III.

Dating Methods

Relative dating and historical dating were the only two methods of establishing the chronology of periods or of artefacts in the more remote past. However, such scientific methods as varve analysis, dendrochronology, and, most recently, radiocarbon dating were developed and used to provide absolute dates, albeit within varying limits of accuracy.

A.

Relative Dating

Relative dating, which is based primarily on stratigraphy (the study of how layers or deposits accumulate in chronological sequence), simply involves placing objects, and by extension events and cultural periods, into an isolated historical sequence. For example, it could be shown that the Iron Age followed the Bronze Age, and that the Bronze Age was in turn preceded by the Stone Age; it could not, however, be established when these periods began and ended.

The secondary basis of relative dating is typology—the grouping of objects into types that share the same attributes, be they material, shape, or decoration, or a combination of these. As a dating method, the value of typology rests on two basic concepts: that objects from a given time and place must share a recognizable style, and that changes in style are fairly gradual. Collections of different but contemporaneous objects can also be grouped together in an assemblage, and various assemblages arranged in sequences and compared with those from other areas.

Other relative chronologies are based on the succession of climatic phases (glacials, interglacials, stadials, and interstadials) that occurred during the last ice age. Pollen from deposits also produces sequences of climatic and vegetational change, but these tend to be fairly localized. Faunal dating—based on the presence of the bones of different species of animal—was also an important method, particularly for Pleistocene archaeology (the study of the last ice age), as remains of species indicative of colder or warmer climates appear and disappear from the stratigraphy.

B.

Historical Dating

Historical dating is based on the equation of the archaeological record with dates provided in written chronologies and calendars compiled by ancient peoples. Many of these calendars, such as those of the Chinese, Egyptians, and Romans, were based on reignal years—the years of rule of their dynasties, kings, consuls, or emperors. The Egyptian dynasties, for example, can be dated by working back from the conquest of Egypt by Alexander the Great that Greek historians recorded as having taken place in 332 bc; from this sequence, it is possible to establish more or less accurate life dates of the Egyptian pharaohs. These texts are still of enormous importance today.

By these means certain objects, such as stones bearing inscriptions referring to events or rulers, or coins of the Roman and medieval periods that carry the name of a current ruler, and the archaeological context in which they are found, can be matched to a specific period. Of course, matching an object to a historical date does not necessarily date the layer in which that object was found—for example, a coin can be passed around or hoarded for decades or centuries—but it does at least give a maximum age for the layer: it cannot be older than the date on the coin (unless the coin is intrusive) but could on the other hand be much younger.

C.

Absolute Dating

C.1.

Varve Analysis and Dendrochronology

Varve analysis and dendrochronology are two methods of absolute dating that are reliable but limited because they are very localized, that is, they can only be applied to restricted geographical areas. Varves are deposits of clay that were laid down annually by melting ice sheets. They vary in thickness from year to year, a warmer year causing increased melting and hence a thicker layer. By measuring the successive thicknesses of a series of varves and comparing the sequence with the pattern in other areas, long sequences can be linked together that stretch back thousands of years.

Dendrochronology (the analysis of the annual growth-rings in trees) is based on a similar principle. A sequence of rings of varying thickness, the variation caused by local climatic fluctuations, can be built up by overlapping samples taken from trees of different ages. Unbroken sequences stretching back to 8000 bc have been established for Germany, for example, so that timbers used in ancient settlements can be compared and their age pinpointed. Results of amazing precision are now emerging. In Britain, analysis of timber from a Neolithic plank walkway in Somerset known as the Sweet Track which was constructed across a swamp, suggests that it was built during the winter of 3807/3806 bc

C.2.

Radiocarbon Dating

Radiocarbon dates are obtained by taking samples from organic materials (such as charcoal, wood, seeds, and human or animal bone) and measuring the amount of the radioactive isotope Carbon 14 (C14) that they contain. Organic materials absorb C14 throughout their lives and steadily lose it after they die. Thus the less C14 that remains in a sample, the older that sample is, and since the rate of radioactive decay can be measured, an accurate date can be given; however, radiocarbon dating is effective only for obtaining dates between about ad 600 and, with an increasing margin of error, about 50,000 bc.

Radiocarbon dating revolutionized archaeology, mainly because of its comparative accuracy and the fact that it can be applied to a range of organic materials from anywhere in the world, regardless of climate. It has established chronologies for areas that previously lacked timescales of any kind. However, the concentration of C14 in the atmosphere is now known to have varied through time, largely because of changes in the Earth’s magnetic field (see Earth: Magnetic Poles). Radiocarbon dates can be plotted against tree-ring dates to produce calibration curves, or graphs that show the changing degree of error in radiocarbon dates over time back to 7000 bc.

C.3.

Other Methods

The Potassium/Argon method, which can date rocks in volcanic areas, is useful for dating early sites, such as those in Africa (notably Olduvai Gorge) at which fossilized bones of early humans have been found. Elsewhere, uranium series dating can be applied to rocks rich in calcium carbonate, such as stalagmites in caves. Thermoluminescence (TL) dating can be used on pottery, the most abundant inorganic material found at archaeological sites of the last 10,000 years, and other inorganic materials such as burnt flint. Optically stimulated luminescence (OSL) can even be used on certain sediments containing archaeological material; deposits in northern Australian rock shelters dated by OSL to 53,000 to 60,000 years ago provide crucial evidence for the early arrival in this continent of humans who were clearly not British convicts (see Aborigines). Electron Spin Resonance (ESR) can be used on the teeth of humans and animals dating from periods far outside the range of C14. For a fuller account, see Dating Methods.