Stage 16: Nîmes – Nîmes

The peloton crossed over the Gardon River several times today on their loop out and back to Nîmes. On the first of these crossings, they made their way across the Pont du Gard, the tallest Roman aqueduct bridge.

The Pont du Gard (source: Benh LIEU SONG, CC-BY-SA 3.0)

The Roman colony of Nemausus did not grow up around a major river, so as it became a regional capital, it needed more water. It was decided that the best nearby source was a spring in Uzès, some 20 kilometers away. Fortunately the Romans were pros at building aqueducts. There was just one problem: the Gardon River. The river sits in a gorge that would need to be spanned to get the spring water to the city, and the needed bridge would have to be massive.

A cross section of the Pont du Gard drawn in 1875 by Alfred Léger. The “pont moderne” is the modern road bridge that was constructed alongside the Pont du Gard in the 18th century. (source)


The Pont du Gard consists of three levels of arches and reaches a height of 49 meters above the low water mark on the river. It weighs around 50,000 tons, solid limestone quarried from a site downstream. The water conduit is carried by the top level and transported around 8 million gallons of water every day to the city.

Stage 15: Limoux – Foix

Today’s mountain stage took us into the town of Foix before heading up the Prat d’Albis. Foix, the hometown of French canal engineer Charles de Freycinet, sits upon the Ariège River, a tributary of the Garonne. But just northwest of the town lies an underground river, the Labouïche.

The underground river (source: site officiel de la rivière souterraine de Labouiche)

The Labouïche sits within a large cave system and is navigable for around 1500 meters, making it the longest navigable underground river in Europe. It was discovered in 1908 by Jules Dunac who, along with his two sons and a couple of young army officers, followed a stream known as the Fajal into the Aïgo Perdent, the Lost Water. Their exploration was taken up by pioneering speleologists Édouard-Alfred Martel in the 1910s and Norbert Casteret in the 1930s, who revealed the currently known extent of the river.

The entrance to the underground river (source: site officiel de la rivière souterraine de Labouiche)

The river terminates in a siphon, a permanently submerged chamber that must be dived to pass. No one has so far managed to cross that siphon. In 2015, Franck Bréhier tried and managed to pass about 100 meters into the siphon before he was forced to turn back.

Stage 14: Tarbes – Tourmalet

The ride up to the Col du Tourmalet takes us up the valley of the Bastan, which was the site of a massive flood in the summer of 2013. The Bastan arises on the western slopes of the Col du Tourmalet and flows through Barèges until it joins the Gave de Gavarnie in Luz-Saint-Saveur. These waters flow down into the Gave de Pau through Lourdes and then through Pau.

Over the winter of 2012 to 2013, precipitation was significantly higher than normal, with a very large accumulation of snow in the mountains surrounding the valley. An avalanche on May 31, 2013, blocked up part of the river, setting the stage for the flood in June. Over the weekend of June 15 and 16, a heat wave developed, melting that tremendous snow pack, and on the 17th and 18th, a storm passed through, dropping an additional seven inches of rain in 48 hours. The Bastan left its banks, overwhelmed the dam at Cabadur and flooded the valley down to Lourdes. Barèges was completely devastated by the flood, and Lourdes suffered significant damage as well.

Stage 12: Toulouse – Bagnères-de-Bigorre

The first of the Pyrenean stages takes us through two bath towns, Bagnères-de-Luchon, just before the climbing started today, and Bagnères-de-Bigorre at the finish line.

The baths at Bagnères-de-Luchon (source)

In 1835, the Scottish physicist James David Forbes took a trip to the Pyrenees and visited hot springs across the mountain range, including those at Luchon and Bigorre. In his travel narrative/geological survey/treatise on thermometers, Forbes had this to say about the springs of Bagnères-de-Luchon:

It is a curious fact that all the chief springs of Bagnères de Luchon issue from the granite within a few feet of one another, although their properties are believed to differ considerably, and their temperatures certainly do. They are kept separate by partitions connected with a vertical wall, into which slabs of stone (which may be removed) are cemented. The springs are called La Reine, La Grotte Supérieure, La Source aux Yeux, La Blanche, and La Froide; but all of these, excepting the two first, are apt to mix with one another; and I even learned that such a mixture was practised in order to give a greater apparent supply to some of the more esteemed of the springs. It is quite certain too that rain water mixes with some of these, which with other facts immediately to be noticed, render observations of temperature here of little avail. My observations were made on the springs as they flowed from beneath the wall just mentioned. These springs are highly sulphureous, and the two whose temperature I measured were copious.

