Relative Dating Introduction: In the early stage of prehistoric studies, dating of any event or site was obtained tentatively. A particular event or specimen is dated in relation to other event or some reference point. Such type of dating techniques are known as relative dating by which one can know or understand whether a particular culture is younger or older than the other one. That is, through relative dating, it is known that culture A is older than culture B but how old A and B are, or how much later B is than A in terms of number of years is unknown. These dating methods have never been able to provide a date in terms of numerical value or years, nor it can calculate the total time span involved in each cultural period. However, with the help of such dating methods a series of events or things can be arranged in a sequential time frame. The relative chronology, in the words of Wheeler (1956), is... the arrangement of the products of non-historic societies into a time relationship which may not have any dates but which has a sequence... Types of Relative Dating: Till the early part of 19 th century quite a good number of relative dating methods have been in used in archaeological studies. These relative dating methods are basically depending upon stratigraphic position of the site or kind of remains associated with the site. Some of the relative dating methods are: 1
i) Stratigraphy ii) Typology iii) Fluorine test iv) Uranium and nitrogen analysis v) Palaeontology vi) Palynology vii) Cross dating viii) Sequence dating All these relative dating methods tend to compare a given event with some other events in a time sequence. Further, it is difficult to believe that all these events occurred everywhere simultaneously. Hence exact contemporaneity with similarly dated objects from other parts of the world can be implied. Stratigraphy : Meaning - Use of stratigraphy is the most important method for establishing relative dating. The simple meaning of stratigraphy is the science or study of rock strata or layers. It is concerned with all characters and attributes of rocks as strata and their interpretation in terms of mode of origin and geologic history. All classes of rocks that are consolidated or unconsolidated fall within the general scope of stratigraphy. Hence it is the analysis of a series of layers that exist in a site in the horizontal dimension, through which the vertical time dimension can be read out. 2
Principle - The method depends on the geological law of superposition. According to this law in an undisturbed stratification of deposited material the lower strata are successively older. That is, the top level is of more recent formation and the lowest level is the oldest. Therefore, whenever a stratigraphic sequence is observed during the excavation of a site, relative ages of the cultural levels can be worked out. The stratigraphic associations of artifact types within and between archaeological sites are regarded as one of the very important method of relative dating. Good stratigraphic excavation at an archaeological site is designed to obtain such a sequence. Part of this work involves detecting whether there has been any human or natural disturbance of the layers since they were originally deposited. Here the cultural formation and natural formation processes play a very important role. For instance, rubbish pits dug down by later occupants of a site into earlier layers, animals burrowing holes and floods washing layers away and re-depositing them elsewhere in secondary context. Armed with carefully observed stratigraphic information, the archaeologist can hope to reconstruct a reliable relative chronological sequence for the deposition of different layers or strata. But of course what we mostly want to date are not so much the layers or deposits themselves as the humanly generated materials within them artifacts, structures, organic remains which ultimately (when 3
systematically studied) reveal past human activities at the site. Here the idea of association is important. When we say that two objects were found in association within the same archaeological deposit, we generally mean that they became buried at the same time. Provided, that deposit is a sealed one, without stratigraphic intrusions from another deposit itself. A sequence of sealed deposits thus gives a sequence and relative chronology for time of burial of objects found associated in those deposits. This is a crucial concept to grasp, because if one of those objects can later be given an absolute date say a piece of charcoal can be dated by C 14 in the laboratory then it is possible to assign that absolute date not only to the charcoal but to the sealed deposit and other objects associated with it as well. Formation of strata - Stratigraphic layer in an archaeological site represents culture in the form of deposits of artifacts as well as eco-facts. 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. The formation of strata either geologically or archaeologically is a long process. Prehistoric people usually occupy open air site and rock shelter. The first comer will be making their home on the natural floor of the rock shelter, but as time passes and one generation follows another, the floor level will become covered with materials such as mud, dirt and debris thrown by the occupant. The descendents or successor of the original settlers or new comers will be living on the top of this deposit and adding in their turn a new layer of the accumulating debris. In this way when a site is 4
occupied continuously by a number of generations, there is every possibility of formation of different layers one after another. The process continued until the floor level stood much higher than it was at beginning and some time it become so high that rock shelter become no longer suitable for habitation and finally abandoned. When climate changes and situation of the hearth is being altered and so on, it frequently happens that the new layers may have a different consistency, or be made of different materials or of different colour from the earlier one. It helps the excavator to mark the layers distinctly. If an excavator cuts section of the rock shelter showing all the layers from top to bottom, he can apply the law of superposition. Accordingly, the bottom layer will be the oldest, the middle layer intermediate and the top layer the most recent. Prehistoric man frequently dropped his artifacts and these become incorporated in various strata. Artifacts collected from the strata also give clue to the industry of the stratum. Thus complete excavations of a site enable the prehistorian to determine the sequence of the strata and the sequence of the industries. When a number of excavations are carried out in a site it enables one to characterize the assemblage of industries of successive dates. He can also determine the sequence of strata. The sequence of culture can be determined by comparing the results of number of separate excavation. Natural Disaster such as earthquakes or volcanic action usually does not disturb the earth s strata. Artifacts assimilated in the strata do not get 5
