Dating is nothing more than ordering time. Time is the quintessential sorter of events. All living beings go through life being on occasion acutely aware of its transient yet eternal, ceaseless yet tenacious quality. Time is the omnipresent judge that indicts all life for existence and condemns it to death. Thus, for the greatest portion of human history, time was seen in terms of an individual or series of lifetimes, with a clear beginning and a clear end. This view of the world applied as much to the wonders of nature as it did to the human being, with such phenomena as the rising and setting of the sun, the moon, and important stars and the passing of the seasons. The ancient Egyptian proverb, “All things fear time; but time fears the pyramids” summarizes the essence of time’s role in human history. Time has always been an enigma somehow understandable to the individual but incomprehensible and unexplainable to others. With the advent of high civilization, time was ordered by the actions of leaders, and a number of king’s lists have survived in the written record to assist the modern scholar in the very difficult task of attaching dates to events of the near-distant past.
This ordering of time throughout the ages serves a purpose, to answer the question: “What is the age?” Or, “How old is it?” In anthropological research, time has always been the great sorting mechanism. Collectors and travelers of classical times, such as Herodotus, studied historic monuments and produced speculative accounts of prehistory. In fact, several dozen classical authors in the first millennium BC ordered time as a succession of ages based on technological progress. A three-age system encompassing the Stone, Bronze, and Iron Ages was the most common time-sorting methodology, but there were variations with copper and gold. Lucretius (95-53 BC) summarized these Western views of dating the past. Yuan K’ang (ca. AD 50), a Han Dynasty Chinese scholar, wrote an account of the historical development of toolmaking, from the use of stone/ jade through bronze to iron.
The principle of a systematic organization of ex situ archaeological materials started with the understanding of the three-age system in the 16th century by Michael Mercati (1541-1593), who was the superintendent of the Vatican gardens and adviser to Pope Clement VIII. The combination of his Renaissance education, his substantial mineral and fossil collections, and his access to the newly acquired American ethnographic artifact collections permitted Mercati to formulate the foundations of modern archaeology. His observations, which were not easily accessible until the 18th century, are all the more remarkable when one considers the intellectual milieu of that era. In Europe during this era, inquiry into the prehistoric past was discouraged, because the Bible was regarded as the supreme authority on human history and the early history of the earth. For example, creationism dominated scholarly writings on the origin of the universe and humanity, and during this period, fossils of marine organisms that were sometimes found in mountains were described as being washed up by the Great Flood. Ancient arrow points and other prehistoric stone tools were thought to have been produced by thunderbolts and other natural phenomena. Prehistoric stone arrow points and axes were believed to have fallen from the sky at the moment when thunder stuck. These implements were called thunder-stones, ceraunia, or pierre de foudre.
It generally was believed that all living plant and animal species were survivors of the Great Flood and that with careful biblical research, especially on the book of Genesis, it was possible to calculate the age of the earth. For example, in 1642, Dr. John Lightfoot, the vice-chancellor of Cambridge University, calculated that the universe was created in 4004 BC, on October 23, at 9:00 am, coincidental with the beginning of the Fall Term. Later in 1658, Archbishop James Ussher refined this estimate and suggested that the earth was actually created on the evening preceding October 23,4004 BC. This is the kind of pedantic (to us) debate that took place, so that although historical sites were being studied, prehistoric archaeology was being interpreted in light of the Bible.
It was only in the early part of the 19th century (1837) that Mercati’s concepts were applied rigorously to a museum collection in Denmark by Christian Thomsen (1788-1865). The material culture in “cabinets” and museums could be now sorted relatively. But, in situ materials also required theory for relative sorting, and this was provided by the Danish atomist and geologist Bishop Nicholas Steno (1638-1686). He was the first to clearly state the three principles of stratigraphy, which have come to be known as Steno’s Laws. They are: the Principle of Superposition: In a sedimentary sequence, the older beds are on the bottom, and the younger beds are on the top; the Principle of Original Horizontality: Sediments tend to be deposited in flat, horizontal layers; and the Principle of Original Lateral Continuity: a flat layer will tend to extend for a considerable distance in all directions. The first principal (superposition) is the one employed in conjunction with fossil markers by early geologists William “Strata” Smith (1769-1839) and Charles Lyell (1797-1875) and archaeologists Pitt Rivers (1827-1900) and Boucher de Perthes (1788-1868) to sort materials stratigraphically, with an assumption of a time progression.
Through human ingenuity, the last 150 years have been witness to great number of techniques for sorting time applicable to the scientific study of the past. These various dating techniques fall into one or more of three categories: absolute, relative, and radio-metric. Absolute techniques of varve analyses and dendrochronology are only such when they can be clearly calibrated to a known year; in all other cases, they are relative dating techniques. Relative dating techniques permit chronological relationships to be ascertained through physical and/or chemical seriation (cation exchange ratio, fluorine dating, patination, pollen analysis) based on spatial relationships (stratigraphy and cross-dating), differential abundances, technological variations, or combinations thereof. Some techniques (for example, obsidian hydration, archaeomagnetism) require a radiometric technique for calibration; all benefit from their use. Other relative dating techniques require dated historical information (astronomical dating).
Contrary to popular belief, radiometric dating techniques, whether they are based on the exponential decay of a radioactive element or the ionizing damage effects of radiation, are only relatively absolute as they can only give a statistical approximation of an absolute age. Radiometric dating techniques that rely on radioactive decay include potassium-argon dating, radiocarbon dating (Carbon 14), and uranium-thorium dating. Radiometric dating techniques that rely on the buildup of damaging ionization from radioactive decay include thermo luminescence dating, optically stimulated luminescence, electron spin resonance (ESR), and fission track dating.
Basically, all dating techniques can be seen as clock types. The analyst must know some basic information about each clock: How does the clock tick? How is the clock set? How is the clock read? With these fundamentals, all dating clocks have the potential to provide a chronological framework. Some dating techniques are shown in Table 1 with respect to their clock functions. Unfortunately, not all dating procedures are created equal, and some methods are more reliable than others.
All dating techniques have limitations with respect to the material within which they function and the age range over which they are applicable. Table 2 lists material and limits for some dating techniques.
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- Grâslund, B. (1987). The birth of prehistoric chronology: Dating methods and dating systems in nineteenth-century Scandinavian archaeology. Cambridge: Cambridge University Press.
- Herz, N., & Garrison, E. (1998). Geological methods for archaeology. Oxford: Oxford University Press.
- Rutter, N. W., & Catto, N. R. (Eds.). (1995). Dating methods for quaternary deposits. St. John’s, Canada: Geological Association of Canada.