Chronology Introduction The emergence of man through the process of biological and cultural evolution is a story of long span of time. For the archaeologist and the prehistorian who deals with that long history of man, time is the most important consideration. The sequence of development of culture or the relationship between events that represent culture can be established only when events can be placed in proper time. Chronology, the study of events in time frame, is hence the central theme of archaeologist, like the geologist who deals with the story of earth history. In fact, chronology is one of the most fundamental issues in and perhaps a characteristic of archaeology. Archaeologists use several methods to assign ages to events of the past. They are engaged in defining the stages of hominid evolution and their artifactual record, and the assignment of a chronology to these stages. Definition of Chronology Chronology is the science of measuring time and ordering of the things in time. According to the Dictionary of Anthropology, the word chronology means the science of computing dates or treaties showing arrangement of events with dates. In other words, Chronology is the arrangement of events, or the materials which represent them, in the order of their occurrence in time. Any study of the origins of human must be set in a chronological context. Dating Methods for Building Chronology A very common question in prehistory is that How old is it? or is it older than or younger than? Prehistorian has been trying to solve such questions by applying several methods of dating. There are specific methods used in each site in determining the chronology. In applying the dating methods in a particular site, it is important to consider a number of factors ; such as i) the nature and availability of sample, ii) the purposes iii) limitations and iv) the kind of problem encountered by the site etc. 1
Types of chronology There are mainly two types of dating; i) relative and ii) absolute or chronometric dating. Relative dating fixes a time frame in relation to other strata or material and not in absolute dates in numbers. It can only define the antiquity in terms of older or younger than something else and makes it possible to arrange a series of things in proper chronological order. But it is difficult to know the total time span involved in the intervals between the things. On the other hand the absolute dating technique exhibit chronology in terms of years. It offers precise and accurate dating. There may be marginal errors which are almost negligible. Some of the common and widely applied absolute dating methods are Carbon-14, Potassium-Argon or K-A40, Thermoluminesence or TL. Dendrochronology etc. In the early stage of prehistoric studies there was only relative chronology. But in the last fifty years, with the emergence of C14 method there has been total change in dating scenario. Relative dating, however, is still applied in those areas where the deposits containing organic materials cannot be dated by any absolute dating techniques. Under relative dating there are different methods such as stratigraphy, typology, Fluorine analysis, Palynology, Palaeontology, Patination etc. Stratigraphy The simple meaning of stratigraphy is the study of strata or layers. Such layers are formed either naturally or due to human occupation in a particular area. The former is called geological stratigraphy and the latter as archaeological stratigraphy. Both are important to derive relative date of a particular site. Use of stratigraphy is the most important method for establishing relative dating. The method depends on the basic geological principle that the earlier deposits lie under later deposits. The Stratigraphic layers in an archaeological site represent culture in the form of deposits of artifacts as well as ecofacts. By analyzing a series of layers in a site that exist in the horizontal dimension a relative vertical time dimension can be derived. The kinds of chronology one can derive through stratigraphy infer a culture in terms of older or younger than another one. But, it cannot ascertain the exact date for any of this culture. However, the basic stratigraphic associations of artifact types within and between archaeological sites are still very important for building chronology. Typology In this method, artifacts are classified according to their form or shape and their relative antiquity, based on the presumsion that crude, poorly preserved tools are earlier than refined and well preserved tools. For example Neolithic tools were more developed than the Palaeolithic tools. Likewise, upper Palaeolithic blade tools were more evolved than the lower Palaeolithic core tools. But this method is not a reliable one, as it cannot give fruitful results universally. But the typological classification when applied locally and in corroboration with stratigraphic method, serves as a very useful tool to understand the different forms and traditions in the manufacture of different classes of artifacts. It also helps in establishing comparison and links between distantly placed cultural remains. For example, the 2
