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Fact Files

The term prehistory - meaning the time before history, came into use in the earlier part of the 19th century and was first used in a book entitled 'The archaeology and prehistoric annals of Scotland' published in 1851. In Britain we regard prehistory as ending with the coming of the 'civilising' Romans in 43AD but in differing parts of the world and in differing cultures prehistory can still exist today.
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Three age system
Although there are the hints of this idea developing in the writings of some early 19th century British antiquarians the breakthrough came in Denmark where Christian Jurgenson Thomsen, given the task of sorting out the collections of the National Museum of Denmark, developed a classification based on three successive ages, of stone, bronze and iron. He suggested that these ages, although defined by technology, represented differing stages of human development and consequently provided not just a classification but also some indication of chronology.
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The concept of stratigraphy was developed in the 19th century and is based on the understanding that where several layers that represent differing events are superimposed, then the layer at the bottom will be the oldest and the one at the top the most recent. The layers visible in the side of a trench can be very easy to understand, a simple sequence laid one on top of another and visibly distinctive. Stratigraphy can however be very complex as such a simple sequence can then be cut through by events such as the digging of a pit or well which may then accumulate its own individual set of layers.
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Aerial survey
Marks or patterns that show the location of buried features such as ditches or walls can appear in two ways. Bare soil marks, most visible in reflective soils such as those found overlying chalk bedrock, are produced at the time of ploughing and the contrasts produced by ploughed banks or ditches will gradually fade. Crop marks are produced when crops growing over buried features develop at a different rate to those growing immediately adjacent. For example, over a soil filled ditch the deeper soil may provide better moisture retention and more nutrients allowing the crop to grow faster and taller. Conversely, over a buried wall the shallower soil may produce a stunted crop and one more liable to parching under dry conditions. In addition, earthworks such as banks and ditches can also be photographed under oblique light conditions (shadow marks).

Although crop and soil marks were first noticed as far back as the 18th century, it has only been possible to record them systematically since the introduction of flying and the availability of faster cameras. Aerial survey provides a means of rapidly mapping extensive elements of past landscapes, such as field systems, provided that the modern land use is suitable for the production of the necessary contrasts.
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Geophysical survey
There are several types of geophysical survey that, by measuring differences in soil properties can identify buried structures or the traces of specific activities.

  • Resistivity, which involves passing an electrical current through the ground and measuring the resistance between probes can identify structures such as walls or floors.

  • Magnetometry measures variations in the soil magnetism and is extremely sensitive to the magnetic enhancement caused by burning. Heaths and kilns provide strong signals and this technique is also very useful for locating ditches or pits filled with soils that are magnetically enhanced by the inclusion of settlement debris. Magnetometers are affected very strongly by the proximity of metal, so wire fences, overhead power lines and buried pipes can all render adjacent areas unsurveyable.

  • Ground penetrating radar is increasingly being used to examine sites that are potentially deeply buried or sealed beneath hard surfaces such as roads or car parks. Radar provides a series of vertical slice images that can then be combined to produce a three dimensional picture of more solid below ground remains.

  • Geochemical analysis in which the chemical properties of soils are analysed to provide indications of past activity while higher levels of organic phosphates may suggest manure heaps or animal pens.
All these methods of 'remote sensing' assist with the location and definition of sites with no disturbance other then perhaps the removal of small surface soil samples. They are therefore known as 'non-invasive'.
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DNA and isotopes
The study of human remains has advanced considerably in recent years due to new techniques including the analysis of DNA and of specific isotopes incorporated in bone structure.

  • Ancient DNA, extracted from bone or teeth if it survives sufficiently intact, can prove gender, even in juveniles whose skeletal gender characteristics have not yet formed. In addition it may provide indications of kinship, the inter-relationship of individual members of a cemetery or grave, while DNA studies of modern populations can show evidence of migrations or invasions.

  • Isotopes: locked into the bones and teeth of a human or animal are isotopes of elements such as oxygen, carbon, lead and strontium, absorbed from drinking water or food during the life of that person or animal. Following the principal that 'you are what you eat' the analysis of specific carbon isotopes can provide indications of diet, for example the amount of meat in a person's diet or the proportions of marine and terrestrial foods.

