An archaeological site is a place in which evidence of past activity or occupation is preserved. Some sites are found by accident for example during ploughing by farmers, digging drainage trenches, road construction, building construction and quarrying. Indeed, the terracotta army in China was discovered in 1974 by farmers digging for a well (Tianchou, 1996). But most archaeological sites are found as a result of a great deal of work and planning.
When locating an archaeological site non-intrusive and intrusive survey methods are used. Non-intrusive survey methods give a broad insight in to the development of the landscape, eg – accurately surveying earthworks and other features – nothing is touched or destroyed just recorded. The results of non-intrusive survey methods can be repeated if the results seem unclear or inconclusive.
Intrusive survey methods are destructive processes and the results cannot be repeated. It is of paramount importance that the contextual recording of every artefact is meticulously done.
The main non-intrusive techniques for finding archaeological sites are:
• Desktop survey;
• Aerial photography;
• Field walking;
• Topography and earthwork surveys;
• Geophysical surveying; and
• Remote sensing;
The main intrusive techniques for finding archaeological sites are:-
• Metal detecting;
A desktop survey is based on the use of existing historical documents, antiquarian sources, maps, archaeological records, literature, pictures and entries on the Sites and Monuments Records or National Monument Record; in order to obtain any hints or information to provide a better understanding and possible extent of the site.
Although many early maps are not always to scale or accurate, they can be useful and by comparing maps of the same area from different periods and projecting them onto modern maps they can reveal past structures and field boundaries that are no longer visible on the surface. As will ancient bridle ways, roadways, cart ways and footpaths, as industry and trade were often conducted at or near crossroads.
Many records are free to access and are found in libraries, museums, County Records and Archive Offices Location and National Sites and Monuments Records offices and in private collections.
Ancient books and manuscripts can reveal much about lost civilisations as can stories and legends (Grant et al, 2002. P8). Heinrich Schliemann’s belief in the accuracy of Homer’s Iliad led to his discovery in 1870 of the ruined city of Troy near the town of Hisarlik, Turkey.
William Stukeley carried out a survey of Avebury in the 1720s and identified a second avenue of standing stones, the so-called Beckhampton Avenue. By the twentieth century the remains noted by William Stukeley had mostly disappeared apart from the Longstones. However, with the use of geophysical survey and excavation in the Negotiating Avebury Project (Gillings and Pollard, 2004; Gillings et al, 2008) it was proved that the Beckhampton Avenue as described by William Stukeley did exist.
Details of any previous archaeological excavations, finds and previous survey results are held in local Sites and Monuments Records or National Monument Records, County Records and Archive Offices or in reports/databases on the internet.
The desktop survey is of particular value as part of the planning process leading to possible excavation of a site. The “purpose of the desktop survey is to determine whether there are likely to be archaeological remains which might be threatened by development” (Grant et al, 2002. P8).
Aerial photography is used to find and interpret sites and usually precedes field survey. O G S Crawford, Archaeologist and Observer in the Royal Flying Corps during World War I pioneered the use of aerial photography (www.ukessays.com/essays/archaeology/archaeological-sites-in-landscape.php), and also the works of C F C Hawes and D Riley. Weather conditions are important, and photographs are best taken on a clear day when the sun is low in the sky as the marks show up better in these conditions (Adkins, 2000). Photographs may be taken either vertically or obliquely.
Aerial photography locates crop marks and soil marks. Cropmarks sometimes only show for a few days a year. Some only show up in drought conditions when crops with access to moisture have the greatest advantage and colour contrast is exaggerated. Aerial photography works best on quickly draining soils such as river gravels and chalk but is less good on clay or areas where the soil retains moisture well. “Cropmarks are the most prolific source of new sites, particularly for the late Neolithic to early medieval periods, and are also used to investigate existing sites” (Grant at al, 2015. P33).
Soil marks are evident where there is a marked contrast between the colour of the topsoil and subsoil. Evidence of ploughed-out monuments can occur as soil marks, for example on chalk, the dark brown of ditch infill will contrast with the chalk rubble of a bank and the lighter brown of the plough soil. Soil marks are sharpest in winter when vegetation is low.
