The Formation of Cropmarks
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 visible differences in crop colour and height during various stages of the growing season.
The aim of the sequence below is to show a site that saw two main phases of activity – in the Iron Age and in the Roman period. A third illustration indicates which of the features are most likely to prompt the appearance of cropmarks, given the right conditions, while the fourth shows what would be revealed if an excavation trench was opened across the site. This last illustration also shows the effect that different types of buried feature can have on the crop growing above them.
Here we see a small Iron Age (around 800BC to AD43) settlement site – a circular enclosure defined by a ditch with an internal bank. A timber fence or palisade runs along the top of the bank. Inside the enclosure is a single post-built roundhouse, its entrance facing towards the sole entrance through the enclosing earthworks. The house is surrounded by what is often referred to as an eaves-drip gulley – a relatively narrow, shallow ditch intended to catch rainwater running off the roof. On either side of the roundhouse is a four-post structure – these are generally interpreted as representing above-ground storage for grain or other agricultural produce.
Jumping forward a few centuries into the Roman period (AD43-410). The roundhouse has gone, replaced by a rectangular house with solid stone foundations, rather than the series of spaced earth-fast posts that would have supported the roundhouse. The enclosure ditch is shallower and has become grassed over, while the bank is a little lower and less steep-sided – these are largely the results of natural erosion processes over time, although episodes of deliberate backfilling of ditches are often encountered in excavations. A series of fences separate farmhouse, garden plots and fields.
Over the millennia, the buildings fall into ruin. Eventually, the land is turned over to farming and ploughed for many years. This gradually removes all surfaces traces of the remains.
Fast forward to the 21st century AD. The entire area is under crop, and ground conditions are right for cropmarks to appear. Centuries of ploughing mean that there is no physical trace of the bank and ditch, or other features, on the surface. The two dark green circles represent cropmarks forming above something cut or dug into the ground – ditches, in this case. By analogy with excavated sites, we would expect the outer circle to represent an enclosure ditch, and the inner one to probably represent a roundhouse, both probably of Iron Age date. The rectangular feature clearly represents something else – the nature of the cropmark suggests something solid – probably building foundations – while the fact that one corner cuts into the roundhouse indicates that the rectangular feature is later, probably Roman, in date.
An excavation trench has been opened over the site, large enough to include elements of all the main features visible as cropmarks. The nature of the cropmarks can clearly be seen – above the ditches, the crop is a little taller, its roots reach a little deeper, and it is ripening a little later than its surroundings. Over the solid building foundations, the roots are shallower, the crop is slightly stunted in height, and is ripening quicker than in the surrounding area. The trench has also revealed the many, slighter, features that have not produced a cropmark – the postholes of the various fences and the roundhouse for example. Many roundhouses were not accompanied by a surrounding gulley, making them virtually invisible to aerial photography.
The Science Behind it All
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 subsoil, with the better-draining soils on gravels or chalk, 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 archaeological remains. 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 in growth that can resemble the shapes and forms of particular types of archaeological monuments. ‘Natural’ causes include patterns in crops forming above geological features such as fissures and frost-cracks.
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