Archaeological air-photography, recording the texture of past human activity in the landscape, can be a tremendously exciting exercise. Crop-marks reflecting the differential growth of crops over buried archaeological features or structures which retain more or less moisture and nutrients than the natural undisturbed soils, need not necessarily result from features created in antiquity. Flying last year with the expert air-photographic pilot Anthony Crawshaw we saw these splendid patterns in fields towards the western end of the Vale of Pickering. The example below with radiating lines extending out from central dark features does not represent the landing pads for alien spacecraft, or for that matter the launching stations for flying pigs, but the radiating paths from central feeding stations for outdoor pigs. The crop-mark reveals compressed paths followed by the pigs which has retarded growth in the crop, there is also some stratigraphy here as well with overlapping features showing changes in the layout of the pig-pens over time.
Crop-marks can also be caused by variations in the subsurface geology. Sometimes these marks are simply very attractive, as in the example below. This has the appearance of a Middle-Eastern Prehistoric settlement with interconnected mud-brick buildings contained within a walled enclosure or lots of tiny fields. The crop-mark here reflects cracks in the underlying limestone geology.