What does aerial photography tell us about the Roman advance?

For those unfortunate enough to witness its terrifying onslaught, the passage of Rome’s campaigning army must have been striking. Smoke curling up where villages had once stood, crows circling sites littered with corpses, the detritus left by the passage of thousands of soldiers. But almost two millennia later, it is much harder to track Rome’s murderous routes through northern Britain.

Yet, there are clues. We have just discussed the literary sources, noting that while they are essential reading, they have multiple limitations. And of course ancient writers did not really seek to give a detailed account of strategy or topography. They did not desire to act as war correspondents.

The fast pace of campaigning armies can mean that little endures of their passing for archaeologists to find. But Roman soldiers built temporary camps as they advanced into Scotland. Few of these contained internal buildings (tents sufficed) but ramparts were required to keep the enemy out and deserters in. These ramparts and their accompanying ditches survive. Eroded, diminished by centuries of ploughing, they can be hard to see from ground. From the air things look very different. An archaeologist trained in aerial reconnaissance can identify the tell-tale ‘ground responses’, the visual clues, that show the presence of even short-lived campaign camps.

There are three basic types of ‘ground response’: shadow, soil colour and crop mark. From the air it is possible to see the play of shadow on undulations in the ground over a larger area, making it possible, for example, to detect systems of banks and ditches. This ground response can be strong in low light conditions, and/or when there is snow or frost.

Soil marks sometimes show the presence of buried structures, particularly after ploughing, which brings elements of the buried feature to the surface. The problem with soil marks is that they indicate that the archaeological feature is being actively destroyed. Crop marks are the third type of ground response. Crop growth will be affected by what lies beneath the ground: a nice moist ancient ditch will foster growth, while an old stone wall will inhibit development.

Dr Rebecca Jones is Head of Archaeology Strategy at Historic Scotland, and a Newcastle University graduate. She is a specialist in aerial photography and Roman camps. Her book, (2012) Roman Camps in Britain, published by Amberly (ISBN 1848686889), is recommended for anyone trying to understand these extraordinary testimonies to Rome’s campaigns in northern Britain.

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