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Cropmarks: How Dry Weather Can Reveal Hidden Archaeological Sites

Across Europe, and the world at large, there are a large number of archeological sites yet to be discovered. Many of these ancient settlements have disappeared from the face of the earth because their structures were made predominantly of wood and earth. Wood decays and any earthen feature has since been eroded by centuries of ploughing. Nevertheless, evidence of prehistoric activity survives below ground, and during spells of extreme drought and heat, these ghosts of the past appear mysteriously through patterns in crop growth. They are known as cropmarks.

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Cropmarks at Grézac, France. Photo credit: cliché J. Dassié/Wikimedia

Cropmarks appear due differential growth in crops caused by the presence of sub-surface archaeological features. For example, a buried stone wall may hinder the movement of water and nutrients causing the plants growing above it to show poor health. Conversely, a buried ditch has deeper top soil than the rest of the surrounding, causing water to naturally collect there. Crops growing above long-gone ditches will grow taller, thicker and greener on account of more nourishment.

The difference in growth is usually very small and not easily discernable from the ground, but from the air a marked differences in tone and color can be seen. Some crops, such as wheat and oats, are particularly sensitive to soil water content and show crop marks clearly but others, such as grass and potatoes, are insensitive and rarely show them. By their nature crop marks are visible only seasonally and some may not be visible at all except in exceptionally wet or dry years.

How crop marks form?

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An ancient settlement with fortification or drainage ditch around it. Graphics by: RCAHMW

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Over the centuries the settlement disappeared and farming took place. Graphics by: RCAHMW

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The ditches got filled in and the soil became deeper in those areas. During intense dry spells crops in the deeper ditches survive because those areas hold more water and nutrients. Graphics by: RCAHMW

Recently, extreme dry spell in the UK revealed traces of a previously unknown henge in Ireland, only 750 meters away from the famous Newgrange monument. The 200-meter-diameter henge is thought to date from the late Neolithic period, up to possibly the Bronze Age, from about 3,000 BCE. According to BBC, no plans are being made to dig on the site, which is a working farm. And once the crop is harvested, the image will disappear.

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Meanwhile, in Clumber Park in Nottinghamshire, the outline of an 18th century estate has appeared. The building that once stood on these imprints is Clumber House, that got destroyed by a series of fires and was demolished in 1938. According to the National Trust, who owns the park, the rooms and corridors became visible as stone foundations left in the ground heated up more quickly than surrounding material, scorching the soil above to a lighter shade. Although the mansion itself is not a new discovery, a previously unknown sundial in front of the lawn has been identified.

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In Gawthorpe Hall, Lancashire, the dry weather has brought back a Victorian garden. While the garden is known to appear in most summers, this year the revelation was spectacular and possibly parts of a previous garden has also been exposed.

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The almost ploughed-down medieval castle mound at Castell Llwyn Gwinau, Tregaron, showing clearly under parched conditions. Photo credit: RCAHMW

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Newly discovered crop marks of a prehistoric or Roman farm near Langstone, Newport. Photo credit: RCAHMW

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Extensive crop marks of Trewen Roman farmstead or villa, Caerwent, Monmouthshire. Photo credit: RCAHMW

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The buried ramparts of Cross Oak Hillfort, Talybont-on-Usk, showing as crop marks in Powys. Photo credit: RCAHMW

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