Stage 6 | Binche - Longwy / A 'pushed-up sleeve' landscape

In the first half of stage 6, we find ourselves on the shores of the Paris Basin, in the Ardennes Mountains. These mountains today have elevations of no more than a few hundred meters, and the terrain where breakaway groups and attackers may be successful.

A mountain belt across Europe

When the Ardennes formed, about 335-300 million years ago, it was part of a spectacular ‘Rheno-Hercynian’ mountain belt, from Poland, through Germany to the Ardennes, and from there to southern England, to continue at the other side of the Atlantic Ocean (which didn’t exist yet) to the Appalachians of the eastern United States. This mountain belt formed because an oceanic basin (the ‘Rheic Ocean’) closed due to subduction between a continent in the (present-day) north (Laurasia, that included Baltica and Avalonia) and a deformed belt of small continents that formed Armorica (read more about Armorica: stage 1) in the south (including the French crystalline massifs of the Vosges, Massif Central, Morvan, and the Armoricaine Massif of Bretagne).

Pushed-up sleeve: folded sedimentary rocks

The subduction zone that existed between Armorica (France) and the Baltica-Avalonia continent (Belgium, the Netherlands, England, northern Germany, Denmark) was diving southward into the Earth’s mantle. As a result, when all the oceanic crust was consumed, Avalonia was shoved down below Armorica, and Armorica started bulldozering over Avalonia. At the southern shores of Avalonia, thick packages of sandstones, clays, and limestones had formed in the course of tens of millions of years, in shallow seas, a bit like the Great Barrier Reef of Australia today. Those sedimentary rock packages were scraped off the rest of the plate that went down below Armorica, a bit like a pushed-up sleeve on your arm. That folded ‘sleeve’, you can now see in the Ardennes. But the sedimentary ‘sleeve’ of Avalonia was not a few millimeters thick, as on your arm, but a few kilometers, so you can imagine that the forces and time required to fold and break those rocks and push them together over a few hundred kilometers into a high, wide mountain belt (see Figure cross-section).

A bit like the Himalaya

We can find a modern-day example that is comparable to Avalonia/Baltica, the Ardennes, and the French Massifs in present-day Asia (see Figure maps). There, the Indian continent (which also contains older continental fragments and subduction zones) is being shoved below Asia. The sandstones, clays, and limestones that existed on India’s northern shores have been, and are being, offscraped and folded up like a sleeve, forming the Himalaya Mountains. The southern parts of Asia, in the Tibetan Plateau, are similar to the French Massifs, but we will see more of that in tomorrow’s stage, when we leave the Ardennes and go to the Vosges! Many in the peloton will be happy that there has been 300 million years of erosion in northern France…otherwise they’d have to climb up to the Tibetan Plateau today!

I am a geologist and I study plate tectonics and the driving mechanisms in the Earth’s mantle, mountain building processes, and the geography of the geological past. I enjoy geological fieldworks all over the world, and translating the results to science and to a broad public. Read more about Douwe.

Douwe van Hinsbergen