Why Can We Still See Marks of Sites Lost Thousands of Years Ago?

To the west of Biggleswade in Bedfordshire is a medieval stronghold. This fortified site was built after the Norman Conquest, possibly during 'The Anarchy', an early 12th century period characterised by the breakdown of law and order, as royal rivals Matilda and Stephen fought over the English throne. Situated on a gravel island, the site overlooked the Ivel Valley, a major communication route in the Middle Ages.

Today thousands of people drive past the site on the A1, the modern route through the valley, unaware of a monument that played an important role at a turbulent time in England's past. So how do we know that this fortification once existed?

An aerial discovery

The site was located in 1954 by Professor Kenneth St Joseph of Cambridge University, who sighted the castle not on the ground, but from the air. Flying over a field, he saw marks in the crops growing over the buried ditches that indicated that a fortified medieval site known as a ringwork-and-bailey castle was present. The site became protected as a scheduled monument 12 years after Professor St Joseph's discovery.

'Cropmarks' like these form when plants over buried archaeology grow at different rates and heights compared to those in the rest of the field. The difference in crop growth is accentuated during periods of dry weather as the plants take advantage of extra moisture trapped in the organic material that has built up in the ditches over time. The best way to see these crop patterns is from the air.

What can be seen?

In the above photograph, taken in July 2010, the ditches of the ringwork-and-bailey castle are visible as green patches in a field of barley. The ringwork would appear to have been a circular platform surrounded by wide ditches. An inner bank was probably topped with a wooden palisade, making the ringwork the main defensive position of the site, containing the most important buildings.

The ditches of the bailey can be seen extending to the top left and bottom right, forming an oval enclosure. There are only 60 ringworks with baileys in England, making this Bedfordshire site a rare survival and so nationally important.

The scheduled area also contains features from the late Neolithic and early Bronze Age periods (2400-1500 BC), but they were difficult to see from the air in the summer of 2010 when the photo was taken. Weather conditions, soil moisture, soil preparation, and crop and soil types all play a part in how well or badly cropmarks develop, and so our aerial photography team have no guarantee that the marks will be visible from one year to the next.

Despite this, hundreds of new sites are discovered from the air every year, and aerial photography is now a central visual resource for archaeological assessment. Once a site is found, our Designation Department considers whether it meets the criteria for national protection. Some, like Professor St Joseph's discovery 60 years ago, do and so are nationally scheduled.

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