Crop marks

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 parch marks,soil marks and frost marks can reveal buried archaeological sites not visible from the ground.

Crop marks appear due to 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. The drought of 2010 produced particularly good conditions for observing crop marks in the UK. Pre-parching stress in crops and grass, and others factors which may affect plant health, can be visualised in the near infra-red.

An alternative approach is thermal imaging, where differential water loss (which is dependent of the availability of water at the roots) can create temperature differences which result in thermal crop marks that are potentially visible at any time during crop growth. Thermal imaging can also reveal archaeological residues as a result of thermal inertia (storage heater effect) or differential evaporation. The interaction of the processes involved can be complex and the prediction of optimal imaging time, for a given site, further complicated by environmental conditions including temperature variation and relative humidity.

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Wikipedia