"We others have the lake. It is vast, ๐ถ๐น ๐ฎ ๐น'๐ฎ๐ถ๐ฟ ๐ฑ'๐๐ป๐ฒ ๐ฝ๐ฒ๐ฟ๐น๐ฒ ๐ฎ๐ ๐ณ๐ผ๐ป๐ฑ ๐ฑ๐ฒ ๐๐ฎ ๐ฐ๐ผ๐พ๐๐ถ๐น๐น๐ฒ. The mountains and hills that border it rise up on all sides..."
The words of Charles-Ferdinand Ramuz illustrate a well-known landscape. The morphology of his "shell", however, is a little less familiar. Here are the details.
The history of Lake Geneva is primarily linked to the collision of the Adriatic and Eurasian tectonic plates. This collision raised the Alpine peaks and the folds of the Jura, but also caused the Franco-Swiss plateau to sink. ๐'๐ฒ๐๐ ๐น๐ฒ ๐ฐ๐ฎ๐ ๐ฑ๐ ๐๐ฒฬ๐บ๐ฎ๐ป ๐พ๐๐ถ ๐ฎ ๐๐ฒฬ๐ฐ๐ ๐๐ป ๐ฎ๐ณ๐ณ๐ฎ๐ถ๐๐๐ฒ๐บ๐ฒ๐ป๐ linked to the formation of the Pre-Alps.
The tectonic influence remains marginal in explaining the relief of the Lake Geneva "shell". Instead, the scientific literature highlights the role played by glaciers. Thus, it is mainly ๐น'๐ฒฬ๐ฟ๐ผ๐๐ถ๐ผ๐ป ๐ด๐น๐ฎ๐ฐ๐ถ๐ฎ๐ถ๐ฟ๐ฒ ๐พ๐๐ถ ๐ฎ๐๐ฟ๐ฎ๐ถ๐ ๐ฐ๐ฟ๐ฒ๐๐๐ฒฬ ๐น๐ฒ ๐น๐ฎ๐ฐ that is responsible for some of its ๐ฝ๐ฎ๐ฟ๐๐ถ๐ฐ๐๐น๐ฎ๐ฟ๐ถ๐๐ฒฬ๐.
For example, ๐น๐ฒ๐ ๐ด๐ฟ๐ฎ๐ป๐ฑ๐ฒ๐ ๐น๐ถ๐ด๐ป๐ฒ๐ ๐๐ฒฬ๐ฏ๐ฟ๐ฎ๐ป๐ ๐น๐ฒ๐ ๐ณ๐ผ๐ป๐ฑ๐ between the small and large lakes would have been formed by large boulders trapped in the lower glacial layers. The advancing Rhรดne glacier would have carried these large boulders with it, "grating" the bottom of the lake.
While the Rhรดne glacier hollowed out the Lake Geneva "shell", it also filled it with sediment as it retreated. Today, ๐น๐ฒ ๐ณ๐ผ๐ป๐ฑ ๐ฑ๐ ๐น๐ฎ๐ฐ ๐ฝ๐ผ๐๐๐ฒฬ๐ฑ๐ฒ ๐๐ป๐ฒ ๐ฝ๐ฟ๐ผ๐ณ๐ผ๐ป๐ฑ๐ฒ๐๐ฟ ๐บ๐ฎ๐ ๐ถ๐บ๐ฎ๐น๐ฒ ๐ฑ๐ฒ ๐ฏ๐ฌ๐ต๐บ, ๐๐ผ๐ถ๐ ๐ด๐ฒ ๐ฐ๐บ ๐ฑ๐ฒ ๐บ๐ผ๐ถ๐ป๐ ๐พ๐'๐ถ๐น ๐ ๐ฎ ๐ญ๐ฎ๐ฑ ๐ฎ๐ป๐. This loss of depth is explained by the deposition of sediments carried by the tributaries of Lake Geneva.
Other more specific remains can be explained by a combination of factors. For example, ๐น๐ฒ ๐ฟ๐ฒ๐น๐ถ๐ฒ๐ณ ๐ถ๐บ๐บ๐ฒ๐ฟ๐ด๐ฒฬ ๐ฑ๐ ๐๐ฎ๐๐-๐ ๐ผ๐ป๐ between Corsier and Versoix is a former mound of uneroded molasse on which a significant layer of sediment has been deposited. In another case, ๐น๐ฒ๐ ๐ณ๐ถ๐ป๐ฒ๐ ๐ฐ๐ผ๐น๐น๐ถ๐ป๐ฒ๐ ๐๐ผ๐๐-๐น๐ฎ๐ฐ๐๐๐๐ฟ๐ฒ๐ off Coppet are former glacial tunnels, filled in during deglaciation.
If you'd like to find out more anecdotes about the morphology of Lake Geneva, visit our website to (re)discover the glacial origin of ๐ฝ๐ถ๐ฒ๐ฟ๐ฟ๐ฒ๐ ๐ฑ๐ ๐ก๐ถ๐๐ผ๐ป in Geneva (60s nยฐ 44), the formation of ๐ฟ๐ฎ๐๐ถ๐ป๐ ๐๐ผ๐๐-๐น๐ฎ๐ฐ๐๐๐๐ฟ๐ฒ๐ at the mouth of the Rhรดne (60s nยฐ 61) or another version of the appearance of Lake Geneva mixing the ๐ด๐ฒฬ๐ฎ๐ป๐ ๐๐ฎ๐ฟ๐ด๐ฎ๐ป๐๐๐ฎ (60s nยฐ 16).
Photo credits :
Swisstopo Jean-Louis Lods
Sources :
– Girardclos, Corboud, Wildi (2015)Limno-geological cruise on Lake Geneva: geological history of the Lake Geneva basin and human occupation". University of Geneva (online)
- J.F. Ramuz, Diary, 1902, in Lรฉmaniques nยฐ100
– Le Temps (2016)The mysteries revealed at the bottom of Lake Geneva".
