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Cropmarks show as differential growth in arable crops caused by the presence of sub-surface archaeological features.  They are most easily visible from the air, and aerial photography has recorded many thousands.  Sometimes they are so clear that they can be seen from the ground or from high buildings or hillsides.

Where a buried ditch or pit exists, such as an ancient field boundary, a defensive ditch or moat, or a ring-ditch around a burial mound, it is likely still to hold some moisture even though it is probably now beneath a layer of uniform ploughsoil.  When crops such as barley or wheat grow, their roots find this moisture.  The stalks which are rooted in ancient moisture-holding features grow slightly quicker and taller than the rest of the crop.  Sunlight, particularly early or late in the day when the sun is low in the sky, can cast a very clear shadow over the different crop heights, revealing the shape and size of the buried archaeological features. Sometimes cropmarks extend over many square miles and whole multi-period landscapes of super-imposed features (‘palimpsests’) are visible.  Archaeological cropmarks should not be confused with ‘crop circles’ which are a modern, created phenomenon (whatever the identity of their creators!).

Aerial photography can record cropmarks, and their ‘mirror image’, parch-marks very effectively. Using manual or computer methods, the photographed evidence can be ‘rectified’ (meaning that any distortion in shape caused by the angle of photography is removed) and the result plotted on a base-map. 

Good cropmarks depend on the amount of contrast between moisture present in archaeological features and within the rest of the soil.  Where the soil is generally wet throughout, they do not show up well.  But where the soil is dry, the crops growing in the damper ditches ripen later sometimes producing a dramatic colour contrast between those growing on the dryer soil.   For this reason, very dry summers are often the best time to see cropmarks: the summers of 1975-76 and 1994-95 were particularly good in Britain.  Because cropmarks tend to show up differently each year, repeated photography is good practice.