The Economist | Nothing, beside remains

Aerial investigators from the Royal Commission on the Ancient and Historical Monuments of Wales have discovered a Roman fortlet beneath a field in Magor, along with the ruins of an ancient farm not far away in Langstone. Historic England, another public body, has dispatched aerial reconnaissance teams from bases in York and Swindon to scour the country for Roman villas and prehistoric forts.

They are on the hunt for “crop marks”, differences in plants’ colour or height that are caused by what lies beneath the surface. Natural and man-made grooves in the subsoil or bedrock hold more moisture than the undisturbed ground around them and are usually filled with richer soils. This means that, in fields levelled by ploughing, crops growing over archaeological remains have access to more water and better nutrients. When pastures are parched, as now, they stay greener and grow taller—as seen in the photograph below of a prehistoric settlement near Eynsham, in Oxfordshire. Conversely, crops growing over old stone walls struggle to find water and wilt in the heat, to form what are known in the jargon as “negative” crop marks

Read more at The Economist