Archaeological Techniques

Invariably, most archaeological landscape evaluations will use a combination of proven methods that will progressively focus upon and define areas of known or potential archaeological interest. The following methods are presented in what is considered to be the most appropriate order of application in field evaluation. The various methods are able to provide different types and levels of information and are critically reviewed with respect to these.

The reporting of each stage may be carried out separately, especially if the various works are executed over a long period of time and/or by different organisations. Ideally the work will be carried out, or at least coordinated, by a lead archaeological organisation that can bring the results together in an appropriately synthetic and coherent way. 'Standards' for assessment, evaluation and excavation work have been produced by the Institute of Field Archaeologists^{212, 213, 214, 215, 216}. However most SMRs will wish to either issue their own specification for the work or endorse a project design or written scheme of investigation prepared by a consultant or archaeological contractor. A summary of the various methods and the ideal sequence of their application are presented in Tables 4 and 5.

Assessment

Two types of off-site assessment are considered: rapid desk-based appraisal and desk-based assessment. Air photograph assessment, plotting and interpretation might ideally form part of a desk-based assessment. However it may equally be viewed as a form of non-intrusive remote sensing that complements the on-site assessment/evaluation techniques of geophysical survey and fieldwalking.

Rapid desk-based appraisal

This has been defined as 'a rapid reconnaissance of the site and records to identify whether a proposal has a potential archaeological dimension requiring further clarification'²¹⁷. The work might typically be limited to a rapid inspection of the SMR holdings to establish whether there are major historical or archaeological issues to be addressed. However, rapid appraisals will not suffice for the purposes of providing a comprehensive baseline assessment that will form the 'Heritage' section of an EIA.

Desk based Assessment

A detailed assessment of archaeological knowledge and potential of all periods can be acquired relatively rapidly for a large landscape area and will form the basis of the 'Heritage' section of an EIA. The assessment should be a thorough collation and review of all extant archaeological and historical information relating to the proposal area and its surrounding area. Typically it would involve the interrogation of the SMR, NMR, local archives and museums services, as well as local studies and archaeological society libraries.

Cartographic evidence can inform upon the exact locations of sites in the last 200-300 years. The distribution and pattern of known sites outside the proposal area may be quite instructive in assessing the likelihood of archaeological sites within it. The assessment will include the interpretation and plotting of features from air photographs and old maps. It may also include information derived from borehole logs and test pits. A 'standard' for carrying out desk-based assessments has been provided by the Institute of Field Archaeologists²¹².

A desk-based assessment may include a rapid walk-over survey of the proposal area to assess the state of known or potential monuments. It may also be used to assess the feasibility of employing further evaluation techniques by recording land-use, ground cover and the positions of pylons and overhead powerlines.

A desk-based assessment should determine the exact nature and location of further necessary evaluation work required to inform the preparation of a mitigation strategy. It should be carried out well in advance of any further work taking place on the site.

Non-Intrusive Prospection And Evaluation

Air Photographs

Two forms of air photography have mainly been employed for archaeological reconnaissance: vertical and oblique. The higher altitude vertical sequences provide comprehensive cover, but are of limited value, being most useful in identifying prominent earthworks in uncultivated upland areas. Far more rewarding are low-level oblique photographs that can identify subtle manifestations such as shadows cast by differential crop growth (which have then been marked in Figure 1), different ripening rates, parching or differential thawing of frozen ground.



The best results are achieved on cultivated land where buried archaeological features enhance or restrict root penetration and moisture uptake causing localised differences in rates of growth or ripening. This effect can be enhanced by drought conditions and consequently the years 1976 and 1996 have proved to be the best for cropmark recording²¹⁸. The theory and limitations of the method have been discussed in detail elsewhere^{219, 220}.

Air photography has the potential to provide rapid assessment of large tracts of land, particularly those with a history of grain crop cultivation. However, the availability of suitable air photographs is subject to flying sorties having been made at the right time of year over the right types of crops in the right sort of weather. Positive results will invariably take the form of ditched linear features and enclosures or large discrete anomalies, whilst small discrete features and unenclosed settlement sites go undetected. The method is of limited use for pasture sites.

Fieldwalking

The method involves a systematic walk-over of the proposal area, collecting and plotting the distribution of significant artefacts on a grid basis^{221, 222} (Figure 2). For very large sites linear traverses between 10m and 20m apart will be appropriate with collections being made at 10m and 20m intervals respectively.