Aerial photography may also show evidence of medieval field boundaries in the form of ridge and furrow marks in the soil which are the remains of a system of ploughing used in Britain and Europe and give a ‘corrugated’ appearance to the ground surface.
While aerial photography is fast and gives good coverage, it can be expensive and can miss features if their signatures are not visible from the air. Care has to be taken with interpretation, as geological features and modern field drainage and underground pipelines also create cropmarks. Trial excavation is often the only way to firmly identify many sites.
Field walking is used for further assessing of a site. Field walking is usually carried out in grids or along lines and provides many opportunities for amateurs and beginners to get involved and works best on either ploughed ground or surfaces with little vegetation. The main purpose is the collection of artefacts from the surface of ploughed fields. All finds need to be recorded accurately so they can be plotted on an overall plan of the site to show the distribution of the results.
Field walking is relatively simple and cost effective. The efficacy varies according to the use of the land, topography, weather conditions and the skill and experience of the field walkers. However, most archaeological sites are too large to be explored entirely, so a targeted approach is more effective.
Topography and earthwork surveys may be required where earthworks within the site are likely to be affected by excavation. The survey allows archaeologists to measure and map the surface of the site in order to locate features or structures and can reveal the outline of building foundations, field patterns or other features not previously identified.
When carrying out topography and earthwork surveys archaeologists use surveying equipment such as GPS or Total Stations in order to relate the survey results to the national system of the Ordnance Survey grid. A Total Station is an instrument set on a tripod that shoots a signal at a target placed on the feature to be mapped. The signal bounces off the target and returns to the Total Station, which calculates the distance and angle of the object. This information is stored in the instrument’s memory and can be downloaded to a computer with software that generates a map. By using a Total Station or GPS the corners of the site grid and any earthworks or structures within the site can be mapped accurately. Very experienced surveyors can work quickly and accurately and with the advances in computer technology this has made some aspects less arduous (www.pastperfect.org.uk/archaeology/ toposurvey.html).
Geophysical survey uses instruments that can detect buried archaeology when their magnetic or electrical properties differ from that of the surrounding soil.
Ground penetrating radar can detect buried masonry structures and can show holes or voids below ground. Is mostly used on urban sites where there are masonry remains, ditches, ancient mines or large landscape features. The survey is carried out by dragging a radar scanner on wheeled trolley across ground in a fixed pattern. The readings are taken as a survey grid and once downloaded and plotted can provide impressive results. The cost of carrying out a survey can be restrictive.
Resistivity surveys involve passing an electrical current through a pair of electrodes set into the ground and resistance to the flow of the current is measured. Where there is buried archaeology there will be higher resistance to the current, where there are infilled pits or ditches there will be more moisture continent which in turn provides a lower reading. However, the success of the survey can be affected by local geology and also weather conditions. The equipment is heavy to use and cumbersome. The probes need to be pushed into the ground at precise intervals which can mean the survey takes some time to complete depending on the size of the site.
Magnetometry surveys detect variations in the magnetic field in the subsoil of a site. Past human activities such as burning, digging and refilling ditches or pits, solid features such as walls or road surfaces disturb the magnetic field and provide higher or lower readings than the surround subsoil. There are various types of magnetometer: Proton magnetometer which takes readings of the absolute magnetic field at given points on a grid; proton gradiometer which is less sensitive but easier to operate; more common is the fluxgate gradiometer which takes continuous readings and can be used to survey large areas quickly; and caesium magnetometers which are extremely sensitive and can produce more detailed results in the appropriate conditions.
In order to interpret the results of the ground penetrating radar survey, resistivity survey and magnetometry survey, the surveys are usually carried out by skilled surveyors with a knowledge of the archaeological site and the way it may be expressed geophysically.
A metal detector is a form of electromagnetic device which can be helpful in detecting buried metal objects. The survey and recording of metallic finds can lead to the discovery of important sites.
