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Before the 20th cent., archaeologists and geologists were largely limited to the use of relative dating techniques. Estimates of the absolute age of prehistoric and geological events and remains amounted to little more than inspired guesswork, as there was no scientific basis for testing such proposals. However, as the basic principles of relative dating progressed during the course of the 19th cent., investigators were able to correctly determine the relative age of many archaeological and geological materials.
Stratigraphic dating is accomplished by interpreting the significance of geological or archaeological strata, or layers. 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. 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. The depth of the strata within the master sequence provides the investigator with the relative date of any particular profile.
Seriation is an archaeological technique involving the description of stylistic changes in artifacts and of changes in the popularity of distinct styles in order to accurately describe the sequence of variation over time. 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. The three-age system devised by the Danish archaeologist Christian Thomsen in the 1830s made use of technological criteria. According to this system, humans passed through three distinct stages of technological development, based on the primary material used to manufacture tools and weapons: the Stone Age, the Bronze Age, and the Iron Age.
Biological criteria can also serve as a means for relative dating. Fossils are useful because certain assemblages of species are characteristic of specific geological eras. Pollen analysis, or palynology, involves the microscopic examination of fossil pollen grains in stratified peat and lake deposits. From this, scientists can establish pollen diagrams (describing the relative abundance of different pollen-producing plants at a given point in time) and floral time charts (showing how climate and flora changed over time). The principle of stratigraphic dating is used to establish the relative age of these floral and fossil assemblages. Through the investigation of many different stratigraphic contexts, a master sequence of fossil and floral assemblages may be devised for a region.
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