Ecluse Gorge, a geological enigma

Today I travelled from France to Geneva and returned later in the day. We travelled along the Rhone river and through Ecluse gorge. One way we went though the tunnel on the motorway and on the other we took the mountain road where there is a tourist sign about the spectacular gorge. Sign #8 of the Vuache Discovery Trail is at the northern side of the Vuache (see Appendix A). This post is based on the content of this sign.

A migratory corridor

Ecluse Gorge is a providential route for birds migrating from northeast Europe in autumn including: wood pigeon, stock dove, common buzzard, European honey buzzard, black kite, red kite, Eurasian sparrowhawk, skylark, common starling, white wagtail, rook, black storks, white storks, and cranes.

Fort l’Ecluse

Ecluse gorge, through which the Rhone flows, entered history thanks to Julius Caesar and the Helvetii. This Germanic-Celtic people, established in the area of present-day Switzerland, migrated towards Saintonge (a province in western France) in the 1^st century BC, after burning their lands. Led by king Orgetris, the Helvetii clashed between Geneva and the Vuache with the fortifications erected by Julius Caesar on the left bank of the Rhone. They were then forced to use the narrow passage of the Rhone gorge on the right bank. In 58 BC, they were defeated by Julius Caesar at Bibracte (northwest of Geneva) and forced to return to their original territory.

The Rhone gorge, the natural outlet of the Lake Geneva basin, was fortified with defensive works very early on. But it was only in the 13^th century AD that the lords of Gex built a fortified house, the target of numerous attacks between the 14^th and 16^th centuries, during which Savoyards, Bernese, Genevans, Spaniards and French clashed.

The lower fort was erected in the 17^th century in the Vauben style, then remodeled in the 18^th century, while the upper fort dates from the first half of the 19^th century. These two structures are linked by an interior staircase of 1,165 steps cut into the limestone rock.

From June 22 to June 24 1940, Fort l’Ecluse, under the command of Captain Favre, resisted the Germans. Its garrison was sent into captivity. A final battle took place in June 1944 between the Maquis of Ain and the retreating occupiers.

Geology of the gorge

In Ecluse gorge (or “cluse”, Appendix B)  the cut in the embankment of the road linking Chevrier to Arcine exposes the rock layers that make up the Vuache mountain. These are limestone rocks, more rarely marl tilted at about 60 degrees, but which were deposited horizontally at the bottom of a deep sea between the upper Jurassic and lower Cretaceous rock layers.

Ecluse gorge which cuts perpendicularly through a mountain range intrigued Horace-Benedict de Saussure in his journey in the alpes (1779-1796). He wanted to understand the origin of this opening. Geologists have long debated how the Rhône managed to punch straight through the mountain barrier rather than flow around it.

Today, geologists, with access to a wealth of new data, propose several hypotheses:
– the presence of a fault with varying degrees of vertical displacement, or
– a different fold structure on either side of the river Rhone, or
– fluvial erosion during the drying up of the Mediterranean Sea 6 million years ago, or
– erosion during the Quaternary period.
But none of these suggestions provides a satisfactory answer to the problem of the formation of this remarkable gorge.

Scientific studies

Since the late 19^th century, there have been numerous studies of these rock layers in order to determine the geological structure of the mountain. A knowledge of the succession of layers, their composition, thickness, and dip is essential before drilling tunnels or installing boreholes.

Paleontological research (the study of ancient animals and plants) conducted on the fossils of the Vuache and surrounding regions for over 200 years has led to the identification of a series of previously unknown fossils. Described here for the first time, these fossils serve as standards (holotypes – the specimen used in the original description of a new species) worldwide for dating these layers. Their names evoke local topography, such as Eclusia and Valserina, two microfossils (foraminifera) discovered here. Remains of ancient crustaceans and fossilized turtle shells have also been found in this area.

