Archaeology: An Introduction - 4th Edition 2002
The
Online Companion: updated November 2007
CHAPTER 4 : Dating the Past
4.2. TYPOLOGY AND CROSS-DATING
4.4. SCIENTIFIC DATING
TECHNIQUES
4.7. THE AUTHENTICITY OF
ARTEFACTS
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4.1. 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.
4.2. TYPOLOGY AND CROSS-DATING
4.2.1. Sequence
dating and seriation
4.2. 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)
4.2.1. 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)
4.3. HISTORICAL DATING
4.3.1. Applying
historical dates to sites
4.3. 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.
4.3.1. 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
4.4.1. Geological
time-scales
4.4.2. Climatostratigraphy
4.4.3. Varves
4.4.4. Pollen
4.4.5. Dendrochronology
(tree-ring dating)
:: The application
of tree-ring dating
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
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)
4.4.1. 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)
- NB: see also Chapter
1.1.2
4.4.2. 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.
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)
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)
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)
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
4.4.4. Pollen
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)
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)
4.5. ABSOLUTE TECHNIQUES
4.5.1. Radioactive
decay
4.5.2. Radiocarbon
dating
4.5.3. Presenting
and interpreting a radiocarbon date
:: The impact of radiocarbon
dating
4.5.4. Potassium-argon
(40K/40Ar) and argon-argon dating (40Ar/39Ar)
4.5.5. Uranium series
dating
4.5.6. Fission-track
dating
4.5.7. Luminescence
dating
4.5.8. Electron
spin resonance (ESR)
4.5. ABSOLUTE TECHNIQUES
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.
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...'
Click for a list of the key factors for Radiocarbon Dating
4.5.3. 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)
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)
4.5.4. 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)
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)
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
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
- 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.'
4.5.8. 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
- 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
4.6. DERIVATIVE TECHNIQUES
4.6.1. Protein and
amino acid diagenesis dating
4.6.2. Obsidian
hydration dating
4.6.3. Archaeomagnetic
dating
4.6. 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.
4.6.1. 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.'
4.6.2. 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...'
(
- See also Chapter 5 6.1
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 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)
4.7. 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)
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