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Dating methodology (archaeology)
Encyclopedia
Dating material drawn from the archaeological record
can be made by a direct study of an artifact
or may be deduced by association
with materials found in the context
the item is drawn from or inferred by its point of discovery in the sequence
relative to datable contexts. Dating is carried out mainly post excavation
but to support good practice some preliminary dating work called spot dating is usually run in tandem with excavation. Dating is very important in archaeology for constructing models of the past, as it relies on the integrity of datable objects and samples. Many disciplines of archaeological science
are concerned with dating evidence.
methods rely on using some physical property of an object or sample to calculate its age. Examples are:
or indirect methods tend to use associations built from the archaeological body of knowledge. An example is seriation
. Ultimately, relative dating relies on tying into absolute dating with reference to the present
. One example of this is dendrochronology
which uses a process of tying floating chronologies of tree rings together by cross referencing a body of work.
In practice several different dating techniques must be applied in some circumstances, thus dating evidence for much of an archaeological sequence recorded during excavation requires matching information from known absolute or some associated steps, with a careful study of stratigraphic relationships
.
: the polarity of the Earth
changes at a knowable rate. This polarity is stored within rocks; through this the rock can be dated.
Tephrochronology
: volcanic ash has its own signature for each eruption. In a sedimentary sequence the associated material within the ash layer can be dated, giving a date for the eruption. If this ash is found anywhere else in the world, a date will already be known (bearing in mind transportation time).
Oxygen isotope chronostratigraphy: this is based on the climatic stages displayed in SPECMAP relating to different cold and warm stages experienced in deep time; for example, point 5.5 in the SPECMAP chronology describes the peak of the last interglacial
125,000 years ago.
s which represents events. Some degree of dating objects by their position in the sequence can be made with known datable elements of the archaeological record or other assumed datable contexts deduced by a regressive form of relative dating which in turn can fix events represented by contexts to some range in time. For example the date of formation of a context which is totally sealed between two datable layers will fall between the dates of the two layers sealing it. However the date of contexts often fall in a range of possibilities so using them to date others is not a straightforward process.
Take the hypothetical section
fig A. Here we can see 12 contexts, each numbered with a unique context number
and whose sequence is represented in the Harris matrix
in fig B.
If we know the date of context 1 and context 9 we can deduce that context 7, the backfilling of pit 8, occurred sometime after the date for 9 but before the date for 1, and if we recover an assemblage of artifacts from context 7 that occur nowhere else in the sequence, we have isolated them with a reasonable degree of certainty to a discrete range of time. In this instance we can now use the date we have for finds in context 7 to date other sites and sequences. In practice a huge amount of cross referencing with other recorded sequences is required to produce dating series from stratigraphic relationships such as the work in seriation
.
in the context representing the backfill of the construction cut, context 3. These artifacts are referred to as "residual" or "residual finds".
It is crucial that dating a context is based on the latest dating evidence drawn from the context. We can also see that if the fill of cut 5 — the wall 2, backfill 3 and trample 12 — are not removed entirely during excavation because of "undercutting", non-residual artifacts from these later "higher" contexts 2, 3 and 12 could contaminate the excavation of earlier contexts such as 9 and 10 and give false dating information these artifacts may be termed intrusive finds.
Archaeological record
The archaeological record is the body of physical evidence about the past. It is one of the most basic concepts in archaeology, the academic discipline concerned with documenting and interpreting the archaeological record....
can be made by a direct study of an artifact
Artifact (archaeology)
An artifact or artefact is "something made or given shape by man, such as a tool or a work of art, esp an object of archaeological interest"...
or may be deduced by association
Archaeological association
Association in archaeology has more than one meaning and is confusing to the layman. Archaeology has been critiqued as a soft science with a somewhat poor standardization of terms.-Finds and objects:...
with materials found in the context
Archaeological context
In archaeology, not only the context of a discovery is a significant fact, but the formation of the context is as well. An archaeological context is an event in time which has been preserved in the archaeological record. The cutting of a pit or ditch in the past is a context, whilst the material...
the item is drawn from or inferred by its point of discovery in the sequence
Sequence (archaeological)
The archaeological sequence or sequence for short, on a specific archaeological site can be defined on two levels of rigour.# Normally it is adequate to equate it to archaeological record. However, the two terms are not exactly interchangeable...
