Today’s Tour de France route crosses the Vercors, a region rich in geology. This is GeoTDF after all. We wouldn’t be here if there was nothing to say. In this region, the geology of the landscape leads to gastronomic highlights and historical events, namely the truffles and maquisards. The Tour de France peloton can hope for a taste of limestone-grown black truffles. These limestones rocks are a remnant of a vast shallow sea that contains traces of dinosaur occupation.
Fast forward in geological time. The encounter of the Eurasian and Adriatic plates and the creation of the Alps vanished this tropical sea. The Alpine tectonic forces folded, faulted and lifted up the limestones layers to form the steep landscape of the Vercors Massif. It is a natural fortress the French resistance used during the World War Two. This is a story on truffles and maquisards in the rugged Vercors.
Three castles, endless truffles
Today, the race starts from Saint-Paul-Trois-Châteaux, a village recognized for the quality of his truffles. This ingredient, at the base of French cuisine, needs specific conditions to grow. In particular it needs a soil rich in limestones, a rock that forms the basement of the region that the peloton will cross today. We already read that wine also grows on the limestone soil in Burgundy (see stage 7), and today it’s truffles. Where would we be without limestone? At Saint-Paul-Trois-Châteaux, the limestone layers are gently folded and mostly flat.
During the race, the Tour de France peloton follows the Eygues, an affluent of the Rhône River, and gets closer to the center of the Alps. On both sides of the road, the limestone beds will be more and more folded and faulted forming peaks and reliefs. The stage ends in Super Dévoluy, that sits on the foot of the Vercors Massif. It is one of the most magisterial examples of the abrupt landscapes Alpine deformation creates.
The fortress
The steep slopes of the Vercors are a natural fortress. It was used as a stronghold by the French resistance during the WWII. Back in the day, a maquisard (i.e. name of the resistant in the region) blessed with a good nose could hope to enhance its meal with the subtle aroma of fresh truffles unearthed from the calcareous subsoil. That calcareous nature? Maybe you guesses it: limestone.
The lost sea
From 200 to 66 million years, the area crossed by today’s stage was a shallow sea connected to the vast Tethys Ocean, the former Mediterranean Sea. It spread between the southern Eurasian and African plates. The tectonic, paleogeographic and climatic reconstructions tells us that France was at a lower latitude than its present-day position. It was actually near the equator. The climate was therefore much warmer.
The tropical shallow seas that were covering southern France are a perfect habitat for the development of a rich marine fauna and flora. In this environment, a wide range of calcareous organism comprising plankton and sea shells are blooming. After their death, these organisms deposit at the bottom of the ocean. They accumulated and by compaction and diagenesis (i.e. physical and chemical changes occurring during the conversion of sediment to sedimentary rocks) created the limestones layers that are now exposed along the race route.
Closer to the surface and on the sea coastlines, much larger creatures occupy the ecological niches. These are the dinosaurs (see stage 16). Before the arrival of the maquisards and of the cyclists of the peloton, these majestic beasts were the first inhabitants of the area. They left us traces of their passage in the form of dinosaurs’ footprints. Ichtyosaurs and Pleisosaurs fossils are found in the sedimentary layers of the Jurassic and of the Cretaceous eras.
From the deep
During the late Cretaceous, around 80 million years ago, the African and Adriatic plates start to converge toward the Eurasian plate. It was a response to the Atlantic Ocean opening. Around 40 million years ago, after the dinosaurs were gone when the Adriatic plate hits the Eurasian continent. This collision creates a large zone of tectonic deformation: the Alps. It’s not the first time you read this, of course. During that collision the tropical seas of Southern France disappeared due to a regional, tectonic derived, uplift. In response to the Alpine forces, the bottom ocean limestones layers are folded, thrusted upon each other’s along fault planes, and carried up to altitude. We saw this on stage 4 too where we find oceanic rock on the Galibier.
This tectonic deformation, combined with the erosion generated by rivers incision, creates medium altitude peaks and abrupt valleys. This rough scenery will, for sure, give the Tour de France cyclists a rough time.
Massive Vercors Massif
The Vercors Massif is a case example of how the Alpine deformation impacted the Jurassic and Cretaceous limestones layers and the landscape. Its geomorphological particularity, that gave birth to its natural fortress shape, resides in the narrow succession of anticlinal and synclinal folds, shifted and superimposed by large-scaled faults. In geological jargon, we call these areas “fold and thrust belts”. There, the tectonic deformation brings to the surface hard limestone layers that are resistant to the erosion. It created elongated cliffs that follow the North South oriented faults plane.
The cliffs are natural barriers separated by narrow and steep valleys, located at the center of the tectonic folds. These valleys are the only crossing point of the area. That made the Vercors Massif a stronghold that is easy to control and defend. This specific geomorphology and geological history explain why the Vercors Massif was a highly disputed strategic military bastion during WWII and a shelter for French resistance. Will the yellow jersey also be able to defend his lead today in the Vercors?
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