Fig. 1 Pierre-Châtel. Roman roads through the Epine range and bridge position. Artwork: P. Artru. A Roman Bridge Across the Rhône Discovered at Pierre-Châtel (France) Philippe Artru Situation and historical background entrance of the gorge and a cable ferry was then in use until 1837 associated with a toll. Some administrative The 3km long, 100–200m deep Pierre-Châtel gorge is and military documents however mentioned the remnants located on the French Upper-Rhône between Geneva and of a bridge across the Rhône at the Bas-Fort site in 1723 Lyon (Fig. 1). The Rhône flows through picturesque cliffs and 1852. Roads enlargements and rocks collapses have cut in Upper Jurassic and Lower Cretaceous limestone now destroyed those remnants and it took the author three formations. Since 2003, the author has conducted a field more years to prove the bridge existence and its antiquity. research program on the main Roman roads of Savoy between the pre-Alps and the Rhône. A road, indicated The access ledge on the Antonian Itinerary and the Peutinger Table came from Italy through the Petit-Saint-Bernard pass and then The carved ledge discovered in the cliff can be compared Lemincum (now Chambéry) heading for Vienna. Another with one existing on the Val de Fier Roman road between one leaving the precedent at Lemincum, reached the col Annecy and Seyssel some 40km north. It also presents du Chat pass and then the Rhône at Yenne upstream from strong similarities with known Roman carved passages Pierre-Châtel. The author’s previous research has shown at Ruaz in the Val d’Aoste (Italy) and the Romanche road that this was also an important road open to vehicles and east of Grenoble. To reach it, the Romans used a natural most probably heading for Lyon but no trace of it had ramp with a 10% slope but only vague remnants of the ever been found in the Pierre-Châtel gorge. In 2005, a road are still visible. The junction between the ramp and systematic survey led to the identification of a carved the ledge, which are currently 30m apart, was probably passage located on a natural ledge some 40m above the provided by a 15–20m high masonry wall, which is also river.1 This road with its typical Roman features could missing. only lead to a bridge as it stopped abruptly in the middle The ledge itself presents three carved passages and three of the cliff north of a former stronghold called the small caves, which could have been used for lodging the Bas-Fort. Past local historians had written of a legendary road caretakers in peacetime or guards when needed. bridge crossing the Pierre-Châtel gorge but only a wooden Among some interesting features are very regularly pick- bridge was attested from 1137 to 1227 at the downstream hewn striated faces and some grooves and holes suggesting 131 Fig. 3 Pierre-Châtel. Right bank remnants. Left: present condi- tion (vegetal cover partly suppressed). Right: reconstruction in Roman times. Drawing: P. Artru. an abutment must have been standing on the bank of the river. Nowadays, the only traces left of it are high underwater steps cut in the limestone with remnants of a foundation wall. The remnants on the right bank On the right bank, in front of the anchoring cavity and hidden by vegetation, the author found a 12m wide trench with a flat bottom inclined at 45° (Fig. 3). The two internal sides of the trench consist of vertical 2–4m high cut faces. This 42m high trench, still intact at a height of 27m, certainly represents the foundations of a massive Fig. 2 Pierre-Châtel. Left bank remnants. Above: present con- dition. Below: reconstruction in Roman times. Drawing: P. Artru. abutment. To realize this work it was necessary to dig into a rocky ridge but the architect took care to leave a 2m the former presence of wooden lateral extensions of the wide wall of natural rock on the ridge’s west side, vertical road built initially or after a partial collapse of the ledge. on both faces. The purpose of this structure oriented at Such extensions, similar to the ‘plank roads’ of old China, 15° from the bridge axis, was most probably to support existed on the Iron Gates Roman road along the Danube a succession of dissymmetric shoulders reinforcing the and on the Romanche road near Grenoble. downstream face of the abutment. Above water, not a single block has been left of this huge construction, The remnants on the left bank which certainly did not collapse entirely as it was strongly anchored laterally and vertically on stable limestone Just before reaching the Bas-Fort promontory, the