The rock walls of the Saint-Maurice gorge have been hollowed out into some very unusual shapes. The Caillette's pothole, a large circular hole, is one of its most impressive examples.
Discover other traces left by the Rhone Glacier in the Bex area
Saint-Maurice's glacial rock bar.
Between Martigny and Evionnaz the Rhone River flows in a narrow valley, which is hardly 2km wide. At Saint-Maurice it is so narrow that the motorway had to be built through a tunnel. The valley then suddenly widens out. It becomes 6km wide near Monthey.
The Saint-Maurice area, characterized by its narrow, high rock walls is typical of a rock bar (3). It has resisted erosion caused by several glacial cycles, although specialists have still not fully explained why. Nevertheless, this obstacle in the glacier's path has itself been affected by the moving ice. It is well known as good site to observe the features of glacial erosion.
The presence of the glacier is revealed by the distinctive features made by the moving ice: the laying down moraines and erratic blocks (4,5), and by the erosion of rocks. This is brought about by the action of rock and sand held in the ice, or by water flowing under or next to the glacier. Thus we can observe roches moutonnées (1,2) that seem to have been polished, glacial striations finely engraved in the rock and grooves (2), which are large and deep striations.
The River Rhone trying to find its way
The present course of the Rhone is not the only path that the river has taken to cross the Saint-Maurice rock bar. Prior to cutting this 250m deep gorge, the Rhone, or an older river coming from the Rhone glacier, skirted round the Chiètres' hill to the east. This older gorge is today filled with sediments. Other similar gorges exist, carved through the hill such as the one that comes out at Caillette's farm; they have also been filled in. (See the blue arrow on the map).
How was the Caillette's pothole formed?
The Caillette's pothole has smooth, rounded edges similar those that can be seen in the gorge of a turbulent river. But what sort of river could have carved such a huge rock feature in the middle of a field?
Water under pressure
The Rhone Glacier, who at the time it flowed over the Saint-Maurice rock bar towards what was to become Lake Geneva, provides the answer.
At the base of a glacier, melt water flows under pressure because of the mass of ice. This water transports large amounts of sand that has been eroded by the glacier. All that is needed is rough rock, a fissure and the water will form a whirlpool (A). The swirling sediment-laden water starts to erode the rock (B).
In some areas it only forms small depressions. In others the hollowing out spirals deeper and deeper: this is how potholes are born (C).
More than just a hole
The Caillette's pothole has impressive dimensions: 5 meters in diameter and over 4 meters deep. However, it has many other interesting features.
For example; looking up you can see that the pothole stretches more than 20 meters in height. But only half of the tube, carved into the rock, is preserved. The other half, orientated downstream has been eroded away, probably during the formation of the pothole. Under the observation bridge you can see the bottleneck carved out by water coming from the pothole.
Swirling water also marked the pothole's edges with streaks that go down in an anticlockwise spiral. At the very bottom, it is deeper on the edges than in the middle where a small polished rock dome can be seen.
The story of a rediscovery
Caillette's pothole has been known about for many years, but nobody had ever seen the bottom until 1962. It had been almost completely filled with earth and boulders. The fact that we can see it today is thanks to a team of volunteers who have returned one of Nature’s masterpieces.
Switzerland's most beautiful pothole?
When cycling through the Saint-Maurice gorge, Mr. Jacques Martin spotted a wooden sign which indicated the presence of a "glacial pothole". Some thirty years later, in 1956, when he was president of the Vevey-Montreux Natural Science Circle, he remembered the sign. He decided to go and investigate the puzzling pothole. The sign had disappeared but the landowner, Mr. Kuonen, guided him to the pothole.
"This glacial pothole has been known for a long time,” Mr. Kuonen tells me, “a glaciology treaty mentions it, and a Swiss-German glaciologist told me it could well be the prettiest in Switzerland."
"Mr. Kuonen,” I told him, “with your permission, I'll come and empty your pothole."
"And when you're done, we'll do a huge fondue here!" He answered.
An explosive work site
Twenty-eight Saturdays over more than a year were necessary to excavate the sediments. The work site, managed by J. Martin, mobilized a team of twenty volunteers: two were women, there were a dozen people under twenty years of age, helped by two Sicilian employees and Mr. Kuonen's son.
The work required was impressive. The pothole contained many boulders that were harder and harder to remove as the hole deepened. Boulders that weighed between 50 and 300 kg were pulled out of the well with a crane. Boulders weighing over 300kg were broken up with dynamite.
"As we approached the bottom, the pothole seemed like a kind of gigantic canon muzzle; the detonations became more intense to the point that we could see the rock shake as if it were an earthquake."
The project was finally successful and on Saturday the 20th April 1963 the pothole was completely cleared.
The 2100 Franc project was funded by the
Circle of Natural Sciences, the town, Bex Tourist Office and the Vaud League for the Protection of Nature.
An investigation for Inspector Stone
Many boulders were cleared out of the pothole in 1962 (read "the story of a rediscovery") and piled up in front of the sign. Some of them are witnesses to a strange story: they were taken from their home, mysteriously transported and abandoned here... Can you help Inspector Stone solve this case?
Investigation Sheet - Inspector Stone
Stage 1 - Find interesting witnesses
Choose one of the big rocks in front of the sign, observe it closely and sort it into the right group.
Find at least one interesting witness.
Rocks made of crystals
Aspect: rough, with texture and crystals (shiny, transparent)
Color: textures of different color (white, grey, black...)
Rocks without crystals
Aspect: smooth, thin textures, no crystals, resembles cement
Color uniform shades of gray (careful of colored moss and lichen)
CLUE 1 - look at the rock in which the pothole is carved: it belongs to the group of rocks without crystals. Therefore the rocks with crystals come from somewhere else!
Stage 2 - Find the guilty party
Who could have transported all these rocks with crystals from their mountain to here?
Think about it and suggest your own theory
Source area of the rocks with crystals
Area of rocks without crystals
We are here, behind the hill
CLUE 2 – Today the guilty party has disappeared (it has gone back to hide in the mountains)
CLUE 3 - The guilty party is capable of climbing up over the hill of Saint-Maurice again.
If you are still in doubt as to who the guilty party is, take a look at the first sign by the cantonal road to unravel the mystery for good…
Lab report - for those who would like more details on the rock witnesses
Jacques Martin who led the pothole's excavation identified some of the boulders as, "granite from the Gotthard massif and Salvan area, protogines (granite) from the Mont Blanc massif, gneiss said to be from Arolla that could also have come from the Dent-Blanche, the Matterhorn or the Weisshorn, some micaschistes from lateral valleys of the left bank of the Rhone, serpentine, some quartzite, maybe from the Sion area, etc.". All these rocks, even if the last two have crystals that are hard to distinguish, can be classified in the group "rocks with crystals", which are known as crystalline rocks.
The "rocks without crystals" are the local rocks seen in the Saint-Maurice gorge. It is a limestone formed in the sea during the time of dinosaurs about 135 million years ago.