During the 1745 Jacobite uprising, it became clear that the Hanoverian forces under the Duke of Cumberland lacked accurate maps of Scotland; their pursuit of Charles James Stuart through the Highlands was considerably impeded by their only partial knowledge of the jagged coastline, lochs and mountains. So in 1747 William Roy, a factor's son from Lanarkshire, was put in charge of the work of producing an accurate survey of the nation: he and a band of colleagues spent eight and a half years enduring the physically exhausting, technically demanding work. The result of their labours was the Military Survey of Scotland, a great map that laid the foundations for the Ordnance Survey.
While surveying, Roy also indulged a passion: his deep interest in Scotland's Roman past. He took detailed plans of numerous forts, camps and the Antonine Wall, the barrier built between the firths of Forth and Clyde in the 140s by the emperor Antoninus Pius. Forty years later, his work on the subject – Military Antiquities of the Romans in North Britain – was published posthumously by the Society of Antiquaries. "Military men," he wrote in his preface, "are naturally led to compare present things with the past; and being thus insensibly carried back to former ages, they place themselves among the ancients, and do, as it were, converse with the people of those remote times."
The Military Antiquities is a joyous book. Aside from a beautiful map of the Antonine Wall, there is page after page of meticulous bird's-eye view plans of Scotland's Roman forts and camps, with the slope of hills shaded in tones of graphite and woodland indicated by delicately drawn individual trees, each with its own shadow. The combination of the Roman geometries and the swollen contours of the landscape often makes these images resemble abstract works of art rather than functional maps. Roy's copious text, though, is much less impressive, for the writings of this scrupulously empirical, careful mapper of the land were fatally infected. In common with his great-and-good antiquarian peers, he had fallen for one of British historiography's most successful and most damaging forgeries.
It began with William Stukeley – himself an intriguing figure in the history of antiquarianism. Born in Holbeach in Lincolnshire in 1697, he was a polymath of his age, studying classics, theology and science at Cambridge and setting up a room there for experiments where he "sometimes surprised the whole College with a sudden explosion". He became a fellow of the Royal College of Physicians, was Isaac Newton's first biographer, and his interests ranged across geology, astronomy and the history of religion (he became rector of Stamford, despite some bracingly heterodox notions which included setting up a druidic grove in his garden). He was, above all, a leading light of the Society of Antiquaries, maintaining a copious correspondence with similarly inclined gentlemen, and though many of his ideas may now seem fanciful, he was a pioneer in the measuring and recording of ancient monuments, laying early foundations for the discipline of archaeology.
Stukeley received a letter, on 11 June 1747, from one Charles Julius Bertram, a teacher of English language in the Royal Marine Academy of Copenhagen. The letter was, Stukeley later wrote, "full of compliments, as usual with foreigners" (Bertram was in fact an emigre to Denmark from Britain). It also mentioned a medieval manuscript that Bertram said he had seen, composed by one Richard of Westminster. The text was a history of Roman Britain along with an "antient map". Stukeley recalled: "I press'd Mr Bertram to get the manuscript into his hands, if possible. Which at length, with some difficulty, he accomplished: and on solicitation, sent to me in letters a transcript of the whole; and at last a copy of the map."
On studying the transcript, Stukeley identified Richard of Westminster with Richard of Cirencester, a known 14th-century chronicler. Richard's work, titled De Situ Britanniae (On the Situation of Britain), drew on known texts about Roman Britain, such as those by Caesar, Tacitus, Solinus and the Antonine Itineraries (the Roman navigational aids, of uncertain date, that charted town-to-town routes through the empire). But the revelation was that Richard of Cirencester appeared to have had access to a host of lost, original sources, as well as an entirely fresh crop of itineraries – indeed a great deal of significant geographical knowledge that had allowed him to come up with a comprehensive map of the British Isles under the Roman empire. Among this wealth of fresh material was evidence of a previously unknown province of Britain. Scholars already knew of the division, in the last years of the third century or early years of the fourth, of Britain into four provinces – Prima, Secunda, Flavia and Maxima – which between them made up the "diocese" of Britain. They also knew of the disputed, possibly nonexistent or only briefly existent Valentia, somewhere in the north of Britain. Richard of Cirencester's map fixed the location of Valentia between Hadrian's Wall and the Antonine Wall and, most excitingly of all, introduced the notion of a further province of Vespasiana, in the Highlands of Scotland.
Stukeley revealed the manuscript's contents in a series of papers to the Society of Antiquaries, published in 1757 as An Account of Richard of Cirencester, Monk of Westminster, and of his Works. "He gives us more than a hundred names of cities, roads, people and the like: which till now were absolutely unknown to us. The whole is wrote with great judgment, perspicuity and conciseness, as by one that was altogether master of his subject," he enthused. The "highland part of Britain", he added, was described "very particularly". The map and new itineraries – one describing the mighty journey between Inverness and Exeter – gave Roman names to places that no one had imagined had had the slightest Roman contact. By applying the information contained in Richard's map to known locations, Stukeley was able to identify numerous Latin place names: Falkirk was Ad Vallum Antonini, Inverness was Alata Castra, Aberdeen was Devana, and the Grampians were Montes Grampium. That was a dead giveaway if anyone had chosen to see it, for the mountains had been named for the presumed site of the Battle of Mons Graupius, where the Roman governor Agricola defeated the Caledonians. "Graupius" was rendered "Grampius" only in the 1476 printed edition of Tacitus's biography of Agricola, such that the range owes its name to this day to a typesetter's mistake. Some of the place names even had a Hellenic flavour: Dumbarton was identified as Theodosia – Greek for "god's gift", though perhaps it was primarily intended to recall the emperor Theodosius.