ledge beds. All its large blocks must have been recovered at an stops abruptly above a 12m wide, 8m high, 1.5m deep unknown date after the bridge destruction. rectangular man-made cavity (Fig. 2). Its form is typical The top of the trench is barred by a 4m high masonry wall, of the cavities made to anchor a stone bridge in a steep the role of which was probably to protect those working rocky bank, but its position and its size imply a bridge on a steep and exposed site from stones falling from of exceptional dimensions. Its depth was 2–3m before particularly dilapidated limestone beds. On the lower a collapse caused by an attempt to widen the tunnel end of the abutment, hard limestone beds emerge from crossing the promontory. Above it, a rough triangular the bottom of the trench. They are cut into steps, which pediment-shaped structure, with the same axis of sym- once formed the base of a vault that crossed the Rhône. metry, has been carved in the cliff, probably to clean it Immediately upstream other steps cut at a 35° angle with from menacing rock pieces. the abutment front face were probably the base for an The 10–12m span of the first arch is deduced from the oblique wall facilitating the flow during high water and 8m height of the cavity, taking into account that the protecting the abutment. cylindrical vault thickness ought to be 2–3m according to stone bridges usual dimensions. The level of the road was Bathymetric surveys 30m above the present road and 42m above the river low waters level (217m above sea level). Most probably this Despite the importance of the remnants found on both arch was needed for the passage of another road along banks, no ashlars from the bridge itself have been found the river, which is still visible now, circling the Bas-Fort and exploration of the river itself became necessary. The promontory. A large pier, so massive that it may be called ‘Compagnie Nationale du Rhône’ (CNR), which exploits 132 Fig. 4 Pierre-Châtel, 3D isobaths diagram of the Rhône (courtesy of CNR) with bridge position, view from upstream. Artwork: P. Artru. the hydroelectric dams and power plants on the Rhône, generously offered to do the necessary bathymetric survey with a specialized boat and team (Fig. 4). Near the Bas-Fort, the isobaths 3D diagram shows that the bottom is nearly flat at a depth of 10–13m (at low waters) and consists of pebbles. The bottom profile shows an almost continuous plateau close to the right bank, followed by an 8m near-vertical face before reaching the pebbles. On the left bank, the slope is less steep and full of collapsed natural blocks. Two-thirds of the Rhône flow are now detoured through a canal, but when the totality of the river was flowing through the gorge, it carried the pebbles away and the depth must have reached 15–17m, even by low waters, and up to 20m during centennial high waters when the flow can reach more than 2,000 3m /s. Such depths and the swift current made the building of a pier in the middle of the river impossible even if it may have been feasible to put the foundations at a depth of 5–6m in front of the pile during the summer low waters of the Roman period warmer climate. The main vault From the above it must be concluded that only one vault extended over the Rhône. It reached a height of 43m (including parapets) above the present winter low waters levels as determined by the access road elevation on the left bank. Total built height may have been 49m if the probable foundation wall is included (same height as the Alcántara bridge and Pont du Gard). The topographic Fig. 5 Pierre-Châtel. Underwater ashlars accumulations along data supplied by the CNR give a distance of 44.40m from bridge axis and downstream. Above: blocks seen from a boat one bank to the other on the bridge axis at low water. close to the right bank trench. Photos: P. Artru. Manipulation cav- ity, rough-hewn face and cornice element with heel. Below: typi- Therefore, the span there was most likely between 40 cal ashlars (1 x 0.50 x 0.50m). Photos: F. Michaud. and 45m. The still existing vault of the Pont-Saint-Martin bridge in the Italian Val d’Aoste, the largest known one in top and assuming a cylindrical design we can deduce the the Roman world, has only a 35.6m span. height of the vault base (springing): 16–19m above the The width of the deck is unknown but must have been low waters level. The length of the bridge was 114m from close to 7m as in some of the largest Roman stone the anchoring cavity wall to the top of the protection wall bridges.