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. Such marks, along with parch marks, soil marks and frost marks, can reveal buried man-made structures that are not visible from the ground.

Crop marks are due to the principle of differential growth. Soil conditions are a key factor influencing vegetation growth. For instance, a buried stone wall can impact crop development above it by redirecting water away from the area and displacing the more fertile soil needed for healthy plant growth. 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 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 as marked differences in tone or colour 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, crop marks are visible only 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 other factors that may affect plant health, can be captured in near infra-red photography.

An alternative approach is thermal imaging, where differential water loss (which is dependent on 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, is further complicated by environmental conditions including temperature variation and relative humidity.

Thermal inertia and differential transpiration/evaporation are involved.