Cropmark
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Cropmarks or Crop marks are a form of archaeological feature visible from the air. Along with soil marks and frostmarks they can reveal buried archaeological sites not visible from the ground.
Cropmarks appear due the principle of differential growth. One of the factors controlling the growth of vegetation is the condition of the soil. A buried wall for example will affect crop growth above it, as its presence channels water away from its area and occupies the space of the more fertile soil. Conversely, a buried ditch, with a fill containing more organic matter than the natural earth, provides much more conducive conditions and water will naturally collect there, nourishing the plants growing above.
The differences in conditions will cause some plants to grow more strongly and therefore taller, and others less strongly and therefore shorter. Some species will also react through differential ripening of their fruits or their overall colour.
Particularly effective crops that exhibit differential growth well include peas, wheat and potatoes.
Differential growth will naturally define any features buried below. Although the growth differences may appear small close up, from the air these are more visible as the small changes can be seen in the context of the normally growing surrounding vegetation. When the sun is low to the horizon, shadows cast by the taller crops can also become visible.
By their nature cropmarks are only visible seasonally and may not be visible at all except in exceptionally wet or dry years. Droughts can be especially useful to cropmark hunters as the differential growth can become apparent in normally hardy species such as grass.
Cropmarks should not be confused with crop circles which are not created by subsurface factors.