by Graham Robb
The southerly winter solstice line leads to the lands of pre-Celtic, Ligurian tribes and, in the opposite direction, through Aquitanian Gaul to the Armorican Peninsula. It would have crossed at least sixty kilometres of open sea, to the consternation of a long-distance surveyor. In those Atlantic waters sprinkled with islands and charted mentally by generations of sailors, an accurate survey line would not have been impossible. Even without precise measurements, it would have been obvious that the sun of the winter solstice set over the headlands of Brittany and the lands of the Osismi, ‘the People at the End of the World’. And perhaps, after all, the measurements were precise: a line projected from the point due south of Alesia on the Via Heraklea reaches the tiny Île de Sein, which, in Breton folklore, was the island to which the souls of the dead processed at low tide. In the days of the ancient Celts, it was the home of nine female Druids. The geographer Pomponius Mela described their peculiar convent in the mid-first century AD. Since they lived at the end of a solstice line, it was only fitting that they demanded of those who sought their wisdom a degree of navigational skill:
Sena, in the British sea, facing the coast of the Osismi, is famous for a Gaulish oracle, tended by nine priestesses who take a vow of perpetual virginity. They are called Gallizenas [probably ‘Galli genas’, ‘Gaulish maidens’] and are said to possess the singular power of unleashing the fury of the winds and the seas by incantations, of turning themselves into any animal they choose, of curing what is elsewhere considered incurable, and of knowing and predicting the future. But they reveal the future only to navigators, and only if they deliberately set out to consult them.27
The probability that the solstice lines of Gaul would pass through so many important Celtic places by chance is exceedingly remote. (Just for the record: within Gaul, even allowing one thousand metres on either side of each line, the likelihood of only five tribal centres occurring on the lines by chance is 1 in 87 million; the probability for the sites that are bisected by the lines is approximately 0.4324, which is effectively zero.) But probabilities are often false prophets. The reality of historical research is that, if a coincidence is amazing, it probably is a coincidence, and when two thousand years have elapsed, material proof is thin on the ground.
The feasibility of the system is not in doubt. Most of the problems confronting long-distance land surveyors who wanted to draw a straight line between two mutually invisible points could be solved by trigonometry. Devices for measuring angles, such as the dioptra-and-protractor, developed in the third century BC, were used primarily by astronomers. Terrestrial surveyors rarely bothered with such refinements. The protohistoric positioning unit was the groma – a vertical stick with horizontal cross-bars and plumb lines which made it possible to draw straight lines and right angles. Little else was needed – just a right-angled triangle with two sides whose lengths were measured in whole numbers. One of the two sides was aligned on the local meridian. Due north could be found by marking the point at which the shadow of the gnomon was shortest, or, more accurately, because of the fuzziness of the shadow, by finding the middle of a line drawn between the shadow points at two symmetrical hours of the day (for example, 10 am and 2 pm). The operation could be repeated ad libitum as the survey progressed. Over long distances, errors would be averaged out and cancelled.
Inaccuracies resulting from the application of flat, Euclidean geometry to the spherical earth are inconsequential over a few degrees of latitude. Within the zones occupied by Gaul, Iberia or Britain, the curvature of the earth can be ignored. From Mediolanum Biturigum to the pass of the Mediomatrici – a distance of over four hundred and fifty kilometres – the deviation would be approximately 2.5 metres or the length of three Celtic swords. This is partly why some of the medieval sailors’ maps known as portolan charts were so remarkably accurate within areas similar in size to those covered by the Druidic survey.28
39. Determining true north
There are some spectacular examples of the effectiveness of these ancient measuring tools. In the second and third centuries AD, Roman surveyors laid out the fortified frontier called the Limes Germanicus. For eighty kilometres, it follows a perfectly straight line through the hills of the Swabian-Franconian forest; on a twenty-nine-kilometre stretch south of Walldürn, the directional error is less than two metres. In the early eighth century AD, using practically the same technology, but with the help of an astronomer, a Buddhist monk, I-Hsing, surveyed a meridian of two thousand five hundred kilometres from the southern border of Mongolia to the South China Sea, which is the distance that separates the northern tip of the British mainland from the Pillars of Hercules. Some qanats (ancient underground aqueducts) in China and on the Iranian plateau stretched for tens of kilometres and were accurate to within a few centimetres, despite requiring a three-dimensional survey. The Celtic pathways had the advantage of existing only as imaginary lines in two dimensions. Astronomical observations would have made them even more accurate, and perhaps, like medieval determinations of the qibla (the direction in which Muslims face when praying), they inspired further trigonometrical refinements.
