Cropmark

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Sketched diagram of a negative cropmark above a wall and a positive cropmark above a ditch

See crop mark for disambiguation.

Cropmarks or Crop marks are a means through which sub-surface archaeological, natural and recent features may be visible from the air or a vantage point on higher ground or a temporary platform. Along with soil marks and frost marks they can reveal buried archaeological sites not visible from the ground.

Crop marks appear due the principle of differential growth. One of the factors controlling the growth of vegetation is the condition of the soil. A buried stone 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 cereal crops, peas, and potatoes.

Differential growth will naturally follow any features buried below. Although the growth differences may appear small close up, from the air the pattern they make is more visible as the small changes can be seen in the context of the normally growing surrounding vegetation as differences in tone or colour. When the sun is low to the horizon, shadows cast by the taller crops can also become visible.

By their nature crop marks 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.

Examples

Examples of archaeological sites where cropmarks have been observed are Balbridie and Fetteresso in Scotland.

In 2009, investigate of crop marks near Stonehenge revealed a variety of 6,000 year old prehistoric subterranean structures.^[1]

Another example is the rediscovery of Altinum, a precursor to the city of Venice, from a combination of visible and near-infrared photos of the area taken during a drought in 2007.

See also

• Archaeological field survey
• Aerial archaeology
• Shadow marks

References

Wilson, D. R . 2000 Air photo interpretation for archaeologists (2nd edn.), London.

1. ^ James Owen (2009-06-15). "Huge Pre-Stonehenge Complex Found via "Crop Circles"". National Geographic. http://news.nationalgeographic.com/news/2009/06/090615-stonehenge-tombs-crop-circles.html. 

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