Now for those of you who watch Time Team, you’re all probably pretty familiar with the fantastic digital imagery they conjure up to recreate what a site looked like in its former glory! We also are introduced to some of the methods archaeologists use in finding where to dig – ‘GeoPhys’ as Tony Robinson affectionally puts it but what about finding sites of interest in general? What are some of the main techniques that archaeologists use to determine the potential of a particular site?
1) Remote Sensing
Satellite photographs of the potential area are examined for clues in the landscape topography that might indicate previous settlement and archaeological structures. Geographic Information Systems (GIS) can be particularly useful in conjunction with satellite imaging as they are a powerful database which can store multiple layers of data against individual grid references on a map. They can show details such as topography, geology, vegetation and archaeological structures.
2) Surface Surveys
Field walking and surveying can help archaeologists find traces of unrecorded sites through evidence such as scatters of building rubble or artefacts. Differences in vegetation may also be an indicator of previous human activity. When studying Mesolithic and Neolithic Britain, scatters of flint and animal bone are usually the only traces of human activity visible. Studying nomadic societies requires meticulous plotting of scatters. Micro-contour surveys of the topography can also be useful and these involve surveying tools to create a picture of variations in height and levels. These types of surveys can reveal hidden features that are not detectable upon first sight.
3) Magnetometer Surveying
The cool looking one! Local magnetic distortions can be caused by previous human activity such as top soil containing haematite iron oxide (in which some of its crystal forms are magnetic). A second kind of distortion is that of the topsoil having been subjected to high levels of heat. Today hand-held fluxgate gradiometers enable this type of surveying technique to be efficient and accurate in detecting magnetic anomalies. Although this type of technology has been well developed today, trouble can occur when the soil contains other metal objects such as iron nails, pipes, and wire fences (and even piercings on the archaeologist him or herself!)
4) Caesium Vapour Magnetometers
CV magnetometers are much more sensitive than conventional magnetometers. They work by using several machines close together on a wooden handcart. Caesium Vapour is pumped and the magnetometer takes rapid measurements around 25 cm intervals. The CV is alkali and therefore sensitive to minute variations in magnetism so it is capable of detecting and defining edges of buried features formed by traces of magnetite! CV magnetometers are less susceptible to interference from background ‘noise’ as such but at £40,000 a pop, the technique is much more dear than the classic!
Unsurprisingly, or maybe surprisingly, the ripening and growth rate of crops can be an indicator of buried archeological features under a field. A buried ditch with infills of humus and topsoil often holds moisture and will create a green line – a ‘positive’ cropmark visible from the air. When plants are buried over a wall a ‘negative’ cropmark (stunted, yellowish growth line) will show. Repeatedly flying over these areas over time can pick up features. This technique works most effectively on quick draining soils (i.e. river gravels) and less so on poorer draining (retain more moisture) areas such as clay soils and areas of deeper topsoil. The type of crop being observed also affects the techniques success – cereal crops such as barley and wheat show up best while peas and beans less so (especially when covered by irrigation or fertiliser). Additionally, geological features such as periglacial cracks and modern field drainage and underground pipelines have to be taken into consideration when interpreting aerial images as these can also produce similar results. Cropmarks are particularly important in uncovering late Neolithic to early medieval period sites.
6) The coolest and most advanced…
I recently saw this on ‘Egypt’s Lost Cities’ a documentary by Dr. Sarah Parcak (A ‘space archaeologist!’) on Yesterday TV. Parcak uses ‘space archaeology’. What is ‘Space Archaeology’ you ask? It uses satellite imaging to show subtle changes at pixel level. The human eye can only see part of the light spectrum while space cameras on satellites have a higher complexity of resolution. Images can also be altered using advanced computer programmes to accentuate certain features on the satellite image. Space archaeology has led to the uncovering of 44 previously unidentified sites including an Egyptian Harem believed to have played a prolific part in King Tutankhamun’s childhood. It has been estimated that less than 1% of Egypt’s ancient wonders have been discovered but now thanks to satellite imagery the impossible is about to happen.
The Archaeology Coursebook: An Introduction to Themes, Sites, Methods and Skills
Jim Grant, Sam Gorin, Neil Fleming