Of course, the fact that the Celts were theoretically capable of plotting long-distance lines does not prove that the lines existed. The forms that proof might take can easily be imagined but not obtained. A Druid of the late Roman empire, seeing his store of knowledge depleted by age, might have inscribed the pattern on a piece of parchment, disguising it as a decorative illumination. The design might have been effaced as the work of a pagan or left to moulder in a monastery, its meaning lost for ever – unless some details of the system had been preserved in a verbal map, like the triads that record the origins of the Gauls, the legends that allow the course of the Via Heraklea to be plotted, or the Irish poem in which Cú Chulainn, son of Lugh, recounts his route to the maiden Emer: ‘[I came] from the Cover of the Sea, over the Great Secret of the Peoples of the Goddess Danu, and the Foam of the Two Steeds of Emain Macha; over the Garden of the Great Queen and the Back of the Great Sow; over the Glen of the Great Dam, between the God and his Druid’ . . . But to place most of those sites on a map is a practical impossibility.
Expeditions into the bleary realms of speculation sometimes return empty-handed only to find the answer waiting at home. I had seen the ‘map’ several times without realizing what it was. It takes the form of written texts that describe one of the great migrations of the Celtic tribes, condensing the events of two centuries into brief but detailed narratives. The basic elements of the tale are known to be true. Between the fourth and second centuries BC, Celtic tribes settled in northern Italy, the Danube Basin, the Balkans and Turkey. It was a long, complex process, and yet, as historians have often observed, it seems to show a common purpose and the hand of a coordinating power.
The historical reality of mass Celtic migrations is well established. In 58 BC, a confederation of Swiss tribes headed by the Helvetii gathered at Genava (Geneva). Their intention was to walk across Gaul and to settle in the lands of the Santones, whose western border was the Atlantic Ocean.29 Preparations began two years before the departure. Every acre of arable land was sown with corn, and when the harvests had been gathered in, the migrants’ resolve was strengthened by the simple expedient of setting fire to every town, village and private dwelling (twelve towns and four hundred villages, according to Caesar’s information).
Even Caesar was impressed by the Helvetii’s logistical competence: ‘Writing tablets were found in the Helvetian camp on which lists had been drawn up in Greek characters. They contained the names of all the migrants, divided into separate categories: those able to bear arms, boys, old men and women.’ He quoted the tablets in the first book of his Commentaries on the Gallic War:
Tribe
Heads
Helvetii
263,000
Tulingi
36,000
Latobrigi
14,000
Rauraci
23,000
Boii
32,000
Total
368,000
of whom 92,000 are able to bear arms
Whatever the reason for the transplantation of 368,000 people from the Swiss plateau to the Atlantic coast – depleted farmland, a sense of stagnation in the military elite or a reversion to the old nomadic ways – these mass-migrations are a cultural trait of Celtic societies. Belgic tribes, who had come from the east, settled in southern Britain at a time when they were prospering in Gaul. Some of the Parisii left the Seine for the Humber and founded a colony in Yorkshire. Many centuries later, most of the Scots who left their homeland for the New World were not victims of the Highland Clearances; they were Lowlanders who had made enough money in the booming industrial economy to become mobile and ambitious.
The greatest of these migrations was the Gaulish diaspora instigated by the Bituriges. Six ancient authors mention it – Livy, Polybius, Diodorus, Pliny, Plutarch and Justinus. Livy’s is the most detailed account. His source was certainly Celtic, and he must have recognized the legend as a verbal map, but without perceiving its underlying logic.
We have received the following account concerning the Gauls’ passage into Italy. It was in the days when Tarquinius Priscus was king of Rome. The Bituriges were the supreme power among the Celts, who form one third of Gaul, and it was always they who gave the Celts their king.
