Stage 7 takes the peloton from the Jura to the Alps. To be precise: we ride from Champagnole in the Jura Mountains to Le Grand Bornand in the Alps. The Alps typically steal the show in bike races. They take the attention away from the lesser-known Jura. That’s not correct. The Jura have a lot to offer. The notoriously steep gradients of the Jura are quite the challenge. But what are the Jura? Why are they so steep? And why are they separate from the Alps? And how can we use is geology for our sustainability goals. Let’s find out! We go from Jura to the Alps.
T-Rex
The Jura are a mountain range that curves around the western half of Switzerland. It is famous for being the origin of the watch brand Rolex and the youngest Swiss canton, also called Jura. And we have the steep limestone cliffs that make for some terrifyingly steep climbs. These limestones are striking. In 1799, the same year that Napoleon rose to power in the First French Republic, they were used to define an entire geologic period: the Jurassic. We read about that yesterday.
More recently, this geologic period has been associated with the rise of the dinosaurs thanks to the Jurassic Park film series. Spoiler alert: the series’ signature dinosaur, the Tyrannosaurus Rex, did not appear until 70 million years after the Jurassic had already ended. Bummer. To put that number into context, Tyrannosaurus Rex is closer in time to the Tour de France Femmes than to the end of the Jurassic!
Limestone, again
Unfortunately, the Jura Mountains themselves do not preserve that many dinosaur fossils. The limestones formed as carbonate sediments in the sea that separated Europe and Italy during the time in which the supercontinent Pangea was breaking apart.
The fossils preserved in some of the limestones lived in shallow tropical waters. This means that in the Jurassic, the Jura looked much like the Bahamas. Every now and then the sea level rose, during which time the carbonate sediments were covered by a more reddish mud (more on that later!).
The formation of the Jura Mountains is the result of the convergence of the African and Eurasian plates. As Africa shoves Italy northward, it bulldozes the rocks of the Central Alps upward, creating the Alps and Jura. We have some more on that on stage 16 of the 2023 Tour de France. But if the Jura formed by the same geologic process as the Alps, why do the riders have to cross the flat Swiss plateau that lies in between them?
Plateau
To understand that plateau between Jura and the Alps, we need to look to the rocks that lie below the Jurassic limestones. Here, we find layers of salt. These salt layers were deposited all over Europe when it was a dry, arid region at the heart of the supercontinent Pangea. This salt is much weaker than the surrounding rocks. Therefore, rather than forming the thrust slices that you also create when shovelling snow, the salt layer caused a very large chunk of rock to slide horizontally over the underlying rocks, before eventually forming a second mountain range separated from the main mountain range by a flat basin.
Modern use of geology
One advantage of this structure is that the Jura are an excellent place to study the rocks that lie below the Swiss plateau. We also implement some great new initiatives for sustainabilty. For instance, the city of Geneva is planning to use the boiling hot water that circulates in the Jurassic rocks that lie directly below the city as a climate-friendly heating system. Since drilling a well to 1 km depth is very expensive, geologists have first extensively studied the rocks that are exposed to the northwest of the city, to understand their hydraulic properties.
Much further to the north, geologists are actively studying the so-called Opalinus Shale, formed from a thick layer of red mud that was deposited during a long period in the middle of the Jurassic when the sea level was higher. The subsequent burial below kilometres of new sediments compressed the mud to make a shale that is so impermeable that the water that when researchers first drilled into them, they found the very seawater in which these clays were deposited 170 million years ago!
Because there is almost no water circulation in these rocks, they are an ideal place to store hazardous materials for millennia: Switzerland recently designated this rock formation to store the nuclear waste, which is currently sitting in a temporary facility that can only serve as storage for a few decades. In short, the Jura Mountains will continue to fascinate geologists for years to come!
For more information on the geothermal project in the city of Geneva, visit https://www.geothermies.ch. For more information on nuclear waste storage in Switzerland, visit https://nagra.ch