Archaeologist often work closely with metal detectorists to assist in making records of artefact discoveries. The introduction of the Portable Antiquities Scheme in England enables the voluntary recording of finds by metal detectorists to local archaeologists which has been useful in the recording of finds which previously would have gone unrecorded. One of the great successes of the scheme was the discovery of the Staffordshire hoard of Anglo-Saxon gold and silver metal work.
While the use of a metal detector may assist in locating metal artefacts on a site the finds are usually limited to artefacts in the topsoil or brought to the surface by ploughing as they cannot detect artefacts at great depths. Whilst most metal detectorists are willing to work with and assist archaeologist there are some who vandalise sites and dig holes, removing artefacts without reporting or recording the finds.
Remote sensing is the technique of using ground based instruments, airborne sensors or satellites to assist in the mapping of archaeological sites from a distance.
Radar (RAdio Detection And Ranging or RAdio Direction And Ranging) uses radio waves and relies on its own transmissions rather than light or from electromagnetic waves. It calculates distance by measuring the time it takes for a reflected signal to return. Radar imagery provides a distinctive perspective of the earth’s surface with its microwave illumination and is helpful in the detection of linear features
Light detection and ranging (LiDAR) uses a laser instead of radio waves and has been utilised for complex mapping of areas. It is a surveying method that measures distance to a target by illuminating the target with pulsed laser light and measuring the reflected pulses with a sensor. The laser is usually attached to a plane. LiDAR can be used to measure heights of objects and features on the ground more accurately than with the naked eye or with radar technology. Vegetation remote sensing is a principal application of LiDAR.
LiDAR has to be line of sight, ie – an unimpeded view of the object needing to be scanned. Radar will penetrate the ‘clutter’, ie – atmospheric fluctuations, weather, etc.
Satellite and Airborne Synthetic Aperture Radar (SAR) imaging has been used since the 1980s. The European Space Agency (ESA) constellation Sentinel-1 provides a free and consistently acquired series of SAR images. Satellite radar imaging is one of the techniques used in generating Digital Elevation Models of the topography of all or part of an archaeological site.
Remote sensing is used jointly with other geophysical surveying methods and desk top surveys and is valuable in the discovery of new sites.
Sampling is an intrusive technique in finding archaeological sites and includes the techniques of geochemical analysis, shovel test pits and auger/soil cores. However, they are not as intrusive or destructive as full excavations of an archaeological site.
Geochemical analysis involves taking soil samples at regular intervals from the surface of the site and measuring phosphate content and other chemical properties. Can reveal what processes or activities were carried out past and construct a detailed picture of features of archaeological interest on a site. Investigations of this type are slow.
Shovel test pits are often used with other forms of investigation, ie – aerial photography to test soil/crop marks; field walking to test whether a concentration of artefacts on the surface represent the presence of buried archaeology. The test pit will be marked on a grid of the archaeological site for ease of identification and placement in any future excavations of the site. They can vary in size but are usually square in nature and dug by hand. This method can be time consuming and even though artefacts may have been discovered in the plough soil or top surface of the pit it does not necessarily follow that buried archaeology will be discovered in the pit but may be close by in an area not tested.
Augers are drill like tools used to take samples/columns of sub-surface soil know as cores and can be used to test the depth of deposits and also for extracting peat for pollen analysis. A single drill sample may answer some question regarding the depth of the deposit but when carried out over an area at regular intervals, may give a very useful view of the types and depths of deposits that may be encountered throughout the site.
The disadvantage of soil samples/cores is that the context of the sample is unknown: for example, an auger may penetrate a grave cut or the floor deposit of a prehistoric roundhouse, but a slight smear of different coloured soil may be all that would be visible in the sample.
In conclusion, there is no technique of discovering an archaeological site which out performs the other and several compliment each other to provide a detailed plan of the site and the possibility of finding artefacts and archaeological features. Ultimately it rests with the experience of the chief archaeologist and his team to decide which techniques and methods to use to locate a site so that any future excavations can be carried out in the most cost efficient and time effective manner, as when excavating it is important to know where best to dig to avoid costly mistakes. ‘It is remarkable how much can be revealed about a site without excavation’ (Green, 1991.p42).