Aerial Archaeology in Essex

The Archaeology Section of the County Council is currently carrying out the Essex Mapping Project, which is part of the RCHME’s National Mapping Programme. The project, which in Essex began 1993 and is funded by the RCHME, has the basic aim of mapping, at a scale of 1:10,000, archaeological and historical information visible on aerial photographs. Cropmarks, soilmarks and earthworks are mapped using set conventions, from both available vertical and specialist oblique photographs from a number of sources. Information about the morphological nature of mapped features is then compiled onto a computer database which will allow sites with similar characteristics and topographical setting to be extracted and their distributions plotted. By comparing excavated sites with groups of similar sites identified by the database, it is hoped that archaeologists will be able to make more informed interpretations about the nature of the sites which appear on photographs.

The development of satellite imagery over the last few decades, indicates the enormous technological leaps which have occurred since the early balloon photography carried out only a hundred and fifty years ago. It also attests, however, the continued desire to witness the earth from the aerial perspective, allowing us not only the ability to record the current state of affairs, but to monitor change over time.

In recent years, pioneering work in Essex has helped to develop the use of aerial photography along the coast, and in particular, on the extensive inter-tidal mud-flats. Flying along the coast at low tide, timber fish-weirs, oyster pits, shipwrecks, and hulks of other boats have been recorded in this way. Many of these areas are extremely inaccessible, and aerial photography allows archaeologists to rapidly cover large areas and locate structures which they can then visit on foot.

The appearance of cropmarks are particularly important in Essex for a number of reasons. Almost all of the remains of prehistoric human activity have been levelled by later agriculture, unlike upland areas of Britain, where prehistoric burial mounds and forts survive as earthworks which can be viewed on the ground. Indeed, with around 50% of the total land-use of the county dedicated to arable cultivation, it is clear why the occurrence of cropmarks is important to archaeologists in Essex. The geology of the county also plays an important role, however, as the gravels and sands which are common along the river valleys and coastal plains, are self-draining. This results in moisture in the topsoil draining away through the gravel, unlike heavier soils, such as clay, where moisture is more likely to be retained. The greater differences between the moisture content of the archaeological features and the surrounding soil results in more defined cropmarks. In particular, Tendring district, the Thames terraces and the Chelmer valley are very productive in terms of cropmarks, although in conditions of extreme drought, the boulder clay areas also produce cropmarks which most years never form. Indeed, it is possible that if the current trend of climatic change continues, tending towards longer, drier summers, the heavier clay areas may begin to produce significant numbers of previously unrecorded sites.

Over the last fifty years, the recording of cropmark sites has radically altered our understanding of the extent and complexity of archaeological landscapes in many parts of the country. Many new sites, however, continue to be discovered on an annual basis. Archaeologists must record these sites whenever possible as their appearance lasts only until the crop is fully ripened. While many sites may reappear year after year, some will appear only in the driest conditions, and then may lie under a non-cereal crop which may not produce a cropmark. In addition, continual ploughing, and modern deep ploughing in particular, can erode the buried archaeological sites which cause the cropmarks to form. It is important, therefore, that archaeologists continue to record sites from the air, not only in order to discover new sites, but also because it presents a rapid and efficient method of monitoring land-use changes and other developments which may threaten archaeology.

Mapping and Interpretation

Once archaeological features have been recorded on aerial photographs, they must be mapped in order to understand their relative shape and size. The position, shape and size of cropmarks are plotted onto modern base maps by comparing the position of objects, such as buildings and field boundaries, which appear both on a photograph and the relevant map. This is especially necessary with oblique photographs which are taken at an angle from the aircraft. The perspective distortion of features on such photographs requires rectification in order to appreciate a site’s actual shape. The resulting cropmark plots can be studied either on a site specific, detailed level (i.e. at a scale of 1:2,500), or by viewing a number of sites over a larger area of land (around 1:10,000 or more). This allows comparisons to be made between individual sites and provides information about the position of sites relative to each other, and the natural landscape.

Often cropmarks represent the remains of several periods of history which are built over each other and subsequently levelled by ploughing. In these instances, it is necessary for the interpreter to identify individual elements which are contemporary, usually on the basis of form and orientation. It is then possible to suggest how use of the landscape has developed over time, without excavation. Cropmark interpretation can often call upon other sources of information, such as old maps, place-names or records of artefact finds, which may suggest or support the interpretation of what a particular cropmark might represent.

The study of the shape and size, or morphology, of cropmarks is used in order to suggest the date and function of cropmark sites with particular shared characteristics. The many different shapes and sizes of enclosures which appear as cropmarks, for example, can be studied and similar groups can then be compared with examples which have been excavated and which archaeologists understand. It can then be assumed that enclosures of similar size, shape and with a similar position in the landscape, might have a similar function and be of similar date. The distribution of these enclosures can then be studied in relation to rivers, soils and other archaeological sites in the area. While cropmark plots can be seen as maps of prehistoric, Roman and medieval activity, to aid the study of the archaeology of the county, they are also used to inform archaeologists when sites are threatened by modern developments such as roads and housing.