archaeology

BACKGROUND

It is increasingly difficult for prehistorians working in the twenty-first century to conceptualise the problems experienced by their predecessors, and approaches to interpretation before the 1960s are consistently criticised. Culture history and diffusionism may - with hindsight - seem excessively preoccupied with classification and social evolution, and to have applied unsophisticated historical interpretations instead of asking fundamental questions about human behaviour.

TYPOLOGY AND CROSS-DATING

It must be made clear at the outset that typology is not, strictly speaking, a dating method, but a means of placing artefacts into some kind of order. Classification divides things up for the purposes of description, whereas typology seeks to identify and analyse changes that will allow artefacts to be placed into sequences.

    * TYPOLOGY IN TEXAS ARCHEOLOGY ‘The type is the basic unit of classification in archeology. In order to establish order and to facilitate analysis, the archeologist divides his data into typological categories.’ (Ellen Sue Turner and Thomas R. Hester: Texas State Historical Association

    * LITHICS-Net Point Type Information: Projectile Point Data Indexed By Morphology (Shape). Guide for the identification of North American stone tools (Art Gumbus)

    * Stronsay flints: ‘The discovery of two tiny flint arrowheads in Stronsay could represent the earliest evidence of human activity found in Orkney – if not Scotland - to date. … Flint experts Caroline Wickham-Jones and Torbin Ballin subsequently identified them as very early forms of prehistoric arrowheads – a type derived from a classification known as Ahrensburgian, found across the plains of north western Europe.’ (Orkneyjar)

Sequence dating and seriation
These techniques both place assemblages of artefacts into relative order. Petrie used sequence dating to work back from the earliest historical phases of Egypt into pre-dynastic Neolithic times, using groups of contemporary artefacts deposited together at a single time in graves. Seriation was developed in the USA to place in order finds from strata or other kinds of assemblages such as potsherds collected from the surface of sites.

    * William Matthew Flinders Petrie (1853-1942) ‘He developed a method for establishing the historical chronology of a site based on identifying different styles of pottery. Petrie was known and respected for his belief in the importance of evidence like pot sherds for informing the archaeologist about life in the past.’ (Petrie Museum, University College London)

    * AN EXERCISE IN SERIATION DATING (PDF file) ‘This method of assigning dates to sites … is based on the fact that a cultural trait, like the type of jeans worn by teens, experiences popularity peaks, in other words, an artifact’s popularity rises to a high point and then trails off, sometimes even to extinction.’ (George Brauer)

HISTORICAL DATING

    * CAVSENNAE / CAVSENNIS Romano-British Town A range of inscriptions and documentary sources brought to bear on dating a small town at Ancaster in Lincolnshire; minor settlements of this kind rarely if ever featured in historical documents (WWW.Roman-Britain.ORG)    * The Thera (Santorini) Volcanic Eruption and the Absolute Chronology of the Aegean Bronze Age A PDF file about Sturt W. Manning’s book A test of time: the volcano of Thera and the chronology and history of the Aegean and east Mediterranean in the mid second millennium BC (Oxford: Oxbow Books, 1999), which focuses on the difficulty of reconciling scientific and historical dates.

If a context containing burnt debris and broken artefacts is excavated on a site from a historical period, it is tempting to search the local historical framework for references to warfare or a disaster in the region, and to date the excavated context accordingly.

SCIENTIFIC DATING TECHNIQUES

    * Dating rock art Superb introduction to traditional and scientific dating methods and their application. ‘The major methodological limitation in rock art studies is that art assemblages can be difficult to date. However, chronological data is crucial to many types of analysis in which rock art evidence is integrated with other archaeological and environmental information. This section will briefly survey the range of dating techniques used in contemporary rock art studies.

    These fall into two broad categories:

    a) Relative dating methods such as degree of weathering, superimposition analysis, stylistic analysis and inter-site patterning.     

    b) Absolute dating methods such as analysis on the basis of subjects depicted, consistent association with datable deposits, the dating of stratified deposits associated with rock art and the direct dating of the art itself.’ ( School of Human and Environmental Sciences, University of New England, Australia)

    * Archaeometry and Stonehenge An example of the application of modern scientific dating to a major prehistoric site (English Heritage)

    * Research Laboratory for Archaeology and the History of Art ‘The department is dedicated to the development and application of scientific methods to the study of the past.’ Lots of informative links (Oxford University)

Accurate knowledge of the age of the Earth was of little direct help to archaeologists, but it emphasised the potential of scientific dating techniques. The first half of the twentieth century witnessed similar progress that began with the dating of recent geological periods in which early hominids lived, and ended with the introduction of radiocarbon dating.

