More sites have been discovered as cropmarks than by any of the other described phenomena, but not all areas produce cropmarks with equal facility. Welldraining soils, such as sands and gravels, produce the best results, though even some clay soils can generate cropmarks under the right conditions. However, the combination of factors which best suit cropmark production - well-draining soil types, dry weather patterns, and extensive areas of cereal production - tend to generate consistent biases in their distribution toward those areas where these factors are more consistently favorable. This potential bias can be further exacerbated by the tendency for those undertaking aerial reconnaissance always to focus on the most productive areas, the so-called ‘honeypot’ effect.
Aerial reconnaissance is good at discovering larger features, such as ditched enclosures, but much less effective for more ephemeral remains such as shallow gullies or posthole structures. This means that there is a further bias in terms of the types of archaeological sites which tend to be recorded and accordingly some periods, such as those involving hunter-gatherer communities, are generally ill-represented in the aerial photographic record. Moreover, even when sites are recorded, the photographic record will be only partial compared with what could be recovered from more detailed work by excavation or possibly by geophysical survey.
Finally, because of the way in which the phenomena are revealed, an aerial photograph will not readily distinguish between different phases of human activity on cropmark sites. Thus, although the two enclosures in Figure 5 Overlap slightly, it is impossible to determine from the photograph which is the earlier.
World History
