Chronology in Archaeology (Relative Dating & Absolute Dating)

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What is the importance of chronology in archeology? Information about the study of chronology, relative dating and absulate dating.

One aspect of archaeological work that is important in both the field and the laboratory is time. By one means or another the archaeologist attempts to discover the time relationships among the cultures or segments of cultures that he analyzes and describes. The archaeologist wants to know if one culture is older than another or if it existed at the same or a later time. That is, he is concerned to establish a relative chronology. He may also want to date a culture in years b.c. or a.d. In this case he is concerned with absolute chronology.

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Relative Dating.

The most common and basic method archaeologists use for determining relative time or age is stratigraphy. This is a method of translating space, or position, into time. In its simplest form, stratigraphy is a system of dating by layers. For example, archaeologists have excavated caves that were used as shelters by prehistoric men and have found layers of debris and earth. In the ordinary course of events, the bottom layer is the oldest and the top layer is the youngest. In one famous instance, excavators found the remains of nine cities in one mound. The seventh from the top is believed to be the city of Troy, scene of the Trojan War. The higher layers represent later cities built on the same site.

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Stratigraphy, however, is not always a matter of layers building up in a vertical column. There are a number of somewhat more complicated forms, one of which is lateral stratigraphy. To visualize lateral stratigraphy, think of a rather shallow garbage dump. Each load of garbage is dumped at the edge of the previous load so that the dump, or midden, grows laterally or sideways instead of up. The layers of this dump are nearly vertical instead of horizontal. The oldest layers are at the end of the midden where the dumping began, and the youngest layers are at the opposite end of the dump. In excavating a midden such as this it would be a great mistake to assume that the lowest-lying materials were the oldest part of the dump. An experienced archaeologist would be on the watch for such a situation and most likely would be able to recognize the lateral stratification in the cleaned profile of the wall of a test trench.

Stratigraphy can be the result of either natural forces or cultural forces or a combination of both. For instance, layers of cultural remains separated by layers of culturally sterile silty soils may represent a series of floods that drove people from their river-valley settlements at various intervals. When the flood was over the people returned and established a new settlement on top of the water-deposited soils. ®Then another flood drove them away, and so on. A similar example would be a case in which layers of cultural remains were separated by culturally sterile windblown sands. This situation would suggest settlements that were abandoned during periods of drought when the rainfall was not sufficient to maintain the vegetation growth that held the soil in place. Blowing dust and sand covered the cultural remains. Then a shift to wetter climatic conditions produced vegetation, and humans again settled in the area. Another example would consist of alternate layers of cultural remains and volcanic ash.

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A somewhat different example of natural stratigraphy is present in situations along seacoasts or large lakes where there have been changes in water level or land level. If the water has fallen in level or the land has been elevated by natural forces, there may be cultural remains associated with the fossil beaches. In such a case the oldest cultural remains are associated with the highest of the old beaches or former strand lines, and the youngest cultural remains are to be found in the youngest beaches. People kept moving down toward the water’s edge.

Belative chronologies can be derived from seriation. This is a somewhat complicated method in which a stylistic variable of a class of artifacts or a type of some sort is ordered in terms of its relative frequency in relation to its level or position within a site or in a group of sites. For example, the number of specimens of one class of pottery found in each of several sites can be a clue to which site was used first. The system has been particularly useful with surface collections obtained in regional archaeological surveys.

In certain localized but important situations the chemical analysis of bone has been used to obtain a relative chronology. The best known of such analytic procedures is the fluorine method. In instances where bones are beneath the earth and the groundwaters contain small amounts of fluorine, the fluorine ions combine with the hydroxyapatite crystals of the bone to form fluorapatite. The longer a bone is buried, the more fluorapatite it contains. Thus the relative chronology of bones in a localized area can be based on the amounts of fluorapatite they contain. So far, the greatest usefulness of this system has been in determining whether bone artifacts and human bones found with those of extinct animals are of the same age as the extinct forms or whether they represent a later intrusion into the deposit containing the remains of the extinct animals.

All the dating techniques thus far described have one thing in common: they provide only a relative chronology. Stratigraphy or seriation can show that one thing or one culture is older than another, younger, or of the same age. But these methods cannot tell how old a thing or a culture is. Some system of determining absolute time is necessary for this.

Absolute Dating.

