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. This post is based on the content of the sign.

A migratory corridor

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

The 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 (see Appendix) 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 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.

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, the cut in the embankment of the road linking Chevrier to Arcine exposes all the terrains that make up the Vuache mountain. These are limestone rocks, more rarely marl titled 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.

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, these areas have been the subject of numerous studies. But what exactly is the purpose of these studies?

Studying the succession of layers, their composition (lithology), their thickness, and their dip (plunge) is fundamental for reconstructing the geological structure within a mountain. This knowledge is essential before drilling tunnels or installing boreholes.

Paleontological research conducted on the fossils of the Vuache and surrounding regions for over 200 years has led to the identification of a whole series of previously unknown fossils. Described here for the first time, these fossils serve as standards (holotypes) worldwide for dating these layers. Their names evoke local topography, such as Eclusia and Valserina, two microfossils (foraminifera) discovered here. Remains of crustaceans and fossilized turtle shells dating back 130 million years have been found in this area.

Explanation of the gorge

“Many rivers flow through a narrow gorge that cuts 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 flow 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.”

Appendix: The Vuache

The Vuache is a range of hills in eastern France, close to the border with Switzerland (the nearest city is in fact Geneva). To the north lies the Jura and to the south the Alps.

At their northern end, the hills drop steeply down to the River Rhône. The Vuache Tunnel, carrying the A40 Geneva to Lyon motorway, cuts through the hills.

Reference

Oard M, 2025, Do rivers erode through mountains?

Posted, October 2026