Marius' canal rediscovered? Geoarchaeological research in the Rhône delta

The Marius canal question

After the bitter Orange defeat by the Cimbres and Teutons in 105 BC, the Senate sent the consul C. Marius to Gaul to liberate territory that had just been conquered by the Romans. Marius set up camp near the Rhône, not far from the Gulf of Fos. To facilitate the supply of the legions, he ordered his soldiers to dig a canal linking the river to the sea, avoiding the natural, sandy and dangerous mouths.

This famous work, described in ancient sources, later gave its name to the seaport of Fos-sur-Mer (Fossae Marianae), whose main remains are today submerged in the Saint Gervais cove². Texts by Strabo, Pliny and Plutarch attest that the canal was in operation for at least two centuries, exploited by the city of Marseille, whose influence extended as far as the delta, and then by the colony of Arles from Caesar onwards³. Since the 19th century, historians and geographers have been searching in vain for traces of the canal to the east of the Rhône delta.

Core drilling in the Marais du Vigueirat national nature reserve

The deltaic areas of the Mediterranean have been the subject of numerous developments since Antiquity, designed to facilitate the accommodation of ships in port basins or navigation between natural mouths, thanks to canals⁴. These can either exploit sections already under water, or use depressed areas that are then artificially overwatered and connected to the river and the sea. They require limited bank development and fill up quickly if they are not maintained or incorporated into later structures. They then blend into the landscape and are particularly difficult to detect and date, except by studying their filling.

In the hinterland of Fos-sur-Mer, the Marais du Vigueirat is an area occupied by ponds and classified as a nature reserve. It lies just back from the ancient coastline, near the Grand Rhône and one of its ancient mouths, the Rhône d'Ulmet. The chance discovery of a batch of Roman ceramics in 2010, followed by the identification by metal rod staking of an ancient watercourse (paleochenal) running through the ponds⁵, has rekindled research into the potential route of the Marius Canal. With the support of the Conservatoire du Littoral, the association Les Amis des Marais du Vigueirat and the French Ministry of Culture (SRA PACA), a research program was set up to carry out geophysical surveys, core drilling and archaeological surveys⁶.

The geophysical surveys (magnetic mapping, ERT) enabled the precise delineation of a paleochannel / paleochannel, some thirty meters wide, which linked the present-day Rhône to an ancient lagoon over a distance of almost 8 km. A team from the LIVE laboratory of the CNRS and the University of Strasbourg has carried out several coring campaigns in the channel fill and the surrounding environment. These sedimentary columns reveal a still poorly understood history of this eastern sector of the Rhone delta in contact with the Crau plain. One of the main milestones is the identification of a paleochannel of the Grand Rhône that was active during the 1st millennium BC. Its course then seems to have been exploited to build a canal, characterized by very fine clay and sand deposits and high magnetic susceptibility. This signature suggests a particularly slow sedimentation, dated by radiocarbon between the 1st century BC and the 4th century AD. It falls within the chronological horizon of the Marius Canal's operation, even if the absence of archaeological data means that it is not yet possible to identify with certainty the remains of the Marian pits in this paleochannel, located just upstream of Fos-sur-Mer.

Three deep archaeological test pits on the south bank of the paleochannel have confirmed the hypothesis of vast, rectilinear pebble platforms, already identified by metal rod stakes. The construction of these platforms - covering more than 6 ha - required an impressive volume of stone to be transported from the Crau plain: they could have been salt tables, whose existence is attested south of Arles in the Late Period.

The presence of ancient ceramics and alignments of piles attest to the fact that the site was frequented between the end of the 1st century BC and the 3rd century AD. Salt production was essential for food, food preservation and livestock farming. The coasts of Portugal and Spain provide precise comparisons of salt production by solar evaporation. But such evidence remains particularly rare in Gaul. These remarkable discoveries justify further research into the canal and the exploitation of its environment.