One of the methods used for determining time according to an absolute chronology is dating by tree rings, or dendrochronology. This technique was developed by A.E. Douglass, an astronomer. Dendrochronology is not just the simple counting of growth rings in a tree stump on top of an archaeological site. It is a much more sophisticated system of dating based on the fact that trees respond to seasonal changes, to rainfall, and to drought, and that this responsiveness is manifested in the growth rings.

In wet years the growth rings are wide and in dry years the growth rings are narrow. Thus certain kinds of trees in situations where they truly and characteristically reflect patterns of climate will possess patterns of growth rings that belong only to specific periods of climatic history.

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A master tree-ring chronology is constructed by first learning the patterns of living trees of known age, say a hundred years old. Then wooden beams from old buildings such as houses built seventy-five years ago are studied in terms of their ring patterns. The growth patterns of the old beams will overlap with the beginning growth patterns of the living trees of known age. If such beams are from trees that were two hundred years old when cut, the master chronology can be extended backward in time another two hundred years. By continuing to cross-date the patterns of tree growth from more ancient samples of wood with the growth patterns of the already established sequence it is possible to establish a master chronology or calendar covering centuries. For the southwestern United States there exists such a tree-ring calendar for the years since the time of Christ. There a sample of wood from an archaeological site of unknown age may be used to date the site. The growth rings of wood from charred roof timbers found in pit houses, small logs from dry cave sites, and roof beams from old pueblos may be compared with the ring patterns of the master tree-ring calendar and dated in terms of the correspondence of patterns. By knowing the date the wood was cut for use in the archaeological site, the site itself or parts of it can be dated. Then this absolute date can be used in conjunction with relative chronologies such as have been described previously.

At the present time the most widely used and satisfactory system for obtaining absolute chronology is the radiocarbon method of dating. Developed in the late 1940’s by Willard F. Libby at the Institute for Nuclear Studies of the University of Chicago, the radiocarbon method revolutionized archaeological dating. In 1950 there was only one radiocarbon laboratory dating a few archaeological samples on an experimental basis. By the mid-1960’s hundreds of luch samples from a multitude of archaeological sites were being routinely dated each year in many different radiocarbon laboratories spread all over the world. The radiocarbon method of dating determines the age of things that lived during the past 40,000 years by measuring the amount of carbon-14 they contain and converting this amount into years. Carbon-14 is a radioactive, unstable form of carbon with an atomic weight of 14. It is being formed constantly in the earth’s upper atmosphere as the result of the bombardment of nitrogen-14 atoms by cosmic rays or neutrons. In the upper atmosphere the carbon-14 combines with oxygen to form carbon dioxide, which then becomes mixed in the earth’s atmosphere with the ordinary carbon dioxide containing carbon-12 atoms. After reaching the earth’s atmosphere the carbon-14 enters all living things, which exchange materials with the atmosphere through their life processes. All living matter contains a constant proportion of carbon-14 because of the equilibrium between the rate of formation of carbon-14 and the rate of disintegration of the carbon-14 contained in the atmosphere, the ocean, and all living things.

When any living thing dies, it ceases to be in exchange with the atmosphere and thus ceases to take in carbon-14. However, the carbon-14 contained at death continues to disintegrate at a constant rate. The half-life of carbon-14 is 5,568 years. This means that the amount of carbon-14 at the time of death is reduced to half that amount in the first 5,568 years. The remaining amount of carbon-14 is reduced by half in the second 5,568 years, and so on, so that the amount of carbon-14 remaining at a given time is proportional to the time elapsed since death. Thus by knowing the carbon-14 content of living matter and the rate at which carbon-14 disintegrates, it is possible to ascertain the elapsed time since the death of a specimen of formerly living matter.

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The archaeological materials that can be dated by the radiocarbon method are wood, charcoal, all kinds of plant materials, antler, burned bone, fur, skin, hair, shell, peat, dung, and many other organic substances. Bone that has not been burned is unreliable because it is easily contaminated by chemical alteration. Bone in general seems to register dates that are too young, whereas shells of various kinds give dates that arc too old. Charcoal, however, is virtually perfect for dating purposes. Bits of charcoal from old hearths or from structural timbers or poles are frequently found in archaeological sites and at different stratigraphie levels.