At the time in question (the early fourth century BC),30 the king of the Celts was a ‘brave and wealthy’ man called Ambigatus. The name appears to mean ‘he who fights on both sides’ or ‘the two-handed warrior’. Under his rule, Gaul had prospered. The harvests had been copious, the tribes had multiplied, and, now that his hair was turning white, King Ambigatus had to face the consequences of his success: Gaul was bursting at the seams. ‘It seemed scarcely possible to govern such a multitude of people.’
Following Celtic tradition, in which property passed through the female line, Ambigatus sent for his sister’s two sons, both energetic and enterprising young men. One was Bellovesus (‘worthy of power’), the other Segovesus (‘worthy of victory’). To each prince, the Druids were to indicate a direction, and the tribes, accompanied by an army large enough to ensure their safety, would follow the paths assigned to them by the gods. Where the augural ceremony took place, Livy does not say. The most likely site of a Biturigan palace is Avaricum (Bourges), but in the solemn circumstances, Mediolanum Biturigum (Châteaumeillant) would have been the most appropriate location: just as the Greeks mapped the home country onto the colonized territory, the first city founded by Bellovesus in the new land would be called ‘Mediolanum’.
The Druids obtained the judgement of the gods. Segovesus was pointed in the direction of the Hercynian Forest, beyond which lay the windy plains of central Europe. His brother Bellovesus was given ‘a considerably more pleasant route’: he was to lead his people into Italy. If Plutarch’s version can be believed, the followers of Bellovesus could hardly wait to expatriate themselves: they had tasted Italian wine, ‘and were so enchanted with this new pleasure that they snatched up their arms, and, taking their families along with them, marched to the Alps’.
At this point, Segovesus disappears from the tale. His contingent of men, women and children set off for the boundless forest of oak in which we shall try to find them later on. Meanwhile, the more fortunate Bellovesus gathered together the surplus population of seven tribes in what was evidently an expedition on the scale of the Helvetian migration witnessed by Caesar.
The first group was made up of migrants from the following tribes: the Bituriges, the Arverni, the Senones, the Aedui, the Ambarri, the Carnutes and the Aulerci. They headed off in the auspicious direction and came to the lands of the Tricastini. The Tricastini lived in the Rhone Valley to the north of Arausio (Orange), in a region where, a few centuries later, the migrants would have been able to satisfy their wine-lust to their hearts’ content. The letters ‘TRIC RED’,31 etched in the stone of the Roman cadastral map of Orange, show that the Tricastini’s territory once extended from the Rhone to the limestone bluffs of the Dentelles de Montmirail.
There, the followers of Bellovesus looked to the east and beheld a daunting obstacle: ‘Beyond stretched the barrier of the Alps.’ In those days, according to the tale, only Herakles had found a way through the Alps, and so they stood, ‘fenced in, as it were, by high mountains, looking everywhere for a path by which to transcend those peaks that were joined to the sky and so to enter a different sphere of the earth’ (‘in alium orbem terrarum’).
Described in these cosmic terms, the migration was a pilgrimage with no return, a mass enactment of the human journey from this world to the next. The Alps were to the Celts what the Red Sea was to the tribes of Israel. The answer came in the form of what the legend calls a sacred duty. Word reached the migrants that Greeks from Phocaea had landed at Massalia and were being attacked by local tribes called the Salyi. Seeing this as a sign of their own destiny, they went to the aid of the colonists and ‘enabled them to fortify the site where they had first landed’.
Having fulfilled their religious obligation, and reaffirmed their affinity with the Hellenic world, the Bituriges and their allies crossed the Alps by the pass of the Taurini (the Matrona) and the valley of the river Duria. Near the river Ticinus, they defeated the Tuscans and then settled in a country that belonged to a people called the Insubres. According to Livy, ‘Insubria’ was the name of a territory in the lands of the Aedui, which seemed a good omen, ‘and so the city they founded there was called Mediolanum’ (Milan).32
40. The pattern of migration
Livy goes on to describe the colonization of northern Italy by all the other Gaulish tribes that followed in the wheel-tracks of the Bituriges and their compatriots. First, by the same Alpine pass, came the Cenomani, then the Libui and the Saluvii. The next group, composed of Boii and Lingones, took a slightly different route, crossing the Alps by the Poenina (the Great St Bernard). Last of all came the Senones – presumably another contingent of the tribe that had joined the initial migration. Polybius gives a similar list, but he describes the Insubres as settlers rather than as the original inhabitants of Milan.
