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Soilmarks are traces of archaeological features which are visible in ploughed or harrowed fields, often for very restricted periods before the crops begin to grow (they may then produce cropmarks or parchmarks).

Soil marks can occur wherever underlying deposits show on the surface. This may be due to deep ploughing which has turned up material from the sub-soil, or where the overlying topsoil is becoming thin and eroded and long-buried features are starting to show through. In some cases standing earthworks have been ploughed and part-destroyed, but still show as soil marks. They are most easily observed from the air, but may be seen in some cases from the ground, or from high buildings or hillsides. Aerial photographic archives contain thousands of examples.
The most obvious trace of soil marks is a colour difference to the rest of the soil. Depending on the geology of the area, soil marks may show up as brown against a white background (common in chalk areas), white against brown, or darker against lighter tones. Organic or burnt deposits may show up as black or even red. In all cases, the judgement of the archaeologist is the essential factor in interpreting their significance.
Archaeology such as plough-damaged field systems, burial mounds, settlement enclosures, Roman villas and former industrial sites can produce soil marks. Geological features which may be of natural origin, but of potential archaeological significance, may also show as soil marks. An example of this might be a dried-up river channel (known as a palaeochannel), which may subsequently reveal rich waterlogged archaeological deposits in its lower layers, or an area of slightly higher ground above winter flood level on an alluvial floodplain, which may be very hard to detect from the ground but which has attracted settlement for thousands of years.
D.R. Wilson ‘Air Photo Interpretation for Archaeologists’ (Batsford, London, 1982).