Why should traces of something that happened centuries, or perhaps millennia, ago be visible today in a field of, say, wheat, barley, or peas?
Buried archaeological features can affect the rate of growth of crops planted into the soil above them. Ditches, pits and other features dug into the subsoil have, over the centuries, become filled by a variety of means. They provide a greater depth of soil than can be found in their immediate surroundings, something that can lead to enhanced growth of the crop immediately above them. Alternatively, a reduction in soil depth caused by the presence of, for example, buried wall foundations, or compacted surfaces such as floors or Roman roads, can inhibit growth. From above, the patterns created can be observed from the differences in crop colour and height during various stages of the growing season.
The key factor in all this is the amount of moisture retained within the soil. A lack of moisture, generally caused by a lack of rainfall before and during the growing season, can exaggerate the effects of greater or lesser soil depth, making archaeological and natural features easier to spot. Visibility varies considerably according to the nature of the soil and sub-soil, with the better-draining soils on chalk or gravels, for instance, more likely to produce cropmarks than clay soils, which tend to be better at retaining moisture. In addition, the type of crop can play a part, with cereals being the most likely to produce distinct cropmarks.
There are a number of potential pitfalls to be borne in mind, in particular the fact that variations in soil depth can occur for a number of reasons, not all of them due to human intervention or of archaeological interest. Pipelines, land drains, recently-removed field boundaries – all can produce cropmarks, while agricultural practices such as irrigation and weedkilling techniques can also affect crop growth, often producing circular or linear variations that can resemble the shapes and forms of particular archaeological monuments. ‘Natural’ causes include patterns in crops forming above geological features such as fissures and frost-cracks.
When there is no crop currently planted and growing, the observation of ploughed fields can still be productive, as the action of the plough reveals differences in colour and texture between the infill of ditches, the material used to construct banks or walls, and the surrounding soil, a phenomenon that also demonstrates the destructive nature of the processes that the aerial archaeologist has to rely on.