Dating the Past

1. BACKGROUND

It is 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.

2. 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)

2.1. 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)

* SERIATION APPLET 'The purpose of this Java program is to allow you to try your hand at determining the proper chronological order the sites should be in according to their seriation.' (MSU EMuseum)

3. HISTORICAL DATING

Prehistorians sometimes overestimate the accuracy and detail of frameworks based on historical evidence; in practice, early written sources may provide little more information than a scatter of radiocarbon dates. The extent of documentation varied considerably in 'historical' cultures and the information that survives is determined by a variety of factors.

* 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.

3.1. Applying historical dates to sites

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.

* In Vesuvius' Shadow New excavation at the most famous ancient site and dated by a historically attested destruction in AD 79: 'Although the Anglo-American Project is interested in the conditions of city life in A.D. 79, the year Vesuvius erupted, we are investigating below the destruction level to understand the whole history of activity and development in VI,1--from its fourth-century B.C. huts to its burial in the late first century A.D.' (Archaeology Magazine).

* Head Street Excavations Excavation in Colchester, England, where traces of destruction dated by the Roman historian Tacitus have been revealed: 'Whenever a Roman house is discovered in Colchester dating to the AD 50s, it almost invariably turns out to have been destroyed by fire. And so it proved at Head Street, where the latest excavation revealed yet another Roman house which was destroyed during the Boudican revolt. The pattern is so consistent that it seems that when Boudica and her followers put the Roman town to the torch during the famous British revolt of AD 60 or 61, the destruction must have been total - every building was burnt.' (Colchester Archaeological Trust).

4.4. SCIENTIFIC DATING TECHNIQUES

The transformation of archaeological dating that began around 1950 continues, but archaeologists may overlook the revolution in scientific dating that had already taken place in geology during the first half of the twentieth century; from this wider perspective, the emergence of radiocarbon dating may seem slightly less dramatic

* 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)

4.1. Geological time-scales

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.

* Tour of geologic time 'Here you can journey through the history of the Earth, with stops at particular points in time to examine the fossil record and stratigraphy.' (University of California Museum of Paleontology)

* RADIOMETRIC TIME SCALE 'The discovery of the natural radioactive decay of uranium in 1896 by Henry Becquerel, the French physicist, opened new vistas in science. In 1905, the British physicist Lord Rutherford--after defining the structure of the atom-- made the first clear suggestion for using radioactivity as a tool for measuring geologic time directly...' (U.S. Geological Survey)

4.2. Climatostratigraphy

While some geologists concentrated on the age of the Earth, others studied distinctive surface traces left behind by changes in the extent of polar ice during the most recent (Quaternary) geological period. They identified a succession of Ice Ages alternating with temperate conditions (glacials and interglacials) which - if they could be dated - would reveal much about the evolution of early humans in the context of changing environmental conditions.

----- Seabed deposits

Cores extracted from ocean floor deposits reveal variations in oxygen isotopes in the shells and skeletal material of dead marine creatures, which reflect fluctuations in global temperature and the volume of the ocean.

* 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)

----- Ice cores

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.

* Holocene Variability from ARCSS/GISP2 compared to other Paleo-Proxy Records (Greenland Ice Sheet Project, University of New Hampshire)

* Mount St. Helens volcano A typical volcano that has a long history of eruptions that can influence short-term episodes of climate change detectable in ice-core records (Volcano World)

4.3. Varves

Sections cut through lake beds in glacial regions reveal a regular annual pattern of coarse and fine layers, known as varves. Variations in climate produced observable differences in the thickness of sediments, and, like the patterns of variation in tree rings, this allows matches to be made between deposits in separate lake beds.

* 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)

Tephrochronology

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

* Tephrochronology Group 'The correlation and geochemical analysis of volcanic ash deposits (tephra) allows the identification and dating of isochronous marker horizons within sediment sequences. Tephrochronology thus provides a precise and well-established dating tool, already widely used in the study of Quaternary environmental stratigraphies.' (Research Laboratory for Archaeology and the History of Art, Oxford University)
* More on Kostenki: How Old is that Tephra? Discussion about dating of a prehistoric site in Russia where volcanic deposits originating in an eruption in Italy were encountered (K. Kris Hirst's Archaeology Blog)

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

Microscopic wind-blown pollen grains survive well in many soil conditions, and pollen that has accumulated in deep deposits - such as peat-bogs - can provide a long-term record of changes in vegetation; suitable samples may be collected from soils exposed by excavation, or from cores extracted from bogs.

* 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)

4.5. Dendrochronology

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.

* Wiener Laboratory for Aegean and Near Eastern Dendrochronology This site includes explanations of dendrochronology and links to research projects (Cornell University)

* Ultimate Tree-Ring Web Pages 'My goal is to make available as much information about dendrochronology as I can possibly find on the Internet, from the basics of tree-ring dating, to reference and bibliographic information, to products and supplies, to books, and more!' (Henri D. Grissino-Mayer, University of Tennessee)

* Sheffield Dendrochronology Laboratory '...has existed for over 25 years. The range of sites and types of material examined has enabled its personnel to develop high standards of expertise and professionalism, and to gain international recognition.' Informative links to methods and research projects (University of Sheffield)

* Laboratoire Romand de Dendrochronologie Beautifully illustrated Swiss site (in French) with explanatory photographs that speak for themselves.

----- The application of tree-ring dating

Unfortunately there are many problems in the direct application of dendrochronological dating. Not all tree species are sufficiently sensitive to display distinctive variations in their ring characteristics, particularly when growing in temperate climates. Wood only survives under exceptionally wet or dry conditions, and large timbers must be recovered to provide sufficient rings for valid comparisons because they rely on patterns that accumulated over several decades.

