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Before the 20th cent., archaeologists and geologists were largely limited to the use of relative dating techniques.
The method begins with the careful drawing and description of strata (the geological or archaeological profile).
The profile from one location is then compared with profiles from surrounding sites.
Stratigraphic dating assumes that the lower layers in any particular profile are older than the upper layers in that profile ("the law of superposition") and that an object cannot be older than the materials of which it is composed.
Igneous masses are dated according to whether they caused metamorphism in the surrounding rock (proof of emplacement in preexisting rock) or whether sediments were deposited on them after they were formed.
The seriation of stratified deposits permits archaeologists to assess the relative age of particular styles.
This information may then be used to surmise the relative age of unstratified deposits (e.g., surface sites).
Technological changes can be used for relative dating of archaeological material.
In geology, a master stratigraphic sequence for a particular region is built up by correlating the strata from different locations with one another.
As new locations are investigated, the geologist attempts to fit the new profiles into the master sequence of geological strata for that region.