close similarity between hand axes and cleavers of South India and South Africa or the Neolithic shouldered axes between South East Asia and Eastern India would open up possibilities of ethnographic links and even prehistoric migration. Fluorine Analysis Fluorine Analysis discloses the percentage of fluoro-apatite in bone specimens found in archaeological sites. This analysis is based on the principle that the longer a bone will be placed in soil, the more fluorine will be caught in and hence can suggest a relative date. All bones whether of animal or of human lying in the same level exhibit similar fluorine percentage in them. Therefore, if the quantity of fluoro- apatite remains same in both kinds of bone, it is sure that they belong to the same age. The bones acquired from a lower level show more fluorine in them whereas the bones coming from the upper level contain less fluorine Fluorine test is generally applied to the mixed deposit of human and animal bones. The method cannot provide an absolute chronological age because the amount of fluorine differs from soil to soil, which gives a differential rate of absorption. But the method has been found quite suitable for the relative dating of bone materials within a particular site. A similar type of technique is the analysis of phosphorous concentration, which works nicely in relation to lake-soil deposit. Patination There is no precise definition for the term patination though it generally means chemical alterations of rock surfaces exposed to atmospheric conditions. The amount of patina on the stone is an index of its age and is valuable for relative placement of the stone artifact in the technological development. The chemical alterations of the stone are usually brought about by the action of iron oxides through time. The observation of the amount of patina on a stone may be of use at sites where there is a long sequence and demonstrates that those tools which lie in the bottom level may have more patina than those in the upper levels. The different types of tools from the river gravel, terraces of rivers or lakes can be differentiated in the relative amounts of patina on the basis of which the relative ages can be assigned on the artifacts. A.J.H. Goodwin who worked extensively on patination in 1960 lists many variables involved in patina formation as well as different types of patination. This method can be used fruitfully for the tools from stratified deposits. Pollen analysis or palynology Pollen grains which are produced in large quantities by plants are almost indestructible and can be identified by their shape when seen through microscope. In a wet site like peat bog or a buried surface, pollen grains are well preserved. Palynologists are able to extract pollen from various strata of a deposit and to examine them under a microscope and identify the genus and even the species to which they belong from their shape, structure and other factors. From this they can determine the kind of vegetation and the climate which correspond with each stratum. Pollen analysis can be very useful not only for relative dating but also for giving absolute dating. Intensive studies of pollen are being made in the United States and Canada. The change 3
in composition of woodland of northern Europe since the last retract of ice has been closely studied and documented by pollen analysis. Palaeontology The bone remains in fossil form found in archaeological sites provides one of the basis for relative chronology. The method is applicable for the areas where change in climate cause migration or extinction as well as brings different animals and plants into a region or when certain species of animals have become extinct since man appeared. Taking these two factors into account one may use palaeontology to establish relative dates. Thus one can assume a temperate climate if such species as Elephas antiquus (forest elephant) are present, whereas Elephas primigenius (a steppe elephant) indicates a steppe or tundra environment of almost glacial conditions. In France an alteration of the forest and steppe varieties of reindeer suggests the alteration of warm and cold stages during the final glaciations. In North America a series of animals like mammoth, horse, camel, bison became extinct after the arrival of man. Although the exact dates for the final extinction of those species are not yet known within a margin of error of perhaps thousand years it is possible to say that man associated with mammoth remains lived before 6000 B.C. Absolute Dating : Carbon (C-14 ) Method Radiocarbon dating is a chemical analysis used to determine the age of organic materials based on their content of the radioisotope of carbon 14. The method was developed in the 1940's by Willard F. Libby and a team of scientists at the University of Chicago. In 1960 Libby received the Nobel Prize in Chemistry for his method to use Carbon-14 for age determinations in archaeology, geology, geophysics, and other branches of science. It subsequently evolved into the most powerful method of dating late Pleistocene and Holocene artefacts and geologic events up to about 50,000 years. In essence, radiocarbon dating is based on the fact that neutrons produced by cosmic radiation enter the earth s atmosphere and react with the nitrogen Isotope N 14. The reaction produces a heavy Isotope of Carbon C-14 which is radioactive and has a half-life of 5730 years. W.F. Libby s equation describing the reaction is N 1 4 + n = C 14 + H. Chemically, C-14 seems to behave exactly as ordinary non radioactive carbon C-12 does. Thus the C-14 atoms readily mix with the oxygen in the earth s atmosphere, together with C-12, and eventually enter into all living things as part of the normal oxygen-exchange process that involves all living things and animals. As long as matter is living and hence in exchange with the atmosphere, it continues to received C-12 and C-14 atoms in a constant proportion. After death the organism is no longer in exchange with the atmosphere and no longer absorbs atoms of contemporary carbon. After the death of an organism the C-14 contained in its physical structure begins to disintegrate at the rate of radiocarbon remaining, one can establish the time when the plant or animal died. Half-life (t 1/2) is measured by counting the number beta radiations emitted per minute (CPM) per gram of material. By radiocarbon method one can date many different types of (organic or inorganic) materials as long as they consist of carbon. The datable organic materials are charcoal, pollen, 4
