Survey work on the Rosablanche, 1917. swisstopo photographic collection

A moving mountain: the enigma of the Rosablanche

In 1915, land surveyors realised that the summit of the Rosablanche mountain in the canton of Valais had moved several metres in the space of just a few years. Today, the cause of the movement is more relevant than ever.

Felix Frey

Felix Frey is an expert in history at the Federal Office of Topography (swisstopo).

The history of movements on the Rosablanche mountain (current summit 3,336 metres above sea level) dates back to 1888. That was when land surveyor Max Rosenmund erected a control point for the Swiss triangulation network on the summit. At that time, the network consisted of clearly visible points throughout the entire country, which were often marked with some form of trig point. Well-known examples of the latter include the ‘pyramids’, like those on the Napf, Gurten and Chasseral peaks. The location and height of the individual control points were determined by measuring angles and performing trigonometric calculations. This work was of major importance, as the triangulation network provided a precise basis for producing maps of Switzerland.

**The Rosablanche was part of the main Swiss national triangulation network.** swisstopo photographic collection

It was only in 1916, just over a quarter of a century later, that the triangulation point on the Rosablanche became a “cause for concern”, in the words of engineer Johann Ganz. A team of surveyors from the Federal Topographic Bureau (now known as swisstopo) made their way to the summit in September 1914 to measure the angles. The results of these measurements showed that something unusual had been happening on the Rosablanche. According to Hans Zölly (1880–1950), the official in charge of the national trigonometric network at the time, “unpleasant surprises” emerged: none of the triangulation calculations in relation to the Rosablanche made any sense. It was initially thought that measurement and calculation errors must be to blame. However, a second set of calculations confirmed Zölly’s observation: the triangulation point on the Rosablanche had moved by around 3.5 metres between 1895 and 1914. The engineer came to a dramatic conclusion: “We were faced […] with the fact that one of our most important trigonometric control points was not actually a fixed point.”

**Survey work on the Gwächten, 1921.** swisstopo photographic collection

Consequently, the Topographic Bureau did everything in its power to determine whether the control point was continuing to move, and how. Almost every summer from 1915 to 1921, engineers and their survey assistants climbed to the top of the Rosablanche. They were literally able to see the summit moving with their own eyes:

At midday, during the warmest hours, everything moves; blocks fall from left and right into the gully or head south from there towards the glacier. You get the impression that the ground beneath the trig point will not be able to resist for much longer, but must give in to the law of gravity and sink into the depths.

Johann Ganz in the Swiss Alpine Club Almanac, 1916.

Calculations carried out in 1921 finally showed that the control point was a good 21 metres lower than when first measured in 1891. The peak, which had once stood out so prominently from the rest of the mountain, had sunk so far that it was now increasingly becoming part of the ridge.

Rosablanche summit with trig point, 1891. — The summit of the Rosablanche in 1891 (left) and 1925 (right). In 1891, the trig point on the peak can still be seen. In 1925, however, the corresponding fixed point of 1888 was no longer on the summit of the Rosablanche, but in the scree field at the bottom right of the picture. swisstopo photographic collection / swisstopo photographic collection

Rosablanche summit with trig point, 1925. — The summit of the Rosablanche in 1891 (left) and 1925 (right). In 1891, the trig point on the peak can still be seen. In 1925, however, the corresponding fixed point of 1888 was no longer on the summit of the Rosablanche, but in the scree field at the bottom right of the picture. swisstopo photographic collection / swisstopo photographic collection

At first, the Topographic Bureau’s engineers assumed that an earthquake must have triggered these movements on the Rosablanche. But as no other triangulation point in the region had shown similar signs of movement, this hypothesis soon appeared implausible. Porous rock and tectonic shifts were also ruled out as potential causes. The land surveyors remained in the dark for many years until a professor of geology from Neuchâtel, Émile Argand (1879–1940), came to their attention in 1920. Argand knew the Alps in that part of the country well and had also been studying the mountain in great detail. In a peculiar twist of fate, both the engineers and the geologist had been closely watching the Rosablanche without either being aware of the other. By 1916, Émile Argand had already concluded that it was glacier melt which had set the mass of rock in motion. The Prafleuri Glacier was situated immediately below the mountain’s highest point. It stabilised the pinnacle not only on the outside, but also beneath the surface: the glacier had been eroding the summit of the Rosablanche for thousands of years while at the same time shoring it up with its thick layers of ice. But the Prafleuri Glacier had begun to melt rapidly at the start of the 20th century, causing the peak to lose its icy foundation – and so, the summit’s movement began. Argand’s findings were also relevant to Switzerland’s system of land surveying: the triangulation point on the unstable summit was no longer suitable as a control point and was replaced by the “magnificent, centrally located peak” of La Ruinette.

**Wiped off the map: the summit of La Ruinette replaced the Rosablanche as a control point in the triangulation network of 1962.** swisstopo

The case of the Rosablanche is a prime example of how Switzerland’s mountains have been kept under ever closer scrutiny since the 19th century. Land surveys, glacier research and alpinism have all helped to document the changes in the Alpine region and to identify them at an early stage. This trend continues today: high-precision height models, the GLAMOS glacier monitoring network and the PERMOS permafrost monitoring network, to name just a few examples, are currently documenting changes in mountain regions in great detail. The dwindling of the Rosablanche in the 1910s was also one of the first incidents to highlight the relationship between ice and rock. The fact that glaciers and permafrost play a significant role in literally holding the Alps together is now becoming ever more apparent as global warming progresses. The massive rockfall of 11 June 2023 on the Fluchthorn mountain, which borders Switzerland and Austria, is the most recent example of this. One million cubic metres of rock were sent crashing into the valley below, leaving the peak around 19 metres shorter than before. The cause: thawing permafrost and the melting of the Fluchthornferner, the glacier which had supported the western side of the mountain. Until it retreated, thereby – like the Prafleuri Glacier before it – setting the mountain aquiver.

Space and time

This article was originally published (in German and French) on the “Space and time” website of the Federal Office of Topography (swisstopo), where readers can regularly discover thrilling chapters from the history of Swiss cartography.

More of: 20th / 21st century Research Technology Nature Article

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