has less fluorine than bone D, then one can assume that bone C is younger than bone D. Bone C would have been in the ground and exposed to fluorine in the groundwater for a shorter amount of time than Bone D. Using fluorine as a relative dating measurement is site specific, as fluorine levels vary with location...and quite likely with time as well. |