relative to datable contexts. Dating is carried out mainly post excavation
Post excavation
In archaeology once the archaeological record of given site has been excavated, or collected from surface surveys, it is necessary to gain as much data as possible and organize it into a coherent body of information. This process is known as post-excavation analysis, and is normally the most...
but to support good practice some preliminary dating work called spot dating is usually run in tandem with excavation. Dating is very important in archaeology for constructing models of the past, as it relies on the integrity of datable objects and samples. Many disciplines of archaeological science
Archaeological science
Archaeological science, also known as archaeometry, consists of the application of scientific techniques to the analysis of archaeological materials. Archaeometry is now considered its own scientific field. The UK's Natural and Environmental Research Council provides funding for archaeometry...
are concerned with dating evidence.
Absolute methods
Absolute datingAbsolute dating
Absolute dating is the process of determining an approximate computed age in archaeology and geology. Some scientists prefer the terms chronometric or calendar dating, as use of the word "absolute" implies an unwarranted certainty and precision...
methods rely on using some physical property of an object or sample to calculate its age. Examples are:
- Radiocarbon datingRadiocarbon datingRadiocarbon dating is a radiometric dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbon-bearing materials up to about 58,000 to 62,000 years. Raw, i.e. uncalibrated, radiocarbon ages are usually reported in radiocarbon years "Before Present" ,...
- for dating organic materials - DendrochronologyDendrochronologyDendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...
- for dating trees, and objects made from wood, but also very important for calibrating radiocarbon dates - Thermoluminescence datingThermoluminescence datingThermoluminescence dating is the determination, by means of measuring the accumulated radiation dose, of the time elapsed since material containing crystalline minerals was either heated or exposed to sunlight...
- for dating inorganic material including ceramics - Optically stimulated luminescenceOptically stimulated luminescenceIn physics, optically stimulated luminescence is a method for measuring doses from ionizing radiation.The method makes use of electrons trapped between the valence and conduction bands in the crystalline structure of certain types of matter . The trapping sites are imperfections of the lattice -...
or optical datingOptical datingOptical dating is a method of determining how long ago minerals were last exposed to daylight. It is useful to geologists and archaeologists who want to know when such an event occurred....
for archaeological applications - Potassium-argon datingPotassium-argon datingPotassium–argon dating or K–Ar dating is a radiometric dating method used in geochronology and archeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium into argon . Potassium is a common element found in many materials, such as micas, clay minerals,...
- for dating fossilized hominidHominidaeThe Hominidae or include them .), as the term is used here, form a taxonomic family, including four extant genera: chimpanzees , gorillas , humans , and orangutans ....
remains - NumismaticsNumismaticsNumismatics is the study or collection of currency, including coins, tokens, paper money, and related objects. While numismatists are often characterized as students or collectors of coins, the discipline also includes the broader study of money and other payment media used to resolve debts and the...
- many coins have the date of their production written on them or their use is specified in the historical record - Archaeomagnetic datingArchaeomagnetic datingArchaeomagnetic dating is the study and interpretation of the signatures of the Earth's magnetic field at past times recorded in archaeological materials. These paleomagnetic signatures are fixed when ferromagnetic materials such as magnetite cool below the Curie point, freezing the magnetic moment...
- Clay lined fire hearths take on a magnetic moment pointing to the North Pole each time they are fired and then cool. The position of the North Pole for the last time the fire hearth was used can be determined and compared to charts of known locations and dates - Magnetic Properties of LeadLeadLead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
used to establish dates. Chemistry Professor Shimon Reich, a specialist in superconductivity, has demonstrated a method for dating artifacts based on the magnetic properties of lead, a material widely used in Israel and elsewhere in antiquity. Reich and coworkers found that at cryogenic temperatures, lead becomes a superconductor, but the corrosion products formed from centuries of exposure to air and water (lead oxide and lead carbonate) do not superconduct. On the basis of magnetic measurements and comparison with artifacts that were known (using other techniques) to be up to 2500 years old, the group showed that the mass of lead corrosion products is directly proportional to an object's age (New Journal of Physics, 2003, 5, 99) - Amino acid datingAmino acid datingAmino acid dating is a dating technique used to estimate the age of a specimen in paleobiology, archaeology, forensic science, taphonomy, sedimentary geology and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed.-Principle:All...
