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Crop Marks
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.
Soil in a buried ditch or pit is still likely to hold some moisture even though it is now beneath a layer of
uniform ploughsoil. When crops such as barley or wheat grow, their roots find this moisture. Those stalks that are
rooted in this moisture-holding soil 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 rest of the soil is dry, the effect can be
dramatic. 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.
Some crops (e.g. wheat and oats) are particularly sensitive to soil water content and show marks clearly but others
(e.g. grass and potatoes) are insensitive and rarely show them. Additionally, well-drained soils (such as sands and
chalky soils) show these marks better than poorly drained clays. Much of NE England is under permanent pasture or
lies on clay soils so cropmarks generally show up less well in this area than in some other parts of the country.
Learn more:
- D.N. Riley Air Photography and Archaeology (Duckworth, London, 1987)
- D.R. Wilson, Air Photo Interpretation for Archaeologists, Batsford, London, 1982, 2nd Edition Tempus, Stroud 2000.
- http://aarg.univie.ac.at
Return to Aerial Photography.
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