When the peloton crosses the geological mysteries of Flanders

Beyond sport, the Tour de France also gives the opportunity to (re) discover our landscapes and sometimes their geological quirks. The Tour de France 2025 thus opens in the Hauts de France. Its runners will cross in particular, during these first days, the Flemish plain. It is the French part of the “Netherlands” which borders the sea of ​​the north of Boulonnais to the frieze. And has been subject to marine submersions for millions of years.

East of the Hauts de France, French Flanders is limited by the foothills of the Ardennes massif; To the south, by the hills which, from Cambrésis to Boulonnais, belong to a bulging that continues in England. Chaling this structure, a modest relief of less than 300 meters high, of which the Blanc-Nez is part, was cut by the overflow of the lake which occupied the North Sea during the periods of great glacial extension: this notch formed the narrow valley that is the Strait of Pas-de-Calais.

To the north, inner Flanders (the woodland) is made up of soft hills, made up of alternations of sands and clays, accumulated in a calm sailor basin 55 million years ago. The shore line was first backed by the hills of Boulonnais-Artois, and gradually moved away by migrating north, a distant effect of the formation of the Alps.

For a few million years, French Flanders has been episodically overwhelmed by sea due to interactions between plates and climate tectonics. Climate change of anthropic origin strengthens the phenomenon.

A landscape from the last glaciation

In the past, torrents were hitting the southern hills of French Flanders, like the Saharan wadis around certain mountain ranges. Many cailloutis tablecloths (REG) have thus scattered all over Flanders. A very hot climate period has froze some of these deposits in the shore of the time, cementing them by ferruginous oxides. These pebbles transformed into breastplates have protected from subsequent erosion the sands they covered, forming the alignment of the mountains of Flanders which surprised geologists and curious: Mount Cassel (176 m) is one of these witnesses.

During the quaternary (for 2.6 million years), the glaciations voted the sea level by more than 100 m, more or less drying the English Channel and the North Sea during each ice cream pic. Archaeologists, paleontologists and geologists knew how to reconstruct from the river terraces of the Somme, 10 glacial cycles of 100,000 years each. The work of the Seine-Nord-Europe canal will also contribute to specify these reconstructions.

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The current landscape of the Flemish maritime plain results from the shaping produced since the end of the last glaciation, that is to say around 20,000 years. Dutch, Belgian and French archaeologists who coordinate their research shows that the natural rise in the sea level is fading, but that the phenomenon is now generated under the preponderant effect of warming of anthropogenic origin.

Dikes from the 12th century

The last marine invasion was well documented by geologists: from the end of III^e century until IX^e Century, they highlighted the burial by a peat and then by marine deposits of traces of agricultural and commercial activities (Salines among others). In response, show inrap archaeologists, humans built during the 11^e and 12^e centuries of the dikes to protect themselves from occasional marine submersion, and have set up rainwater collection spaces to spare a freshwater resource.

Until then, in fact, each owner defending his field could only do so by returning excess water to his neighbors. This situation, which would generate conflicts, led from the 12th^e century Philippe d’Alsace, count of Flanders, to try to set up a common, ordered and controlled management system. But you have to wait for the XVII^e A century for a Dutch engineer, Cobergher, acquires the residual lagoon (Les Moëres) to build a mechanical device, the principle of which is still current. Only the energy shape and the control of maneuvers, self -regulated today, have since changed.

Today, polding – the drying up of coastal marshes to make cultivable land – is increasingly embarrassed by the rise in sea level, by the development of town planning and industrialization. The period of sustained rains, generating floods during the winter of 2023-2024 highlighted another limit: the processes of police were developed to drain agricultural land; On this occasion, it was also used as a regional sanitation work to drain the entire maritime plain. However, this system has never been sized for such a goal!

A management system now exceeded

The management principle is known as Wateringues. Each owner of a piece of land is responsible for cultivating and maintaining it: it rejects excess (rain and streaming, therefore soft), in a drain (Watergang) which surrounds its plot and pays a fee for them to be collected by a larger channel (Ringslot) which leads them to conventional canals, managed, by sailables from France.

The main problem is that today, at high tide, the flow of water to the sea can no longer be done naturally by gravity, because of the climb of the sea level. The water is therefore temporarily stored behind locks, either while waiting for the low tide, or to be taken up by pumps which will reject it above these locks. The episode of winter floods 2023-2024 has shown that a development is not diverted if we have not understood the functional system of the territory.

In this context, it is a question of rethinking in its entirety the development of this territory, carrying very heavy, economic, social, environmental issues. The depression of moeres, in particular, is today officially located under sea level: it should therefore be (gradually) released from any habitat and gradually transformed into a pool of rainwater and runoff. Various forms of economic valuation are to be studied. Only problem: the negotiators of the Utrecht Treaty (1713) passed the border in the midst of this depression … which implies dealing with this problem in a European framework.