Samples of the charcoal—or any other organic substance that is to be dated—are carefully collected and sent to radiocarbon laboratories. There the sample is prepared chemically for the measurement of the carbon-14 content. Unfortunately this chemical preparation destroys the sample for archaeological purposes other than dating; hence rare artifacts of wood and other organic materials usually are not dated. Fortunately, however, there are usually other sources of organic materials from a given archaeological site. Thus the problem of choosing whether to get a date or to preserve a rare or artistic artifact does not often arise. When the sample to be dated has been properly prepared in the radiocarbon laboratory it is placed in a specially constructed, exceedingly sensitive radiation counter that is something like a Geiger counter. Such a counter measures the amount of carbon-14 in the sample by registering the number of carbon-14 disintegrations over a period of time. The longer the run of the counter the greater the accuracy of the measurement.

The final result is a radiocarbon date, which is expressed, for example, as a.d. 850 plus or minus 30 years (often abbreviated 850±30). These plus-or-minus years attached to the date are a way of stating the probable error of the dating method. In the example given above there are 66 chances in 100 that the true date of the sample is within 30 years one side or the other of a.d. 850—that -is, between 820 and 880. By doubling this particular error, the probability would be increased to 96 chances out of 100 that the true date of the sample was within sixty years (plus or minus) of a.d. 850.

Although radiocarbon dating dominates the archaeological scene today, other systems of determining absolute chronology are useful in particular situations. Among these methods is glacial varve chronology. Glacial varves are thin layers of clays deposited annually in basins of meltwater by the retreating ice of continental glaciers. Cultural remains associated with late-glacial and postglacial features, which are in a fixed position in relation to the varve count, can be dated by this method. However, this system has not worked well except in the Baltic region of northern Europe.

The potassium-argon method is most useful in dealing with dating problems of times so remote that they are beyond the range of radiocarbon measurements. The method is based on the radioactive decay of potassium 40 into calcium 40 and argon 40 and utilizes known proportions in terms of known rates of change. A notable application of this method was the dating of the geological context in Africa’s Olduvai Gorge, where L.S.B. Leakey found fossil remains of Zinjanthropus and Homo habilis. The’ layer containing these remains was found to be 1,750,000 years old by scientists at the University of California, Berkeley.

Although it had been under development for some years, the thermoluminescence technique for dating pottery achieved the required standard of accuracy only as recently as 1965. The technique depends on the fact that radioactive elements (primarily thorium and uranium) in clay bombard other substances in the clay and raise electrons to unstable levels. When the clay is fired in the kiln, each electron falls back to its stable position and emits a photon of light. If a fragment of ancient pottery is reheated in the laboratory, small amounts of light are given off. The amount of thermoluminescence indicates how much radiation damage each electron has received. Hence the amount of thermoluminescence is a measure of the time that has elapsed since the pottery was first fired. New accuracy in use of this technique is the result of refinements developed at the University Museum, University of Pennsylvania. The improved method consists of bombarding pottery to be analyzed with X-rays, and of using a series of samples from one small piece of pottery for each assessment of age.

In colonial and historic archaeology the dating of undocumented sites or parts of sites is complicated by lack of depth of time. Often, not enough time has elapsed for observable stratification to occur. For such sites it is possible to establish chronology based on dated historic materials of known style and manufacture. Pipes, glass beads, silver ornaments, buttons, and iron knives are among the objects that have been used to construct chronologies of this sort. When objects that have been dated in historic time overlap in one site with older artifacts, it is possible to link history with prehistory.

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One highly specialized method of dating in terms of absolute chronology is ideally suited to colonial and historical archaeology as well as to classical or other fields in which artifacts of glass are present. Glass that has been under water or buried in the ground for a long time becomes encrusted with variegated scales built up in iridescent layers. Although the destructive process causing these weathering crusts is not known exactly, scientists at the Corning Museum of Glass discovered that the number of decomposition layers on a given piece of glass is equal to the number of years that piece of glass has been submerged in water or buried in the earth. By counting the crust layers, which can be seen with a microscope, it is possible to tell how long an artifact of glass has been buried or submerged and thus to determine the date of the burial or submergence. For historic sites on land or underwater sites of shipwrecks that are only a few hundreds of years old, the method of dating by glass can scarcely be surpassed.

An unusual method of determining absolute time has been used with success on islands off the coast of Peru where artifacts were found in position in deep deposits of guano, or dung. Research indicated that the guano was deposited in annual layers that could be counted. Thus an artifact found at a depth of 1,000 layers beneath the fresh surface of a deposit of guano would be 1,000 years old.

There are a number of almost equally exotic ways in which chronology can be determined. Moreover, new concepts and methods of archaeological dating are being developed. The foregoing review, however, suffices to show that archaeologists use a great variety of dating methods and that they get help from chemists, botanists, geologists, and many other scientists.

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