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, [1] soil marks and frost marks, can reveal buried archaeological sites that are not visible from the ground.
Crop marks are 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 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. [2] Pre-parching stress in crops and grass, and others 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 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. [3] 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.
Thermal inertia and differential transpiration/evaporation are involved. [4]
The usefulness of cropmarks to archaeologists has largely been a fruit of inspection from aircraft, but the possibility was suggested by Rev. Gilbert White in The Natural History of Selborne (1789), in a note appended to his Letter VI, to Thomas Pennant, apropos of local people's success in searching for bog oak for house construction, by discovering these trees "by the hoar frost, which lay longer over the space where they were concealed, than on the surrounding morass." To White it suggested the query "might not such observations be reduced to domestic use, by promoting the discovery of old obliterated drains and wells about houses; and in Roman stations and camps lead to the finding of pavements, baths and graves, and other hidden relics of curious antiquity?"
Examples of archaeological sites where cropmarks have been observed are Balbridie and Fetteresso in Scotland.
In 2009, investigation of crop marks near Stonehenge revealed a variety of 6,000-year-old prehistoric subterranean structures. [5]
Another example is the rediscovery of the Roman city Altinum, a precursor to the city of Venice, from a combination of visible and near-infrared photos of the area taken during a drought in 2007, which stressed the maize and soy crops. [6]
The multi period site at Mucking was discovered as a result of aerial photographs showing cropmarks and soil marks. The earliest photographs to reveal the site were taken by the Luftwaffe in 1943. [7] The importance of the site was recognised following photographs taken by Kenneth Joseph in 1959 [7] (published in 1964). [8] In 1982, Margaret Jones (site director at the Mucking excavation) said that some sites were being interpreted on crop mark evidence alone. She said that some features do not produce crop marks and that some crop marks, when excavated, turn out not to be what they seem. [9] [ full citation needed ]
Stonehenge is a prehistoric monument on Salisbury Plain in Wiltshire, England, two miles (3 km) west of Amesbury. It consists of an outer ring of vertical sarsen standing stones, each around 13 feet (4.0 m) high, seven feet (2.1 m) wide, and weighing around 25 tons, topped by connecting horizontal lintel stones. Inside is a ring of smaller bluestones. Inside these are free-standing trilithons, two bulkier vertical sarsens joined by one lintel. The whole monument, now ruinous, is aligned towards the sunrise on the summer solstice. The stones are set within earthworks in the middle of the densest complex of Neolithic and Bronze Age monuments in England, including several hundred tumuli.
Woodhenge is a Neolithic Class II henge and timber circle monument within the Stonehenge World Heritage Site in Wiltshire, England. It is 2 miles (3.2 km) north-east of Stonehenge, in Durrington parish, just north of the town of Amesbury.
Aerial archaeology is the study of archaeological remains by examining them from a higher altitude. In present day, this is usually achieved by satellite images or through the use of drones.
The Goseck Circle is a Neolithic structure in Goseck in the Burgenlandkreis district in Saxony-Anhalt, Germany.
Shadow marks are a form of archaeological feature visible from the air. Unlike cropmarks, frost marks and soil marks they require upstanding features to work and are therefore more commonly seen in the context of extant sites rather than previously undiscovered buried ones.
In archaeology, survey or field survey is a type of field research by which archaeologists search for archaeological sites and collect information about the location, distribution and organization of past human cultures across a large area. Archaeologists conduct surveys to search for particular archaeological sites or kinds of sites, to detect patterns in the distribution of material culture over regions, to make generalizations or test hypotheses about past cultures, and to assess the risks that development projects will have adverse impacts on archaeological heritage. The surveys may be: (a) intrusive or non-intrusive, depending on the needs of the survey team and; (b) extensive or intensive, depending on the types of research questions being asked of the landscape in question. Surveys can be a practical way to decide whether or not to carry out an excavation, but may also be ends in themselves, as they produce important information about past human activities in a regional context.
In archaeology, a ring ditch is a trench of circular or penannular plan, cut into bedrock. They are usually identified through aerial photography either as soil marks or cropmarks. When excavated, ring ditches are usually found to be the ploughed‐out remains of a round barrow where the barrow mound has completely disappeared, leaving only the infilled former quarry ditch. Both Neolithic and Bronze Age ring ditches have been discovered.
Altinum was an ancient town of the Veneti 15 km southeast of modern Treviso, close to the mainland shore of the Lagoon of Venice. It was also close to the mouths of the rivers Dese, Zero and Sile. A flourishing port and trading centre during the Roman period, it was destroyed by Attila the Hun in 452. The town recovered, but was later abandoned when sea-borne sand began to cover it over. Its inhabitants moved to Torcello and other islands of the northern part of the lagoon.
Crop mark may refer to:
Mucking is a hamlet and former Church of England parish and civil parish adjoining the Thames Estuary in the Thurrock unitary authority area, in southern Essex, England. It is located approximately 2 miles (3.2 km) south of the town of Stanford-le-Hope. In 1931 the civil parish had a population of 498.
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Wilsford Henge is the site of a Neolithic henge, west of the village of Wilsford, Wiltshire in the United Kingdom. The site was discovered from cropmarks in aerial photographs. The monument lies within the Vale of Pewsey, a short distance south of the large henge known as Marden Henge.
Margaret Ursula Jones was an English archaeologist, best known for directing major excavations at Mucking, Essex.
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