Dating Methods in Archaeology: Must Know for Geology Students!

Establishing the age of objects, landscapes, or contexts is one of the primary tasks of the archaeologist. It has also proved to be among the most difficult. Archaeology's twin roots in the humanities and in sciences such as geology and paleontology provided two types of answers to the question, how old? Although antiquarians, historians, and philosophers long pondered the history of people before writing, it was only in the nineteenth and twentieth centuries that the discipline of archaeology was able to make great strides in developing reliable means of assigning age and chronological relationship.

From the humanities, particularly through the study of material culture, archaeologists have developed schemes of relative dating based on the close analysis of the forms and functions of artifacts. In this endeavor strong links with classification (taxonomy) and typology were forged that depended on the development of consistent rules for describing artifacts and the contexts in which they were found. The earliest relative chronologies, such as the three-age system, were founded upon description, taxonomy, and the development of typology. But in the absence of a means to establish relative age, such classifications and typologies were, in essence, ahistorical and potentially circular in their logical forms.

Geology and paleontology provided the basis of a solution. The principle of relative dating (that is, establishing that one thing is relatively older or younger than another thing) was based on the notion of stratigraphy. In this sense it was understood that following the law of superposition (by which what is on the top is assumed to be younger than what is on the bottom), the relative ages of artifacts could be established on the basis of their relative positions within a stratigraphic profile. Advances in the degree to which antiquarians and archaeologists in the nineteenth century understood the principles of stratigraphy (and site formation) were matched by an increasing sophistication in the ways in which they were able to apply them.

The work of Scandinavian archaeologists such as jens jacob worsaae, sophus müller, and oscar montelius and Egyptologists such as w. m. flinders petrie was particularly significant in the development of the relative-dating technique of seriation. These advancements were supported by painstaking research into the nature and distribution of artifact assemblages across space and time (as defined by the chronological relationships established through stratigraphy and typology). Such research allowed archaeologists to document a phenomenon in the human use of material culture that had temporal implications. Put simply, seriation acknowledges that artifacts change in their forms and in their styles of decoration over time.

Archaeologists such as Petrie were able to use empirical evidence from excavations to establish that change in such forms and styles had a history that was generally repeated-an artifact in its earliest appearance was rare, it then became more popular (and hence more numerous), and, as fashion moved on, it eventually became rare once again before finally vanishing. Petrie's great contribution to the method of seriation was to harness his empirical information to construct frequency diagrams for artifacts of various types at various stratigraphic levels, thus creating "battleship curves" that plotted the "life histories" of artifact form and style.

Notwithstanding the great advances made in relative chronologies based on the analysis of artifacts and their stratigraphic contexts, archaeologists were significantly hampered by their inability to quantify time. Without quantifiable time they could establish the direction of history, but they could not explore significant elements of process that require a means of establishing rate and duration. The establishment of high human antiquity, which was first achieved in the mid-nineteenth century by scholars such as hugh falconer, joseph prestwich, édouard lartet, and jacques boucher de perthes, was limited in its impact precisely because it was essentially based on relative chronology. Archaeologists had to wait until well into the twentieth century before reliable means of establishing the absolute ages of objects or their stratigraphic contexts could be established, but when this was achieved the practice of archaeology was transformed to its core.

Of course, archaeologists had long been aware that ancient societies had their own sense of time and succession, creating calendars that, when deciphered (as in the case of mesoamerican calendrical systems), provided absolute dates with great precision. Indeed, it was the linking of these calendars and lists of kings and dynasties (especially in Egypt and the Middle East) that allowed Petrie and Montelius (and, of course, vere gordon childe) to construct a notional "history" for late-European prehistory. However, although it was widely accepted that constructing such histories was a valuable undertaking, it was also understood that the reliability of such histories rested on assumptions about duration and processes that were difficult to independently test. Dates derived from the study of ancient coins and inscriptions certainly assisted (and continue to assist) the archaeologist in refining chronology, but of themselves they did not overcome the limitations of the approach.

This accomplishment had to wait for the application of science-based dating methods to archaeology.During the twentieth century archaeologists were presented with a constant flow of techniques for dating either archaeological objects or archaeological contexts or both. The first of these, dendrochronology, or tree-ring dating, was pioneered by a. e. douglass in the first two decades of the century. Although it was originally developed in the Southwest of the united states, the technique has been used in other parts of the world with varying degrees of success (Baillie 1982).

The story of dendrochronology mirrors that of other science-based dating technologies in the twentieth and twenty-first centuries in that the techniques themselves become the subject of continuing research and development. Although the great proliferation in absolute dating technologies that occurred in the twentieth century has often been explained as stemming from a desire to encompass more time with greater precision, it is also true that the archaeological scientists who create and employ such techniques also need to devote considerable research time to understanding their nature, prospects, and limitations.

This is perhaps most apparent in the development and application of radiocarbon dating. Given that this was the first dating technology that depended on the establishment of regular, time-dependent processes (in this case radioactive decay), archaeologists and archaeological scientists have been researching C-14 dating since its development by willard libby in 1952. Radiocarbon dating has become the most widely used absolute dating technology all over the world. Indeed, one of its very great strengths has been its capacity to create a "world prehistory"-a framework within which archaeologists could compare what was happening in parts of the world that had, at that time, little or no shared history (Bowman 1990). However, virtually from the time the technique was first applied to archaeological contexts, practitioners have recognized and worked to correct limitations in the technology-an effort that has led to the development of a thriving industry in dating research and the education of archaeologists in the business of collecting samples and interpreting dates. After some fifty years of research we now have enhancements of Libby's original technique that can deliver more accurate absolute dates over longer time periods.

Some limitations in radiocarbon dating have been overcome by the development of new technologies such as luminescence dating (Aitken 1985), which themselves have become the subjects of ongoing research. Given that dating is so central to the business of doing archaeology in the early twentieth-first century, any reputable undergraduate archaeology textbook contains exhaustive descriptions of techniques for dating materials as diverse as the enamel on teeth or the products of volcanic eruptions in the very remote human past.

The identification of regular decay processes occurring in nature is an ongoing task designed to assist the archaeologist in obtaining absolute dates from seemingly intractable materials and to improve our confidence in the reliability and precision of such dates. Notwithstanding the great success achieved by dating specialists in the twentieth century, we should never forget that although the development and application of the technologies is important, the task of making sense of them remains firmly in the province of the archaeologist.