woods, twigs, seeds, bones, antler, shells, fish remains, insect remains etc. The non organic materials are sediments and soils, ice core, metal casting ore, underground water, pigment from wall painting and work arts etc. The advantages of the C-14 method are: the abundance of datable material, low cost, readily-accessible facilities etc. and the disadvantages are: the materials dated by C-14 typically quite porous (easily contaminated by old/young carbon), must be very careful with pretreatments and sample selection. The radiocarbon dates are expressed in BP, and are accompanied by a statistical margin of error which corresponds to the measurement of a random phenomenon. The calibration is necessary to transform these results into calendar dates. Radiocarbon (C-14) dating is the most widely accepted technique for studying the chronological relationships of archaeological complexes. Using the radiocarbon method as a source of objective information, archaeologists are able to build Stone Age chronologies as well as establish the primary Chrono-cultural boundaries. Potassium Argon Dating This method is similar to carbon dating. The earth s crust contains potassium of which Isotope K40 decays to A40 at a known rate. The ratio of Potassium Argon may be measured to ascertain the date of minerals and rocks in a deposited. This method is able to cover a wide range of time even far greater than C-14 method. Because of the half-life of the radioactive potassium is 1330 million years. The method has proved quite useful in dating some hominid fossils as employed in the site of Olduvai Gorge in East Africa where the remains were as old as 1.75 million years. The advantage of the method is that it works well in case of the sites which are 500,000 years old. But the disadvantage of the method is that it can be applied only to those rocks and minerals which are rich in potassium. Since those rocks are available only in volcanic areas, the method is restricted to those areas. Thermoluminescence Dating The use of thermoluminescence for dating purposes was developed in the 1960s mainly at the Oxford Research Laboratory for Archaeology and the History of Art by M. Aitken and co-workers. Initially designed to date archaeological ceramics, it was subsequently extended to other mineral materials, such as burnt flint. The method was based on the fact that objects such as pottery that have been heated in the past could be dated by measurement of their Thermoluminescent (TL) glow. Thermoluminesence is the emitted light from pottery, which can be measured. If the ground up pottery is reheated, it emits light. The phenomenon results from radioactive influence of the metallic elements like uranium and potassium present in the clay and surrounding soil. By the use of TL dating methods and the results obtained it is possible to provide a new chronological framework for archaeological and anthropological knowledge. For example, the new chronology based on TL dating enable in revising some prior assumptions about the evolution of Lithic industries and the nature of hominids present in the Near East at various stages of the Middle Palaeolithic. Dendrochronology 5
Dendrochronology is a very important absolute dating method and was first developed by Dr. A. E. Douglass of Arizona University. The age of wooden objects can be determined by means of Dendrochronology or tree ring analysis. The method is based on the fact that every year trees form an annual growth ring which can be counted. It is possible to determine the calendar years of tree-ring formation and the felling dates of trees, which helps to determine the age of wooden objects with a precision that has not been matched by any other method. Dendrochronology has therefore become well established in the field of archaeology, art history and cultural heritage. The recent development of Dendrochronology related to archaeology and art history has provided a reliable framework for chronometric dating and reconstruction of past climate and environment. This dating method with latest methodological advances helps us in defining the calendar year in which the tree-rings were formed and in interpreting such dating in terms of the age of a wooden object. The major drawback of this method is that it has limited application as it helps in dating of wooden object which are only few thousand years old. However, it has been best applied in south western United States. Besides, all these dating techniques, there are several other methods such as optical- and infrared-stimulated luminescence (OSL) dating and electron spin resonance (ESR) dating of tooth enamel of larger mammals, which are also of equal importance. In recent years, the application of absolute dating methods such as Thorium/Uranium, Uranium-series has become increasingly useful in constructing a chronological framework for many parts of the world. *** 6