Crop marks

Ground monuments that are no longer recognizable above ground, or only with difficulty, become recognizable in the aerial photograph through various features at different times of the year and/or in certain climatic contexts[1].

The following characteristics are generally distinguished:

Soil discolorations

Many ground monuments can usually be identified as soil discolorations in freshly plowed fields in the fall or spring. Pits, wells or ditches were filled after their abandonment by erosion or human hand. This filling then often shows a different color than the still unchanged, surrounding soil. But also paths and former ramparts can be seen in the arable soil, if the now flattened fills were made of foreign material (stone packs, material taken from deeper layers, etc.).

Crop marks

Today's technically highly equipped agricultural industry creates large field areas on which crops are planted very homogeneously. Plant density (e.g. for corn) has also been further increased in recent years. Under optimal conditions, plants grow at the same rate and mature at the same time. However, if there are different soil conditions within an area, this has a varying effect on length growth and fruit maturity, especially during periods of extreme weather. Cereals in particular then show such soil differences through different plant sizes or coloration. But also in corn, rapeseed or soybean can be discovered growth characteristics [2] [3].

A distinction is made between positive and negative crop marks.

If plants grow faster and/or taller than their surrounding plants due to better nutrition, this is called a positive crop marks. This also includes plants that remain green longer than their surrounding, earlier maturing neighboring plants due to better water supply. Such differences are often easily seen in aerial photographs.

Example of positive crop marks of a Late Latène quadrangular enclosure in the maturing winter 
Fig. 1: Example of positive crop marks of a Late Latène quadrangular enclosure in the maturing winter cereals. Recognizable are not only the ditch courses, but also postholes of the interior construction (date of recording 28.06.2014).

On the other hand, negative crop marks result from reduced growth, paler leaf coloration or earlier maturation due to less favorable soil conditions. These can be caused, for example, by wall remains still present underground, floors or old paths.

Example of negative crop marks of a Roman villa rustica in grain
Fig. 2: Example of negative crop marks of buildings of a Roman villa rustica in the grain (date of recording 05.07.2017). In the ripening wheat you can see well the different growth heights and the different coloration of the plants standing on the foundation wall remains of the Roman buildings.

Shadow characteristics

Smaller elevations (such as flattened burial mounds) or slight depressions (e.g., caused by an almost completely filled-in ditch) are often still visible in the aerial photograph when the sun is low. The cast shadow created by the difference in elevation, which becomes particularly prominent when the sun is low, exposes such ground irregularities. Positive and negative crop marks become even more visible through shadow characteristics.

Snow characteristics

Ground monuments can also become visible from the air during the winter months through the formation of frost or snow features. Snow features come in two types. In the first type, the first snow often melts on heat-retaining soil structures (such as walls) while remaining on surrounding soils. It is just the opposite in a dew period: The snow remains longer on the places where the cold is stored longer in the soil, while the neighboring soils are already free of snow.

Another type of snow feature is created by snow drifts. When the wind is stronger from one direction, the snow is carried away and redeposits in places where there is no wind. While in the places where the snow has been blown away, the darker soil becomes visible again, the more windless places (such as depressions or the windward side of burial mounds) are highlighted by a snow accumulation.

Actually to the shadow characteristics belongs the effect that with continuous snow cover and the particularly low-standing sun in the winter half-year elevations and depressions become clearly visible due to the long cast shadows.

Humidity characteristics

Moisture or moisture characteristics form in soil structures with different water storage capacities. As soon as the soil dries again after a rain, the soils with the better water storage remain darker in colour for longer, while the neighbouring soils are already drier and thus lighter in colour.

Example of moisture features of a leveled quadrangular enclosure of the late Latène period
Fig. 3: Example of moisture features of a levelled quadrangular enclosure of the late Latène period (date of recording 08.04.2018). The backfilled ditches of the enclosure are marked stand out as dark bands in the ploughed field due to the higher moisture content.

Water characteristics

Another type of feature occurs during floods or inundations. While much of the ground disappears under the even surface of the water, slight elevations (e.g. of burial mounds) still peek out from the water surface.

[1] A detailed overview with many examples can be found, among others, at Braasch 2005 page 12-37 and Song/Leidorf/Heller 2019 Kap. 2.4
[2] Krause 2014
[3] Krause 2013