The significance of any finds will depend to some extent upon what is already known about the area and the date of the finds in question. In very broad terms prehistoric and Roman finds will be significant in any concentrations if there is no previous evidence for activity at those periods.

Typically, a prehistoric find would consist of flint, fire-cracked pebbles and possibly ceramics, whilst Roman material would probably consist predominantly of pottery and building materials. For later medieval and post-medieval periods the significance of fieldwalking results will largely be based upon concentrations of finds. Night-soiling of the fields from these periods onwards has resulted in many cultivated landscapes having a background of artefactual rubbish mixed into it. Only where higher relative concentrations of finds are recovered might these be considered significant (e.g. settlements and industrial/production sites).

With the right strategy fieldwalking can evaluate large areas at relatively low cost. It has the potential to generally locate and date settlement and production or manufacturing sites. For unenclosed or truncated sites, particularly those of prehistoric date that evade detection by other remote sensing techniques, this is the only non intrusive method of detection in the field. However, areas to be walked must have been recently ploughed. Used on its own this method might give a false impression, particularly in terms of negative results, artefact visibility and sample bias.

Early prehistoric sites are particularly underrepresented, whilst many late prehistoric and Roman rural sites have low finds counts from open area excavation. It is possible that many of these would not be detected by fieldwalking. Indeed most are initially detected by air reconnaissance or geophysical survey. Whilst distributions of artefacts provide an indication of the general area of a site, the exact locations of in situ remains may not be reflected by artefact distributions because of the migration of these due to long-term ploughing or topsoil movements.

Earthwork and Photographic Survey

Earthwork survey will only generally be employed when field inspection has identified the existence of remains of a former settlement or land boundary that could be lost. Depending upon what may be specified, the survey may take the form of a contour survey or, more traditionally, involve annotation by hachures indicating the subtle changes of slope that might not be picked up by contouring.

Geophysical Survey

Of the various geophysical survey methods available only four are regularly applied in the evaluation of rural landscapes^{223, 224, 225}; these being magnetic susceptibility, magnetometer scanning, detailed magnetometer survey and resistivity survey. The latter two are those most commonly used while ground-penetrating radar does not see much use in large-scale rural situations.

There remains some uncertainty as to why the iron oxides in soils have different levels of induced magnetism, though the most popular theory is that it is fire induced. What is clear, however, is that anthropogenic effects do increase magnetic susceptibility. This means that the relative measurement of it over an area has potential for broadly identifying areas of intense past human activity. Measurements can be made quite rapidly (Photo 4) in the field on a 10-20m grid, the results being plotted as contours of shaded blocks (Figure 3). The method is highly suited to providing evaluations of large areas. It may indicate the presence of unenclosed sites, invisible to air reconnaissance or geophysical survey, or sites with a low finds count that might otherwise go undetected by fieldwalking. Alternatively, it could help nominate target areas for the more detailed investigation techniques.





Against this the results on their own offer no precise information regarding the nature and date, or indeed the reality, of the potential site and the method should not be used in isolation from other evaluation techniques.

Magnetometer scanning (Photo 5) and magnetometer area survey detect local magnetic anomalies caused by the magnetically susceptible components of buried archaeological deposits. Most typically these are the fills of deep cut linear or curvilinear features defining former fields, enclosures, features and deposits resulting from intense burning (e.g. kilns and hearths). Surveys are conducted using a fluxgate magnetometer with digital data storage and transfer facilities that enable survey work to be conducted at walking pace. For scanning purposes, two or more operators traverse a site at 15-20m intervals plotting the presence/absence and intensity of magnetic responses, as well as the concordance of responses between separate instruments.



Like magnetic susceptibility, the method is fast and highly suited to dealing with large areas. It can help nominate target areas for the more intensive magnetometer area survey by giving a general indication of the presence/absence and intensity of magnetic anomalies. Results can offer some detail on the nature, orientation and extent of potential archaeological features and sites. However, the method cannot be used in isolation from other evaluation techniques as scanning results alone can be open to misinterpretation and the interval between traverses is large enough to miss smaller discrete features. The method can be less effective on certain types of geology (particularly igneous rocks and some drift deposits).

Using the same principles and equipment as for magnetometer scanning, detailed area survey is a much more intensive geophysical investigation, usually conducted on a 20m grid basis, in survey blocks of no less than 3600m². Traverses within 20m grids are at 1m intervals with reading being taken at 0.5m intervals (800 readings per grid; 20,000 readings per ha). Data are computer processed by dedicated software packages and presented graphically as either grey scale contour, dot-density, 3D-wireframe or X-Y plots.