    In addition, isotopes of lead, oxygen and strontium absorbed at early stages of development, can, because they are linked to localized geology and drinking water, indicate place of origin. This type of information can consequently demonstrate migration at an individual level.
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Environmental studies
It is now possible to investigate the past environment using a wide range of materials such as pollen grains or the shells of land snails, the preservation of which will depend on varying soil and burial conditions. Waterlogged deposits within which the exclusion of oxygen may slow down or even halt natural processes of decay, offer the best potential for environmental study. Such deposits may preserve a wide range of organic materials such as wood and other plant remains, pollen and even insects. Together these will indicate localised vegetation and the modification of the natural environment and can also provide evidence for climatic change.

Soil types: Within dry environments the survival of evidence is strongly linked to soil type. Acidic soils such as those found on heathland and moors offers ideal preservation conditions for pollen but will degrade or destroy bone and snail shells. In contrast, alkaline soils such as those found on chalk and limestone will preserve bone and shell but not pollen. Wood and other plant remains will survive under almost any soil conditions when charred. The evidence for environmental change is usually extracted from soil samples taken on site by specialists and analysed under laboratory conditions.
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Pottery studies

Shape and form
Since its introduction in the British Isles in about 4000BC pottery has always been valued by archaeologists as vessels through time can show evidence of changes in manufacture, raw material, shape and decoration. These all provide clues about date, function and the movement of people and ideas. Pots are classified according to shape ('form'), the type of clay used and what has been deliberately added to it ('fabric' and 'inclusions') as well as the way in which surfaces have been decorated, for example using incised or stamped patterns, smoothing or glazes. It is these distinguishing characteristics that are used to distinguish between a fragment of for example a hand made, local clay, undecorated, cooking pot of about 1000BC (Bronze Age) from one of a wheel thrown, imported, lead glazed jug of the 13th century AD.

Food remains
It is possible in some cases to determine what has been cooked in a particular pot by studying burnt-on food remains but, more recently advances have been made in the analysis of lipids, animal fats and plant waxes that impregnate porous pots. Lipid analysis has provided the first clues to the function of particular types of highly distinctive prehistoric vessels and has also provided otherwise unavailable information about prehistoric diet.
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Dendrochronlogy is a method of determining absolute date from surviving pieces of timber, whether part of the fabric of a historic building or a tree preserved in a peat bog. The method is based on the simple fact that trees put on annual growth rings, visible in cross section. These can, in certain slow growing species such as oak, vary considerably in thickness according to fluctuations in temperature and rainfall in a specific growing season.

Dating procedure: Starting with a tree of known felling date, rings are counted backwards and their variations recorded. This sequence can be compared with one taken for example from a historic building the construction of which overlaps the older part of the first sequence. By matching variations in rings and overlapping sequences a master 'curve' can be established, the one constructed for oak in Ireland now going back over 7000 years. With this in place any fragment of that species having sufficient identifiable rings can be matched and, if it includes the outer growth ring, the year of felling can be determined. For example the Sweet Track, a Neolithic trackway in the Somerset Levels was built from timber felled in the winter/spring of 3807/6 BC.

The variation in growth rings also provides a valuable indication of climatic change and periods of environmental stress have been correlated with documented natural disasters such as the Bronze Age eruption of Santorini, possibly in 1626BC.
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Radiocarbon (C14) dating

Radioactive isotopes
All living organisms, absorb carbon while they are alive, including radioactive isotopes of carbon such as C14. Once the organism within which it has been absorbed ceases to live and take in new carbon, its radioactivity starts to decline at a known rate. Half of its radioactivity will have decayed away in 5730 years and half of what remains in another 5730 - this length of time being known as its ‘half life’. If the remaining level of C14 can be measured then it is possible to estimate how long ago the organism ceased to live. This method can be applied to any surviving material that contains sufficient carbon, the most commonly used materials being charcoal and bone.

Carbon dating
When first introduced this method was thought to provide absolute dates. So, in 1950 a radiocarbon date of 3950bc was thought to represent a calendar date of 2000BC. However, by radiocarbon dating objects of known historic date it has been shown that radiocarbon dates are consistently younger than they may appear and require calibration to be converted to calendar dates.

Radiocarbon dates are expressed in several ways but it is important to note that BP after a date refers to before present, present being fixed as 1950. Also, that the use of bc (lower case letters) refers to a raw, or uncalibrated date, while BC (upper case) is one that has been calibrated. Figures such as +-40 after a date indicate the date range within which the central date has a standard statistical chance of lying within.
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Published: 28-01-2005

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