Of the twenty or so dating methods employed in twenty-first-century archaeology, the US Congressional Office of Technology Assessment (OTA) report on Technologies for Prehistoric and Historic Preservation (US Congress, OTA 1986) listed only seven and highlighted two radiocarbon and archaeomag- netic techniques; see Taylor 2000, pp. In manyregions ofthe world, radiocarbon, dendrochronology, obsidian hydration, and archaeomagnetism are the most common physical dating methods used, presumably in that order, but often together as a group providing redundant checks on one of the most important goals of archaeology—providing a framework of time, a chronology, upon which a rational reconstruction of the past can be built.
Regardless of an archaeologist's theoretical or political perspectives, the first goal is to order regional prehistoric or historic sites, a single site, or a portion of the site into meaningful cultural and chronological segments.
Indeed, the methods used by Quaternary geologists to date sediments include most of the methods used by archaeologists and discussed here (see Fig. Stratigraphic studies of archaeological sites are designed to define objectively and categorize the sediments and soils, the contact units between them, and the amount of time they represent, as well as their relationship to the surrounding sediment history.Archaeological stratigraphy is based on the geological concepts of the law of superposition, which states that older sediments are emplaced at a lower level than more recent sediments.Therefore, sites, features, and artifacts residing in lower levels are, by definition, older than those in upper levels.This conceptual framework in archaeology appears to havebegun with John Frere in 1797 attempting to understand the relationship between stone axes in a sedimentary sequence in England: 'The manner in which...[the hand axes] lie would lead to the persuasion that it was a place of their manufacture and not of their accidental deposit...A dissatisfaction with this rather unilineal explanation of the past stirred archaeologists to look beyond simply constructing chronologies and to begin to understand process and social relationships in the past.
Dating methods and the construction of cultural chronologies, however, have remained the basement upon which archaeologists build their understanding of social process through time.Stratigraphic relations have always been the primary method to infer the relative age of artifacts within a site.Stratigraphy is defined here as the study of the spatial and temporal relationships between the sediments and the soil (Waters 1992, pp. Much of this section's discussion can be more robustly understood by referring to the entry Geoarchaeology; an awareness of the geological processes of archaeological site formation is crucial to an understanding of dating archeological sediments.It may be suggested that the different strata were formed by inundations happening at distant periods' (Frere in Rapp and Hill 1998, p. This idea that features or artifacts found within a site in the same stratigraphic level are contemporaneous forms the foundation upon which archaeological stratigraphy is based. 2 shows a simple stratigraphic profile of a single excavation unit with the relative positions of natural and cultural features, and the position of radiocarbon dates recovered from the various levels.In early twenty-first century archaeology, the various relative and absolute dating methods such as radiocarbon or archaeo- magnetic dating are used to verify this assumption and produce a site chronology (see Fig. Note that the radiocarbon dates indicate that, in general, the stratigraphy is intact in that the oldest dates are at the bottom and the latest dates at the top, except for one date (CAMS 43177) at 3690 ± 50 BP which seems to be out of sequence and suggests 'small-scale' disturbance probably caused by the digging of the pit structure during the later ceramic period (Shackley et al. Time-sensitive artifacts, such as previously dated pottery or projectile point types, can be used as index fossils within the stratigraphic column to create a relative chronology within the site.The stratigraphic Simple stratigraphic wall profile of a unit at Mc Euen Cave (AZ W:13:6 ASM) showing position of bedrock, undisturbed strata, pit fill structure, and the position of recovered radiocarbon dates. 2000example discussed above also used index fossils, in this case projectile point types, to add another chronological piece of information. In this instance the stratigraphy, radiocarbon dates, and index fossils all contributed to an understanding of the site chronology and the integrity of the archaeological deposit.