Climatostratigraphy

—– Seabed deposits

    * Temperatures from Fossil Shells ‘An example of the ingenious technical work and hard-fought debates underlying the main story is the use of fossil shells to find the temperature of oceans in the distant past.’ (American Institute of Physics)

A datable record of climatic change in relatively recent periods has been recovered from cores, up to 3 km long, extracted from the ice sheets of Greenland and elsewhere.

Varves

    * VARVES: annually-deposited sediment ‘1912 Gerard DeGeer developed the Swedish Varve Chronology, the first accurate dating of the late-glacial and Holocene.’ (Department of Geosciences, University of Arizona)

Deposits of volcanic ash encountered in stratified contexts on archaeological sites offer opportunities for dating.

Pollen (NB: pollen analysis has been superceded as a DATING method by radiocarbon since the 1950s)

    * Pollen analysis ‘Each sample can be analyzed for pollen grain and spore content, with each grain or spore being identified as the prepared slide is traversed on a mechanical stage under the high-power microscope. Then a pollen diagram, graphical expression of pollen analysis, can be constructed with consideration of sampling error.’ (MSU EMuseum)

It has been recognised since at least the fifteenth century that trees produce annual growth rings - their physiology was understood by the eighteenth century - and that they could be counted to calculate the age of a tree when it was felled. Because the thickness of these rings is affected by annual climatic factors, distinctive sequences of rings may be recognised in different samples of timber and used to establish their contemporaneity.

—– The application of tree-ring dating

    * Crossdating Tree Rings ‘You will be able to interact with this presentation, including trying skeleton plotting for yourself!’ (Laboratory of Tree-Ring Research, University of Arizona)

    * Dendrochronology (Tree-Ring Dating) of Panel Paintings ‘Many European paintings are painted on solid wooden panels or boards, typically oak for Netherlandish paintings. The wood is usually split radially so that, in ideal circumstances, a sequence of annual growth-rings from pith to sapwood is present. These sequences are then matched one against another by the dendrochronologist and compared with growth sequences whose dates are known from living trees. Absolute dates can thus be assigned to specific annual rings. Sometimes the geographic origin of a board can be determined as well.’  (detailed illustrated explanation by Peter Ian Kuniholm)

Radioactive decay

    * What is radioactive dating? Part of a clear introduction to geological dating methods from Australian Museum, Sydney

    * PRIME Lab Purdue Rare Isotope Measurement Laboratory: explanations of the principles of Accelerator mass spectrometry, and a wider look at the uses of radioacive isotopes.

Radiocarbon dating was one peaceful by-product of accelerated wartime research into atomic physics and radioactivity in the 1940s. The rate of decay of 14C, which has a half-life of 5730 (±40) years, is long enough to allow samples of carbon as old as 70,000 years to contain detectable levels of radioactive emissions, but short enough for samples from periods since the late Stone Age to be measured with reasonable precision.

Presenting and interpreting a radiocarbon date

    * CALIB Radiocarbon Calibration ‘CALIB makes the conversion from radiocarbon age to calibrated calendar years by calculating the probability distribution of the sample’s true age. Graphics and a variety of options are available through the program’s menus.’ (Minze Stuiver and Paula Reimer)

    * Why radiocarbon measurements are not true calendar ages How radiocarbon calibration works (from Radiocarbon Web-Info)

    * Der Tod startet die Stoppuhr Death starts the stop-watch: ‘Everytime a living being dies a stop-watch starts ticking. Science can read this watch and thus determine the age of a find.’ A dated but well illustrated description of radiocarbon dating (WebMuseen, Germany)

Most organic materials are suitable for dating; the lower the carbon content, the larger the sample needs to be.

—– The impact of radiocarbon dating

    * THE CONTRIBUTION OF RADIOCARBON DATING TO NEW WORLD ARCHAEOLOGY PDF file of article by R E Taylor from RADIOCARBON, Vol 42, Nr 1, 2000, p 1–21: ‘The application of the 14C method to archaeological materials is generally considered to be a watershed event in the history of archaeology and, in particular, in prehistoric studies… Perhaps the most forceful statement was the view of the late Glyn Daniel that the development of the 14C method in the 20th century should be equated with the 19th century change in the Western world view that accompanied the revelation of the great antiquity of the human species…’ (Delaware Department of Transportation Archaeology Series No. 155)

Potassium-argon is ideal for dating early hominid fossils in East Africa, for they occur in an area that was volcanically active when the fossils were deposited between one and five million years ago; pioneering results in the 1950s doubled previous estimates of their age.