Detailed magnetometer survey has the advantage of being a precise and rapid method with the potential to map the deep-cut linear and heat affected features of a site (Figure 4). It can be particularly useful in evaluating sites where air reconnaissance is absent, or for enhancing and complementing cropmark plots from air photographs. Whilst the plotted data cannot date a site, the mapped anomalies can be diagnostic, or at least suggestive, of certain period type-sites. Detailed magnetometer survey is the most regularly applied method of ground-based remote sensing in archaeological evaluation and is usually the 'prime consideration' in geophysical evaluation ²²⁵.



The effectiveness of this method does vary for different types of geology. The method is generally very effective on sedimentary and metamorphic geology, but can be variable over drift deposits, responses usually being dependent upon the magnetic mineralogy of the parent rock. Results are invariably poor on igneous geology²²⁵. Results will also be upset by sites with a large component of ferrous metal debris in the soil, as well as service pipes, cables and pylon bases (although overhead power lines are not usually a problem). The method will not detect small features such as pits, post-holes and cremations.

The resistivity method detects buried features by systematically measuring local changes in electrical resistance of the soil over a site. Walls, roads, paths and other stone-made or void features do not conduct electricity and therefore have high electrical resistance. Conversely, moisture-retaining deposits such as ditch fills, are relatively good conductors of current and have low resistance. The technique logs data on a similar grid basis to magnetometry, but because it involves placing probes into the ground at 1m intervals it is consequently slower. Whilst it cannot be used for scanning in the same speedy way as magnetometry, it can be employed in limited traverses or strips to identify the course of say a Roman road or other large responsive linear features.

Whilst the method will detect a range of features in ideal circumstances, it is best employed for the detection of buried stone and brick buildings, solid floors and courtyards and roads. The method consequently sees much use on, or in the search for, ecclesiastical and fortified sites. It is slower than magnetometry and consequently more expensive, although technological advances are gradually increasing the speed and efficiency of the method. It will generally not detect the presence of small cut features such as gullies, pits and post-holes.

Metal detecting has generated a long running debate in archaeology, mainly because much of the work carried out in the past has been by hobbyists and might be more appropriately termed 'treasure hunting'. Yet the method does, in the right hands, and with the right strategy, have an important role to play in the detection of hitherto unknown, or poorly located, archaeological sites. Whilst it would be impractical to apply the method intensively to a very large landscape of unknown potential, results can prove quite revealing when the method is employed expediently in targeted fashion following initial scanning by fieldwalking or geophysical techniques.

Metal detecting has proved to be a particularly useful tool in complimenting ploughsoil assemblages from fieldwalking, revealing the location of former settlement, industrial/production sites and the identification and interpretation of intangible battlefield sites²²⁶.

Intrusive Evaluation

Trial Excavation

Intrusive evaluation is ultimately the only sure way of establishing the detail of potential archaeological sites or features indicated through non intrusive, remote sensing techniques. Depending upon the prevailing circumstances, trial excavation could take the form of small hand-dug test pits or larger machine stripped trenches. It is certainly advisable to carry out some testing of certain geophysical 'field' patterns as, on some geology, magnetometer data can give similar responses for older post- medieval land drainage regimes as it does for Iron age /Roman British enclosure complexes²²⁷.

For uncertain targets it will be necessary to implement a strategy of open area sample stripping in order to stand some chance of establishing the extent and nature of the underlying site. Many unenclosed sites, typically those of earlier prehistoric date, are represented by small post-hole arrays and shallow gullies that do not show up as cropmarks or geophysical anomalies. The existence of such sites will only ever be realised by excavation.

Trial excavation (Photo 6) can confirm the existence (or non-existence) of a purported site and possibly establish its extent, date and sensitivity to proposed land uses. Moreover, it will provide more precise information upon which to base further mitigation proposals.



Ideally trial trenching should be carried out before the application in order that the conditions of the planning consent are informed and appropriate (Figure 5). However, the work, which usually involves up to 5% of the site being stripped, and up to 10% if no other evaluation methods are employed²²⁸. Thus the method can require the temporary removal of large areas of ploughsoil, invariably at inconvenient times agriculturally, and so is often deferred. A 'standard' for carrying out evaluations has been provided by the Institute of Field Archaeologists²¹³.