In 1759, Charles Bertram published the Richard of Cirencester manuscript as part of his Britannicarum Gentium Historiae Antiquae Scriptores Tres (Three Ancient Writers on the History of the British People). Thanks to Stukeley's passionate advocacy, its authenticity as a genuine medieval document was not questioned – despite the fact that, as Stukeley himself recorded, his requests to be shown the original manuscript were, mysteriously, fruitless. The best he got was a copy of the handwriting of the first few lines, "which I shewed to my late friend Mr Casley, keeper in the Cotton library, who immediately pronounced it to be 400 years old". If there were any immediate doubts about the discovery, they were confined to the truthfulness of Richard as a historian rather than extending to the intentions of Charles Bertram, and Roy was only one of many antiquaries who wasted oceans of ink trying to square his own accurate on-the-ground observations with the document's fantasy geography. De Situ Britanniae had a fresh burst of life when, in 1809, it was brought out in a new edition, with an English translation by Henry Hatcher, whose preface defended Richard as "scrupulously exact".
Some of the document's spurious Roman names indelibly persist. The hill-range that runs like a spine through northern England from the Derbyshire Peaks to the Northumberland Cheviots had no single name by the early 19th century. However, when William Daniel Conybeare and William Phillips came to compose their pioneering work of 1822, Outlines of the Geology of England and Wales, they decided that it would "be useful to distinguish this ridge of mountains by some collective appellation". The authors noted that "Richard of Cirencester's description of the Roman state in Britain" had "denominated them the Penine Alps". (Bertram almost certainly had the idea from the 16th-century antiquary William Camden, who had likened the range to that other mountainous backbone, the Apennines of Italy.) Conybeare and Phillips announced that as the hills had "clearly a title to this, as their earliest known, if not their original designation, we shall therefore henceforth call them the Penine Chain."
It took until the mid-19th century for Charles Bertram's work to be definitively revealed as a forgery. Doubts grew in the 1850s, and then, between 1866 and 1867, the Gentleman's magazine ran a series of splendidly acidulated articles by Bernard Bolingbroke Woodward, a fellow of the Society of Antiquaries and librarian-in-ordinary to the Queen, which finally demolished its claims to authenticity. His grounds were numerous: Richard's Latin was "more or less good idiomatic English put into Latin words, and apparently by the help of a dictionary", while he had clearly been working from a dodgy edition of Tacitus – "a very badly edited printed one of the 17th or 18th century". Indeed, in order to have consulted Tacitus at all, the putative Gloucestershire monk would have had to have read the works in manuscript, which in his day languished ignored in libraries in continental Europe. Some of the place names he had put forward were derived from medieval linguistic roots. He had repeated mistakes that Camden had introduced in the 16th century. In short, it had "every mark of being the production of such a man as Bertram translating bad English into worse Latin".
And yet, it was a clever and stupendously successful deception; it wove its inventions seamlessly into the accounts of Britain by known classical authors, and lavishly fed the 18th-century antiquarian interest in the origin and etymology of place names. Its revelations were significant and surprising, but not so fanciful, at least to its immediate audience, as to raise suspicion. As to Charles Bertram himself: he died, aged 42 or 43, in 1765, his deception intact. Little is known of him: his silk-dyer father was one of a number of Britons who decamped to Copenhagen in the retinue of Princess Louise, George II's daughter, when she married Prince Frederick of Denmark. Charles was born in 1723, studied at the University of Copenhagen, and was the author of English-language grammars and textbooks for Danish speakers, and an "Essay on the Excellency and Style of the English Tongue". Among his English-language aids is a collection of moralising maxims: sayings such as, "The World oftener rewards the Appearances of Merit, than Merit itself"; "Tis a great Weakness to be credulous, nothing being more common than Lying"; and, "The too great Goodness of a virtuous Man exposes him to Tricks and Deceits." Also: "Patience is the surest Remedy against Calumnies: Time, soon or late, discovers the Truth." Bertram's motivations for perpetrating the forgery can only be guessed at. Stukeley mentioned that he had havered for a year before sending his first letter, which perhaps suggests some doubt that the hoax could be pulled off. Perhaps he wanted the attention and scholarly kudos; perhaps he was all the time laughing at the gullibility of Stukeley. His own words, from his Latin preface to his edition of "Richard of Cirencester" of 1759, are both revealing and oddly wistful. "It contains," he wrote of the document, "excellent fragments of a much better age, which you would seek in vain to find elsewhere".
• This is an edited extract from Under Another Sky: Journeys in Roman Britain by Charlotte Higgins, published by Jonathan Cape.