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Parch Marks and Soil Marks
Parch marks are caused by thinner crop growth over solid features such as masonry showing up in aerial photographs, and depend on thin soil and dry weather conditions to create a contrast between the area over the archaeology and the rest of the field, park or garden. The hot summers of 1975-76 and 1994-95 in Britain were especially good for producing parch-marks, and many new sites were discovered.
Parch marks are most commonly caused by buried stone or brick structures such as walls or paved areas. The stone inhibits the crop or grass roots in the overlying topsoil and the result is an area of weak growth that can show as a white or brown mark reflecting the shape of the archaeological structure underneath. In some cases, complex plans of villas, churches, Roman roads, long-demolished houses and industrial structures can be revealed in this way. As with cropmarks, further investigation using geophysics can reveal even more detail before any excavation takes place.
Soil marks are traces of archaeological features visible in ploughed or harrowed fields, often for very restricted periods before the crops begin to grow (they may then produce cropmarks or parchmarks). They can occur wherever underlying deposits show on the surface, due to deep ploughing turning 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.
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.
Archaeological features such as plough-damaged field systems, burial mounds, settlement enclosures, Roman villas and former industrial sites can produce soil marks. Geological features 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.
Both parch marks and soil marks 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.
Learn more:
- D.R. Wilson, Air Photo Interpretation for Archaeologists, Batsford, London, 1982, 2nd Edition Tempus, Stroud 2000.
- http://aarg.univie.ac.at
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