Most of this is historically true. North-eastern Italy was Celtic from the early fourth century until the Roman victory of 191 BC. Many settlements that became Roman towns were founded by Celts. Senigallia, on the Adriatic coast, preserves the name of the Senones, who also gave their name to Sens. Mezzomerico was once Mediomadrigo (from the Mothers of Middle Earth). Bologna, Brescia, Ivrea, Milan and Turin are all Celtic names. The Massalian episode probably dates from the same period. A century or so after the foundation of Massalia, when new trade routes were opening up along the Rhone, Massalian merchants would have sought the protection of tribes to the north. The most powerful trading port on the Gaulish Mediterranean would certainly have had to reckon with ‘the supreme power in Gaul’.
41. Tribal centres and the migration to Italy
The names in boxes are those of tribes who migrated to Italy. A solar line – sometimes more than one – passes directly through a Gaulish tribal centre in twenty-five cases and within 1000 metres of the site’s perimeter in twelve further cases (on average, 750 metres). In Italy, beyond Milan, the system suggests a general direction of migration rather than an exact trajectory.
Several minor oppida on the lines have been omitted (e.g. Les Baux-de-Provence, Mondeville by Caen, Malaucène, Vézénobres, etc.), as have tribes whose names or capitals are unknown (e.g. the Budenicenses and the inhabitants of Marduel and Tarusco). Some tribes or tribal names may differ from those of the fourth century BC.
Speculative journeys along the lines will reveal many other likely sites for which there is, at the time of writing, insufficient archaeological evidence (e.g. Dôle, Jouarre, Luxembourg, Montpellier, Najac, Treviso).
Bearings – taken where possible from the nodal points of Mediolanum Biturigum and Alesia – are those provided by the tangent ratio of 11:7 (57.53°, 122.47°, etc.), with the excep
tion of the Alesia–Bibracte–Gergovia line, which, for geographical reasons, is 28.2° from north rather than 28.8° (see here).
The creation of a settlement obviously depended in part on topography, and so some leeway was presumably allowed. With a slightly broader margin, several more tribes could join the total of thirty-seven. Major oppida occurring more than 1000 metres from a line are indicated on the map: Arvii (1.5 km), Cenomani (1.5), Remi (at Bibrax) (1.7), Bergistani (2.4), Laietani (2.4), Atacini (3.1), Seduni (3.5), Ruteni (3.6), Consoranni (4.0), Atrebates (4.1), Morini (4.1), Nervii (4.1), Sagii (4.4), Ausetani (4.8), Osismi (4.8), Sequani (5.0), Veragri (5.0), Aduatuci (5.3). In retracing ancient surveys, especially over such long distances, common sense calls for certain adjustments. Future explorers of the system should feel free to experiment.
The legend is entirely plausible, except in one respect: the god-given itinerary looks very odd indeed. Unless Bellovesus and his followers mistook the lacy ridge of the Dentelles de Montmirail for a range of mountains, the territory of the Tricastini is a strange place from which to contemplate the distant Alps. Either they ignored the direction assigned to them by the Druids, or the gods’ instructions were exceedingly complicated: south-south-east, a deviation towards Massalia, then north-east, and finally south-east to New Mediolanum (fig. 40).
This all sounds like a post facto justification of military conquest, a boastful bardic tale designed to make the events fit a spurious divine plan. Yet Livy, like most ancient historians, accepted the divinatory basis of the expedition. Justinus, too, in describing the Celtic colonization of Italy and later migrations to the east, said – presuming that Druids used the same method as Roman augurs – that the Celts ‘penetrated into the remotest parts of Illyricum under the direction of a flight of birds, for the Gauls are skilled in augury beyond other nations’.