* 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)

5. ABSOLUTE TECHNIQUES

5.1. Radioactive decay

The successful development in the early twentieth century of radiometric methods relying upon radioactive decay for dating geological periods offered hope that a similar technique might be found to give absolute dates for prehistoric archaeology.

* 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.

5.2. Radiocarbon dating

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.

* Radiocarbon This is the principal periodical for C14 dating - follow the links to further information and individual laboratories.

* Oxford University RLAHA Radiocarbon Accelerator Unit

* Beta Analytic 'World's Largest Radiocarbon Dating Service; AMS, Mass Spectrometry, C14, and more...' Lots of informative links from the homepage of this commercial radiocarbon dating service.

* Radiocarbon WEB-info Detailed explanations of the technique from University of Waikato radiocarbon lab, New Zealand

5.3. Presenting and interpreting a radiocarbon date

Because interpretation is so complex, all radiocarbon dates included in an archaeological publication must be presented in a standard format.

* 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)

----- Radiocarbon samples

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

* The Prehistory of Lums Pond: The Formation of an Archaeological Site in Delaware - Vol. II: X. RADIOCARBON ANALYSIS 'Methods: Collection and Processing: Radiocarbon samples were recovered from a variety of proveniences across the site. These included charcoal from concentrations within features; dispersed charcoal from arbitrary levels within features; dispersed charcoal from arbitrary stratigraphic levels not associated with features; and bulk soil samples from stratigraphic levels.' (Delaware Department of Transportation Archaeology Series No. 155)

----- The impact of radiocarbon dating

Radiocarbon dating has grown exponentially, and many problems and inaccuracies have been isolated and examined, some leading to major adjustments of the results. Without doubt, it has made the greatest single contribution to the development of archaeology since geologists and prehistorians escaped from the constraints of historical chronology in the nineteenth century.

* 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)

5.4. Potassium-argon (40K/40Ar) and argon-argon dating (40Ar/39Ar)

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.

* Chronological Methods 9 - Potassium-Argon Dating 'The Potassium-Argon dating method is an invaluable tool for those archaeologists and paleoanthropologists studying the earliest evidence for human evolution.' Clear introduction from course materials produced by Brian M. Fagan (University of California Santa Barbara)

5.5. Uranium series dating

The dating of rocks back to the Pre-Cambrian by measuring the proportions of uranium to lead or uranium to helium was possible because isotopes of uranium remain radioactive for such a long period.

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

5.6. Fission-track dating

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.

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

5.7. Luminescence dating

The physical phenomenon of luminescence may be used to date artefacts that were made from (or include) crystalline minerals which have been subjected to strong heating. The first successful application was to clay fired to make pottery, but it is commonly used now for dating flint tools that have been burnt, for example by being dropped accidentally into a fire.

* 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)

5.8. Electron spin resonance (ESR)

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.

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

* Professor Rainer Grün: abstracts Insights into the application of ESR dating in archaeology can be gained from these summaries of works by a leading exponent; PDF files of some are available from his list of publications. (Research School of Earth Sciences at the Australian National University)

6. DERIVATIVE TECHNIQUES

Derivative methods may only be used for dating if their results can be related to a time-scale or reference curve that has been established by absolute dating methods. If it is not affected in any way by its environment the result can be described as absolute. In contrast, dating the change of one form of amino acid to another is derivative because the rate of alteration varies, and is heavily dependent on the temperature and humidity of the context where the sample has been buried.

6.1. Protein and amino acid diagenesis dating

Bones, teeth and shells contain proteins that break down after death, and the most commonly investigated products of decomposition are amino acids. Amino acid racemization dating (AAR) measures changes between these amino acids' L- and D-forms; their ratio is an indication of age.

* 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)

6.2. Obsidian hydration dating

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.

* Introduction to Obsidian Hydration Studies 'Once a hydration layer has been measured, it can be used to determine the relative ages of items or, in some circumstances, can be converted into an estimated absolute age. In order to transform the hydration rim value to a calendar age, the rate of the diffusion of water into the glass must be determined or estimated. The hydration rate is typically established empirically through the calibration of measured samples recovered in association with materials whose cultural age is known or whose age can be radiometrically determined, usually through radiocarbon dating methods...' (Northwest Research Obsidian Studies Laboratory, Oregon; a number of lab reports and articles are available to download as PDF files)

6.3. Archaeomagnetic dating

The Earth's magnetic field undergoes continuous change. The position of magnetic North wanders around the North Pole, and even reverses completely to the South Pole for extended periods on a geological time-scale. From any reference point its position is measurable in terms of two components: movement up or down (inclination or 'dip') and from side to side (declination).

* 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)

7. THE AUTHENTICITY OF ARTEFACTS

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.

* Labor Ralf Kotalla 'Worldwide oldest private Laboratory for genuine Analyses': Thermoluminescence analysis for ceramics and cores of cast metalwork - especially to detect forgeries. Follow links to 'Analysis' and 'Articles'.

* Spectroscopic Dating and Classification of Wood PDF file: 'It is important to be able to detect the use of old wood for recently-made fakes. ... The surface layer of any wooden artefact has undergone chemical changes due to UV light and other environmental agents. Since spectroscopy is a chemical analysis, it can detect these differences. ... If an object is made of wood that is already old, both curves are nearly identical.' (Gottfried Matthaes, Wooden Artifacts Group).