Buried treasure

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Scotland's archaeological heritage can be hidden for years, depending on crops, weather, even the light. But there is a team dedicated to making sure the land's secrets don't remain unseen

THE little Cessna pivots on a wing- tip over the Howe of Fife. Loch Leven slides out of eye-shot as, 2,000ft below, the lie of the land spins slowly for our scrutiny – woods and plantations, castles, utilitarian farmyards and, particularly, ripening crop fields tram-lined by tractors. Clutching his camera, Dave Cowley opens his passenger seat window, momentarily filling the cabin with a flurry of slipstream, and points to a faint but still discernible circle showing in one field: "That's almost certainly the ditch around a Bronze Age burial ground; the dot in the centre would be the burial pit. The mound would have been ploughed away long ago."

It's the kind of feature which wouldn't have been noticed from the ground, and an illustration of how fundamental aerial photography has become in recording our archaeological heritage. One of a relatively recent breed of flying archaeologists, Cowley is aerial survey projects manager for the Royal Commission on the Ancient and Historical Monuments of Scotland (RCAHMS), the agency charged with recording, interpreting and collecting information about all aspects of our built environment, from prehistory onwards.

The ancient remains below us have been obscured at ground level by many centuries of land use, and are visible from above purely – and temporarily – due to differentials in the colour and pattern of ripening arable crops. For a brief period only, dependent on crop growth, weather and, to an extent, light, what lies beneath is revealed to the airborne surveyor's practised eye. As we fly on, further crop mark evidence becomes visible – the outlines of what could be a 5,000-year-old henge or a 2,000-year-old Iron Age farmstead, traces of medieval field patterns, the lines of 19th-century field drains and of a country estate's long vanished 18th-century designed landscape. We are "reading" history layered into a tantalising palimpsest of land use over centuries and millennia.

RCAHMS, currently celebrating its centenary, has been conducting airborne surveys since 1976. It is estimated that at least 50 per cent of known archaeological sites have been recorded from the air – a proportion which becomes as high as 90 per cent in Lowland areas such as Fife, Angus, East Lothian and parts of Perthshire, where there is often nothing left on the surface and archaeologists are dependent on these tell-tale crop differentials to see buried features.

Aerial photography, agrees Cowley, has had a revolutionary impact on his profession: "But it's a revolution which plays out in slow motion, because it's very much an accumulative pattern. What was recorded by my predecessors in 1976 is only a small proportion of what we now know in totality." He refers to one feature we've just overflown, a quadrant of a circle that could be part of the ditch which once enclosed a prehistoric settlement, the corresponding bank having long been razed. We're only seeing part of the circle because about a quarter of it is concealed beneath a car park while another section lies under an "unresponsive" root crop, which gives no trace of its existence. "That's an example in microcosm of the problems of seeing these things: one field may be given over to an unresponsive root crop, the next may have a responsive cereal crop. You'll see the archeological feature in the responsive crop, but not in the adjacent one. If you multiply that over the landscape as a whole, you'll maybe see one in ten fields in a responsive crop every year."

Cowley compares the process to trying to assemble a jigsaw, "but someone's thrown the pieces all over the place and, year on year, you pick up pieces of it and start putting it together, but there are always some bits missing. There's almost a sort of game element: you're trying to filter geology and agriculture from archaeology, you're trying to second-guess the weather, you're trying to second-guess the crops, you're trying to get airborne in a country with very changeable weather patterns. All these elements conspire to make it a very real survey challenge."

Cowley and our pilot, Ronnie Cowan of the Edinburgh Air Centre, have just returned from a very profitable flight over Islay and Campbeltown, the latter area revealing a multitude of hitherto unrecorded prehistoric sites, largely thanks to unusually dry weather on the west coast earlier this summer. "Many of these sites had never been seen before," he says, "and if we got a typical west coast summer for the next ten years, the opportunity to see them simply would not arise.

"Because crops ripen at different rates, with spring and winter sowings ... you really need to keep looking. Something that becomes visible one week might not be visible the next, or a heavy shower of rain might mean that it isn't visible any more that season."

A small GPS set Cowley brings on the plane keeps a record of the flight path, later to be downloaded to locate everything spotted and photographed. Within a few months the information will be accessible online via RCAHMS' Canmore database, which holds details of some 270,000 archaeological sites, monuments, buildings and maritime sites. In addition to more than 100,000 of its own aerial images, the Commission's collections boast more than a million aerial photographs taken by the RAF and Ordnance Survey, among others, some as early as the Second World War. These RAF and OS images, points out Cowley, are now invaluable, not just as a record of the changing Scottish landscape during the second half of the 20th century, but also in recording rural areas which are now under forestry, or have been erased by upland pasture improvement or obscured by urban sprawl.

Cowley, who is usually accompanied by RCAHMS colleague Kevin MacLeod, confines his flights to the summer cropmark season; another colleague, Robert Adam, concentrates on photographing architectural and industrial sites from the air. Apart from providing a comprehensive archaeological record, the aerial survey results can also be consulted by planning authorities. "The number of these sites that are likely to be threatened by development, be it a new road or a housing estate, is actually a tiny proportion, but the important thing is that they are there on record."

Maintaining that record is very much about continuing vigilance, and an element of seizing the day. As we bank and head homeward over Kincardine, below us a combine harvester is leaving its own temporary field traces. Once that tell-tale crop is harvested, the evidence has been lost for another year.

• For further information, see www.rcahms.gov.uk