Uranium series dating

    * Open University Uranium-Series Facility Information on archaeological projects (Department of Earth Sciences, The Open University)

This method involves counting microscopic tracks caused by fragments derived from fission of uranium-238 in glassy minerals, whether geological or of human manufacture. In practice the most useful samples come from zircon or obsidian, which was used extensively for making tools.

Luminescence dating

    * Forschungsstelle Archaeometrie Follow link to Luminescence (Max-Planck-Institut für Kernphysik, Heidelberg)

    * Dating and Research Projects ‘The laboratory has extensive experience of dating archaeological ceramics and burnt stones from sites in Scotland and overseas. The value of TL dating of such materials frequently lies in the association between the event being dated and an archaeologically important event in the development of the site. For example the last heating of a hearth stone dates the abandonment of a prehistoric settlement. More recently there has been a marked increase in interest in optical dating of sediments, with many groups within the Scottish Universities having application interests.’ (Scottish Universities Environmental Research Centre, Luminescence Facilities)   

    * Luminescence dating ‘Luminescence dating is a relatively new alternative approach to Quaternary chronological problems. Both quartz and feldspar rich sediments, which are otherwise undateable by conventional radiocarbon methods, can be absolutely dated within a range of 10 to 300,000+ years.’ (Sheffield Centre for International Drylands Research)

    * Palaeolithic tools from the surface of optically stimulated luminescence dated alluvial fan deposits of Pinjaur Dun in NW sub-Himalayas PDF file of a case-study: ‘We therefore need to search for new evidence that may be available for working out a true chronology of the Sohan type tools and their sites, particularly in the absence of absolutely datable material. If in some cases, the absolute age of the surface on which some stone tools are found is known, it will certainly provide us with a lower limit to the date of fabrication/use of these tools.’ (ANUJOT SINGH SONI, VIDWAN SINGH SONI, CURRENT SCIENCE, VOL. 88, NO. 6, 25 MARCH 2005)

Like thermoluminescence, ESR is a ‘trapped charge’ dating method, but it is applied to different kinds of samples, and the method of measurement is also different. ESR does not release trapped electrons, but subjects them to electromagnetic radiation in a magnetic field, which causes electrons to resonate and absorb electromagnetic power. The strength of resonance reflects the number of electrons that have become trapped since the crystals were formed.

DERIVATIVE TECHNIQUES

    * Amino Acid Racemization Dating in New Zealand: An Overview Large PDF file. AAR ‘…is used to determine relative dates of biological materials such as bone, shell and teeth and has been used in an archaeological context for over 30 years. During this time a number of significant results have been generated but many have been questioned and the technique remains controversial. In spite of this the possibility of reliable AAR dating is attractive. The technique potentially serves as an independent method for dating faunal material, which is useful in the context of providing support for chronometric information produced by other methods.’ (Judith Robins, Martin Jones and Elizabeth Matisoo-Smith, Auckland University)

Obsidian - a natural volcanic glass - was a popular alternative to flint for making flaked tools in many parts of the world. As soon as a fresh surface of obsidian is exposed, for example during the process of making it into a tool, a microscopically thin hydration rim begins to form as a result of the absorption of water.

Archaeomagnetic dating

    * Archaeomagnetism ‘Archaeological materials that contain magnetic particles are kilns, pots, hearths and most sediments. Heating and cooling such materials (or depositing in air or water in the case of sediments) causes the geomagnetic field to be recorded by the magnetic particles present. This recorded magnetisation can be measured many years later and so give a date that is directly related to anthropogenic activity. The technique can be applied in the last 3000 years in the UK, however, it is not an independent method of dating and requires a reference curve to convert the magnetic direction measured into a date.’ (Archaeomagnetic Dating Laboratory, Department of Archaeological Sciences, University of Bradford)

    * Archaeomagnetic Analysis of a Roman(?) Kiln/Drying Oven, F159, Nosterfield Report on the application of this technique to an excavated structure in Yorkshire, England: ‘’A total of 13 samples of fired stone and 1 of fired clay were removed from F159 for the purpose of archaeomagnetic analysis and dating. Specimens were oriented in situ using the button method, combined with spirit levels and a sun compass. Demagnetisation tests showed that the magnetisation in the material is highly stable. The mean archaeomagnetic vector in the samples was compared with the UK Master Curve to suggest that last firing occurred in the date range 100-170A.D.’ (© Archaeological Planning Consultancy Ltd)

When major museums buy items for their collections they become involved in expensive commercial dealings in the fine art market. The profits to be made encourage not only illicit plundering of ancient sites but skilful forgeries. Scientific dating techniques can provide reassurance; when what is needed is confirmation that an object is not a modern fake, rather than a precise date, full control of all the variables that affect accuracy is not necessary.