Explanation of the gorge

Although rivers should flow around mountains, “many flow in gorges that cut through a mountain range, a ridge, or a plateau. Such gorges are called water gaps. It looks like the river cut the gorge, but how could it? Surely, if the river had carved the landscape slowly over long ages, it would have flowed around the barrier instead of flowing through it. So was the gorge eroded first by something we don’t see happening today?” (Oard, 2025)

Although none of the uniformitarian suggestions above provides a satisfactory answer to the problem of the formation of Ecluse gorge, “The biblical Flood provides a simple solution. After the whole globe was covered, the mountains rose and the valleys sank, so the water ran into the current oceans (Ps. 104:8, NASB). At first, the water would have flowed in enormous sheets, and this explains the many planation surfaces that look like someone took a giant plane to the terrain and shaved it flat.”

“As the water level reduced, flow concentrated into huge channels, which had the potential to erode valleys and gorges. This was the Dispersive or Channelized phase. Since water gaps are located on the surface of the Earth, and formed after much erosion of the land had already taken place, they were cut during the Dispersive phase. They would form rapidly whenever channelized floodwaters flowed perpendicular to a barrier.”

So, water gaps are not forming now.

Conclusion

During the global flood, deep sedimentary layers were laid down and then elevated, fractured and tilted to form an elevated mountain range, which was much higher than the existing mountain range. As the flood subsided sheet erosion removed the uppermost layers and then the runoff was channelized through a weak part of the plastic recently-deposited sediments to form Ecluse Gorge. After the floodwaters subsided, the Rhone river flowed through the gorge.

That is why the gorge is much larger than what could be caused by the erosion of the hard limestone rock in the mountain range by the Rhone, even when it was flooded.

In a similar way, the catastrophic flood (which was associated with meteorites, volcanism, plate tectonics and uplift/sinking of the earth’s crust) can explain most geomorphic enigmas across the world.

Appendix A: The Vuache

The Vuache mountain range in eastern France stretches 11 kilometers from Ecluse Gorge to Chaumont, reaching an altitude of 1106 meters. It is aligned northwest to southeast, close to the border with Switzerland (15km from Geneva). The range is part of the Jura massif, separated from the main Jura ranges by Ecluse Gorge and the Rhône river.

The Vauche fault runs through Arcine and is parallel to the mountain range. This strike-slip fault (a fracture in the earth’s crust where two blocks of rock slide horizontally past each other along a vertical or near-vertical plane) extends from the Jura mountains to Annecy. The northeastern flank of the fault was uplifted by nearly 800 meters meters (to form the Vuache mountain range) and the block slid approximately 10 km northwest along the fault.

At the northern end, the mountain drops steeply down to the river Rhône. The Vuache Tunnel, carrying the A40 Geneva to Lyon motorway, cuts through the mountain range.

Appendix B: Cluse

In geomorphology, a “cluse” refers to the part of a valley, generally narrow, that opens downstream by cutting through hard strata perpendicular to their direction.

Cluses are found primarily in limestone massifs. A cluse generally allows passage from one valley to another without having to cross a mountain pass. Thus, by creating natural communication routes, cluses have enabled the development of villages or towns at their mouths or in valleys difficult to access by other means.

Other clauses in France include:

• Clue de Barles (Bès)
• Clue de Pérouré
• Voreppe Cluse (Isère)
• La Clusaz (Name)
• Cluse de l’Arve (Arve)
• Le Boël (on the Vilaine river)
• The Cluse des Hôpitaux (dead gorge)
• Chambéry Cluse (dead cluse)
• Nantua gorge  (dead gorge)
• Parade of the Lock (Rhône)
• Cluse d’Entreportes (municipality of Lent)
• Gorges of the Val du Fier
• Pierre-Châtel Parade
• Double gorge of Plantaurel (Touyre)
• Double gorge of the Agly (Clue de la Fou and Gorges de Galamus)
• La Clue (Plaisians, 26170)
• Cluse de Bange (Chéran)
• Val de Fier gorge (Val de Fier)

Reference

Oard M, 2025, Do rivers erode through mountains?

Posted, October 2026