Why Does a Buried Wall Show Up in a Field of Wheat? The Crop Knows

A crop growing over buried archaeology does not grow uniformly, because what is beneath the plough soil affects how much water the roots can reach. A buried ditch or pit is filled with softer material holding more moisture and more nutrients than the surrounding subsoil, so plants above it grow taller, greener and ripen later. A buried wall or road surface leaves less soil above it and a barrier to roots, so plants above it grow shorter, thinner and ripen earlier. Those differences are almost invisible standing in the field and are obvious from directly above.

The effect appears only when several things coincide:

• •A crop that responds visibly, with cereals the best
• •A dry season, so the difference in available water matters
• •Free-draining subsoil, such as gravel or chalk
• •The right stage of growth, frequently a narrow window
• •Low sun or the right light, for photographing the difference in height
• •Somebody in the air at the time

Exceptional summers produce exceptional results, and the discipline has a list of them. The drought of 1976 in Britain revealed enormous numbers of previously unknown sites as crops failed differentially across buried features, and the flying done that summer transformed the archaeological record of several counties. The summer of 2018 did it again, revealing sites in Wales, Ireland and England including a previously unknown henge and numerous settlements, and the images circulated widely. The reason is straightforward, since a wet year gives every plant enough water and the differences disappear, while a dry year makes the buried feature the only source of the difference.

Two other kinds of mark reveal buried features and knowing them together helps in reading an aerial photograph. Soil marks appear on bare ploughed ground where the plough has brought up material of a different colour from a buried feature, showing as pale or dark patches directly, and they need no crop at all. Parch marks appear in grass rather than in a crop, where shallow soil over a buried wall dries out first and the grass browns above it, which is what revealed several features on famous lawns during recent dry summers. Frost and snow marks appear where differing ground temperature makes snow linger or melt in a pattern.

Decades of aerial photography have built an archive that is enormously valuable and systematically biased, and knowing how matters. Sites show up on free-draining soils and not on heavy clay, so the distribution of known sites reflects geology and flying effort as much as where people actually lived. Areas under permanent pasture, woodland or buildings produce nothing. Features appear only where a crop is grown. The bias is well recognised and other methods now fill gaps, with airborne laser scanning working through woodland, and geophysical survey and satellite imagery adding coverage where nothing grows.

A buried ditch holds more moisture so the crop above grows taller and greener, and a buried wall holds less so the crop is shorter and ripens earlier, differences that are invisible from the ground and obvious from above. They appear only in dry seasons on free-draining soil at the right growth stage. The resulting archive is biased towards those soils and towards arable land.