Cropmarks show as differential growth in arable crops, and sometimes grass,  caused by the presence of sub-surface archaeological features. They are most easily visible from the air, and are usually observed using aerial photography.

If the effect of buried features on a crop is favourable, for example with a buried ditch, the crop grows faster and more luxuriantly, producing a positive cropmark, usually a darker green mark or line. Similarly, if the conditions become less favourable in comparison with the rest of the field, for example over a buried wall foundation, the growth is checked and a negative cropmark is created, which usually manifests as a pale mark, often called a parchmark.

Different crops will respond in slightly different ways, according to their root structure.

Cropmark

Positive cropmarks showing the triple ditches surrounding the Roman fortlet at Scaftworth, Nottinghamshire, and clear indications of a larger earlier fort. RR28a just shows as a very faint parchmark between the two dotted white lines

©Eric Houlder LRPS, 2017