- Obsidian hydration datingObsidian hydration datingObsidian hydration dating is a geochemical method of determining age in either absolute or relative terms of an artifact made of obsidian....
- a geochemicalGeochemistryThe field of geochemistry involves study of the chemical composition of the Earth and other planets, chemical processes and reactions that govern the composition of rocks, water, and soils, and the cycles of matter and energy that transport the Earth's chemical components in time and space, and...
method of determining age in either absolute or relative terms of an artifactArtifact (archaeology)An artifact or artefact is "something made or given shape by man, such as a tool or a work of art, esp an object of archaeological interest"...
made of obsidianObsidianObsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth... - Rehydroxylation datingRehydroxylation datingRehydroxylation [RHX] dating is a developing technology for dating fired clay ceramics. It is based on the principle that after a ceramic is removed from the kiln, it immediately begins to recombine chemically with moisture from the environment and thus increases in weight. This weight increase...
- for dating ceramic materials
Relative methods
RelativeRelative dating
Relative dating is the science determining the relative order of past events, without necessarily determining their absolute age.In geology rock or superficial deposits, fossils and lithologies can be used to correlate one stratigraphic column with another...
or indirect methods tend to use associations built from the archaeological body of knowledge. An example is seriation
Seriation (archaeology)
In archaeology, seriation is a relative dating method in which assemblages or artifacts from numerous sites, in the same culture, are placed in chronological order. Where absolute dating methods, such as carbon dating, cannot be applied, archaeologists have to use relative dating methods to date...
. Ultimately, relative dating relies on tying into absolute dating with reference to the present
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...
. One example of this is dendrochronology
Dendrochronology
Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree-rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year...
which uses a process of tying floating chronologies of tree rings together by cross referencing a body of work.
In practice several different dating techniques must be applied in some circumstances, thus dating evidence for much of an archaeological sequence recorded during excavation requires matching information from known absolute or some associated steps, with a careful study of stratigraphic relationships
Relationship (archaeology)
An archaeological relationship is the position in space and by implication, in time, of an object or context with respect to another. This is determined, not by linear measurement but by determining the sequence of their deposition - which arrived before the other...
.
Age Equivalent Stratigraphic Markers
PaleomagnetismPaleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...
: the polarity of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
changes at a knowable rate. This polarity is stored within rocks; through this the rock can be dated.
Tephrochronology
Tephrochronology
250px|thumb|right|Tephra horizons in south-central [[Iceland]]. The thick and light coloured layer at the height of the [[volcanologists]] hands is [[rhyolitic]] [[tephra]] from [[Hekla]]....
: volcanic ash has its own signature for each eruption. In a sedimentary sequence the associated material within the ash layer can be dated, giving a date for the eruption. If this ash is found anywhere else in the world, a date will already be known (bearing in mind transportation time).
Oxygen isotope chronostratigraphy: this is based on the climatic stages displayed in SPECMAP relating to different cold and warm stages experienced in deep time; for example, point 5.5 in the SPECMAP chronology describes the peak of the last interglacial
Interglacial
An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...
125,000 years ago.
Stratigraphic relationships
Archaeologists investigating a site may wish to date the activity rather than artifacts on site by dating the individual contextArchaeological context
In archaeology, not only the context of a discovery is a significant fact, but the formation of the context is as well. An archaeological context is an event in time which has been preserved in the archaeological record. The cutting of a pit or ditch in the past is a context, whilst the material...
s which represents events. Some degree of dating objects by their position in the sequence can be made with known datable elements of the archaeological record or other assumed datable contexts deduced by a regressive form of relative dating which in turn can fix events represented by contexts to some range in time. For example the date of formation of a context which is totally sealed between two datable layers will fall between the dates of the two layers sealing it. However the date of contexts often fall in a range of possibilities so using them to date others is not a straightforward process.
