Stage 5 | Lille - Arenberg / The Flemish Basin and ice age loess

- In cooperation with Patrick de Wever -

The peloton will ride through northern France, a wide open landscape of meadows and agriculture that hide sedimentary rocks from the Paris Basin and the Flemish Basin. These consist mostly of sandstones and shales (compresses claystone), chalk, and below that the older rocks of Avalonia such as diorites - a type of granite but without quartz, which forms in big magma chambers - that stem from the Silurian period (440 million years ago). And these rocks have been covered by packages of loess: wind-blown, very fine-grained, and very fertile sediments that formed in the steppen south of the giant glaciers of the ice ages. In northern France, there are not many rocks visible at the surface, but they are quite shallow: they are excavated in quarries.

Construction geology

In almost all areas in the world, you can get a good impression of the geology of the surrounding subsurface by studying the walls of buildings. In ‘poorly-exposed’ areas - which in geological terms means that the rocks close to the Earth’s surface are only rarely visible - you can get a pretty good impression of the composition of the shallow subsurface by looking at the walls of the buildings. Even if it may look a little strange when you’re not looking through the display windows of shops, but at the facade next to it. But make sure you use ‘old’ buildings, say, from before 1950. The rocks in buildings of the last decades come from all over the world. For instance, the sand that was used for the artificial ‘palm island’ in the United Arab Emirates was shipped in from Australia. It’s not going to be easy to be a geologist in 10 million years from now….

Blue-grey loess makes for red bricks

The geology of Northern France is clearly visible in the buildings along the route of today’s stage. A well-known feature is the ‘rouges-barres’, the red-white banded walls. These consist of a row of the bright white chalk limestone that we saw yesterday in the cliffs of Calais. On top are then three rows of red bricks that are made from the loess of the ice ages. By the way, did you know that red brick is not necessarily made from red clay? Moet loess and clay is bluegrey in color, but when it is heated to more than about 700°C, iron minerals in the clay oxidize. And when they do, they color the brick red. And in addition to the rouges-barre, the walls often contain bluegrey sandstones (which when weathered turn brown). The roofs consist of shales, or tiles: that’s basically the same stuff, but the first was geologically baked, and the second was baked by us.

The cobble road of Paris-Roubaix

But today will mostly be about the nightmare of the mechanics that have to prepare and fix the bikes: the famous cobbled roads of northern France. These roads, or ‘drèves’, a word derived from Dutch and that also underlies the word ‘drive’, was made by farmers in the region who did not want to get stuck in the mud. They used hard sandstones, and even harder diorite from the Silurian of Avalonia: the ‘Henegouwen porphyry’ that is mined in a quarry across the Belgian border. ‘Porphyry’ means that there are ‘chunks’ in the rock, which in this case consist of plagioclase minerals. These started to crystallize the first when the magma cooled - so-called ‘phenocrysts’. But also the roads of northern France cannot withstand the progress of society: for repairs today, rocks are used that come from Scandinavia. And they are protected: in several cases the local population cannot use them anymore. Only the riders in the peloton can, who come from all over the world to France, to race.

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 a broad public.

Douwe van Hinsbergen