displaced, due to such natural movements of earth. Prehistorians are not concern with such disasters. As these are not disturbing them, but the prehistorian are really concerns with the problem arising through burrowing animals such as rabbits. Rabbits usually drive their burrows through the wall of the rock shelter. As such artifacts present in the rock walls gets displaced and the industries consequently gets mixed. Application - Most of the archeological sites have been dated by using stratigraphical method. One such example is the identification of existence and position of Mesolithic phase. Until 1865, the Mesolithic was not identified as a distinct cultural stage in man s history. The Mesolithic gained recognition as a separate cultural identity only after the discovery of the type site at Mas-de-Azil, a cave in France near the famous Magdalenian site. Excavation at Mas-de-Azil showed that Mesolithic tools occurred over a sterile layer containing rich Magdalenian (Upper Paleolithic) implements. Above the Mesolithic tools, a separate layer was found which contained Neolithic tools. So, the existence of a transitional phase in between earlier upper Paleolithic and later Neolithic phase was realized. 6
Sratigraphic method can also be applied in open air sites. Due to successive occupation of these areas, layers may form one after another and there some mound may be formed. Excavating such mounds reveals stratigraphic sequence and according to geological law of superposition, chronological sequence can be worked out. In prehistoric sites like Nevasa in Maharashtra, Lothal in Gujarat, excavation of mounds exposed the cultural levels ranging from 3000 B. C. to historical period. Excavation of river terraces also reveal succession of cultures in stratified context. In India, terraces of Narmada, Godavari, Malaprabha, Ghataprabha are found very potent in having cultural deposits of Indian stone age. Demerits - The kinds of chronology one can derive through stratigraphy infer a culture in terms of older or younger than other one. But, it cannot ascertain the exact date for any of these cultures. Secondly, it is not possible to obtain stratigraphic sequence everywhere; therefore its applicability is limited. Stratigraphic sequence in many places can be exposed only by applying excavation technique, which is very costly and time consuming. Merits - i) It gives the proper culture sequence. ii) It helps to understand the industries of different cultures. 7
Fluorine Test : Fluorine Test, another method of relative dating was first devised by a Frenchman A. Carnot in 1893, but later on it was perfected by Dr. Kenneth P. Oakley. Fluorine is a non metallic element. It is pale yellow in colour and is a highly reactive gas. Fluorine test is not isotopic and it does not measure elapsed time. It depends on the fact that bone and tooth (calcium phosphate) absorbs fluorine dissolved in percolating water and forms fluorapatite compounds. At any given site, bones (human and animals) of a particular age will yield the same percentage of fluorapatite. Prehistoric sites often consist of many bone remains. Fluorine Analysis discloses the percentage of fluorapatite in bone specimens found in archaeological sites. Principle - The basic principle of the method is 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 fluorine 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 bone remains coming from the upper level contains less fluorine. Relative ages of different bones at the same site thus can be established by measuring their fluorine contents. 8
Application - Fluorine test is generally applied to the mixed deposit of human and animal bones. The method cannot calculate an absolute chronological age because the amount of fluorine differs from soil to soil. The amount of fluorine that gets into a bone depends upon how much is present in the ground water, and this varies a great deal from place to place, which gives a differential rate of absorption. The useful method of assessing whether several bones found in association in the same stratigraphic deposit are in fact of the same relative age is chemical dating by studying fluorine, uranium, and nitrogen content. But the methods have been found quite suitable for the relative dating of bone materials within a particular site. For example, fluorine test played a major role in exposing the famous Piltdown forgery by demonstrating that the jaw of alleged early specimen of man was of modern age. Here we may recall the words of Colin Renfrew and Paul Bahn The most famous application of the method was in the case of Piltdown forgery. In the early 1900s pieces of human skull, an ape-like jawbone, and some teeth were found in a lower Palaeolithic gravel pit in Sussex, Southern England. The discoveries led to claims that the missing link between apes and human had been found. Piltdown man (Eoanthropus dawsoni) had an important place in the textbook until 1953, when it was exposed as a complete hoax. Fluorine, uranium and nitrogen dating at the British Museum (Natural 9
History) showed that the skull was human but of relatively recent age (It was subsequently dated at about 620 years old); the jawbone came from an Orangutan and was a modern plant. Both the skull and jawbone had been treated with pigment (Potassium dichromate) to make them look old and associated (Renfrew, Colin & Bahn, Paul 1991:104). A similar type of technique is the analysis of phosphorous concentration, which works nicely in relation to lack of soil deposit. Like fluorine, uranium or nitrogen content of the bones also can be measured. Since fluorine and uranium levels in the bone increase with time, while nitrogen decreases, such measurements helps to place in sequence the cultural phases with which the different bones may be associated. In fact the analysis of fluorine, uranium or nitrogen is regarded as one of the very important techniques for relative dating. *** 10