Archaeological section
In archaeology a section is a view in part of the archaeological sequence showing it in the vertical plane, as a cross section, and thereby illustrating its profile and stratigraphy. This may make it easier to view and interpret as it developed over time....
fig A. Here we can see 12 contexts, each numbered with a unique context number
Single context recording
Single context recording was initially developed by Ed Harris and Patrick Ottaway in 1976, from a suggestion by Lawrence Keene. It was further developed by the Department of Urban Archaeology from where it was then exported, in the mid 1980s by Pete Clarke to the Scottish Urban Archaeological...
and whose sequence is represented in the Harris matrix
Harris matrix
The Harris matrix is a tool used to depict the temporal succession of archaeological contexts and thus the sequence of deposition on a 'dry land' archaeological site. The matrix reflects the relative position and stratigraphic contacts of observable stratigraphic units, or contexts. The Matrix was...
in fig B.
- A horizontal layer
- Masonry wall remnant
- Backfill of the wall construction trench (sometimes called construction cut)
- A horizontal layer, probably the same as 1
- Construction cut for wall 2
- A clay floor abuttingRelationship (archaeology)An archaeological relationship is the position in space and by implication, in time, of an object or context with respect to another. This is determined, not by linear measurement but by determining the sequence of their deposition - which arrived before the other...
wall 2 - Fill of shallow cut 8
- Shallow pit cut
- A horizontal layer
- A horizontal layer, probably the same as 9
- Natural sterile ground formed before human occupation of the site
- Trample in the base of cut 5 formed by workmen's boots constructing the structure wall 2 and floor 6 is associatedArchaeological associationAssociation in archaeology has more than one meaning and is confusing to the layman. Archaeology has been critiqued as a soft science with a somewhat poor standardization of terms.-Finds and objects:...
with.
If we know the date of context 1 and context 9 we can deduce that context 7, the backfilling of pit 8, occurred sometime after the date for 9 but before the date for 1, and if we recover an assemblage of artifacts from context 7 that occur nowhere else in the sequence, we have isolated them with a reasonable degree of certainty to a discrete range of time. In this instance we can now use the date we have for finds in context 7 to date other sites and sequences. In practice a huge amount of cross referencing with other recorded sequences is required to produce dating series from stratigraphic relationships such as the work in seriation
Seriation (archaeology)
In archaeology, seriation is a relative dating method in which assemblages or artifacts from numerous sites, in the same culture, are placed in chronological order. Where absolute dating methods, such as carbon dating, cannot be applied, archaeologists have to use relative dating methods to date...
.
Residual and intrusive Finds
One issue in using stratigraphic relationships is that the date of artifacts in a context does not represent the date of the context, but just the earliest date the context could be. If we look at the sequence in fig A we may find that the cut for the construction of wall 2, context 5, has cut through layers 9 and 10 and in doing so has introduced the possibility that artifacts from layers 9 and 10 may be redeposited higher up the sequenceRelationship (archaeology)
An archaeological relationship is the position in space and by implication, in time, of an object or context with respect to another. This is determined, not by linear measurement but by determining the sequence of their deposition - which arrived before the other...
in the context representing the backfill of the construction cut, context 3. These artifacts are referred to as "residual" or "residual finds".
It is crucial that dating a context is based on the latest dating evidence drawn from the context. We can also see that if the fill of cut 5 — the wall 2, backfill 3 and trample 12 — are not removed entirely during excavation because of "undercutting", non-residual artifacts from these later "higher" contexts 2, 3 and 12 could contaminate the excavation of earlier contexts such as 9 and 10 and give false dating information these artifacts may be termed intrusive finds.
See also
- Harris matrixHarris matrixThe Harris matrix is a tool used to depict the temporal succession of archaeological contexts and thus the sequence of deposition on a 'dry land' archaeological site. The matrix reflects the relative position and stratigraphic contacts of observable stratigraphic units, or contexts. The Matrix was...
- Obsidian hydration datingObsidian hydration datingObsidian hydration dating is a geochemical method of determining age in either absolute or relative terms of an artifact made of obsidian....
- Vole Clock
- Oxidizable Carbon Ratio datingOxidizable carbon ratio datingOxidizable carbon ratio dating is a method of dating in archaeology and earth science that can be used to derive or estimate the age of soil and sediment samples up to 35,000 years old...
Literature
- M. Jacoby, Chemistry in the Holy Land, Chemical & Engineering News, 5 March 2007, page 20, published by American Chemical Society