Forbes’s main interest in the springs was their temperature. He would later devote a good portion of his career to the study of heat conduction, so the apparent warming of the springs at Bagnères-de-Luchon was absolutely fascinating:

The springs of Luchon have undergone most surprising changes. Camperdon, a writer of credit, and himself physician for thirty years at this place, assures us that the Source de la Reine was cold until 1755, when (on occasion of the great earthquake of Lisbon) it assumed a temperature of 41° Reaumur. The hot springs of many parts of Europe were affected by the same event. What shows that these springs are much connected with the sources of ordinary springs, is a curious fact mentioned to me by M. Barrau, one of the Inspecting Physicians of Luchon. In 1835, after great rains (in the month of May if my memory serves me rightly), the same spring, La Reine, delivered four times as much water as usual, and its temperature fell 16° Reaumur. This continued for twenty-five days, when it resumed its former state.

Forbes also cared a lot about the accuracy of thermometers. Thus, he (and I) appreciated his correspondent’s method of checking his thermometer before taking the temperature of the springs.

Boisgiraud’s observations were made with great care; I had an opportunity of verifying the accuracy of the copy I obtained of them by the originals. M. Boisgiraud informed me that he had verified the zero of his thermometer by frequently plunging it in melting snow.

Stage 11: Albi – Toulouse

Coming back after the first rest day, we had from Albi, home of the pigeon-eating catfish, to Toulouse, capital of Occitània. A ford of the Garonne at Toulouse, known as the Bazacle, from the Latin vadaculum, “little ford,” lies just down the road from today’s finish line.

The mills at the Bazacle as seen in 1642. (source)

The first mills at the Bazacle, built in the 11th century, floated in the river. These were replaced by standing mills and a dam was constructed across the river during the 12th century. The mill owners founded the Société des Moulins du Bazacle in the 13th century. The owners received shares in the company and a percentage of the future revenue while the company took ownership of the mills at the Bazacle, making the Société des Moulins one of the first examples of a joint-stock company in the world. Supported by the wealth of the mill’s, Toulouse’s industrial district grew up around the Bazacle. At the end of the 19th century, the mills were converted into a hydroelectric power plant that still operates to this day.

Stage 10: Saint-Flour – Albi

Stage 10 takes us into Albi where the shallows of the Tarn River are stalked by “freshwater killer whales,” a population of European catfish that have developed a taste for pigeon.


European catfish hunt pigeons in the shallows of the Tarn (source: Cucherousset et al. 2012, PLOS ONE)

Catfish were introduced to the Tarn in 1983 and have firmly established themselves in the river. It has been suggested that the growth of the catfish population depleted the populations of prey fish, pushing the catfish to seek out a new food source. An excellent and abundant prey for any urban predator are the pigeons that inhabit major cities around the world. But how is a bottom-feeding catfish supposed to catch a flying pigeon? This video, taken by the authors of the PLOS ONE paper that reported on this behavior, shows how they manage it.


The pigeons of Albi frequent a small island where they drink and wash up in the Tarn. The catfish have learned that they can thrust themselves out of the water, beaching themselves on the sandy shore, and catch an unsuspecting bird. This behavior looks a lot like a killer whale or a crocodile, but it doesn’t appear to have been learned by catfish populations outside of Albi. The combination of an urban river, an abundant pigeon population that comes close to the water’s edge, and an introduced predator — all of which are direct consequences of human influences on the environment — has created a novel ecological interaction that reveals the tremendous potential for evolution in the built environment.

Stage 9: Saint-Étienne – Brioude

This hilly stage took us from Saint-Étienne, near the Loire, to one of that river’s main tributaries, the Allier, home to “l’ombre de la rivière,” the “shadow of the river” — the grayling.

A grayling (source)

The grayling is related to salmon and is found across northern Europe, where it is a popular target for fly fishers. The Allier hosts one of the southwesternmost populations of grayling on the continent. This population is disconnected and genetically distinct from those in the rest of the continent, including that in the Rhône.

Stage 8: Mâcon – Saint-Étienne

A thrilling stage took us from Mâcon on the Saône, which is thus in the watershed of the Mediterranean Sea across the European continental divide to Saint-Étienne on the Loire, which flows into the Atlantic Ocean. Along the way, we passed by Villefranche-sur-Saône home to the church of Notre-Dame-des-Marais, Our Lady of the Marshes.

The church of Notre-Dame-des-Marais. (source: Andrzej Harassek, CC-BY-SA 3.0)

Local legend has it that in the 13th century or thereabouts, some shepherds were tending their flocks in the wet meadows surrounding the Saône when they stumbled across a statue of Mary sunk in the marsh. They pulled it out and decided to build a new church near the location, which became Notre-Dame-des-Marais.

A painting in the church depicting the discovery of the statue in the marsh. (source: Quintius, CC-BY-SA 4.0)

Stage 7: Belfort – Chalon-sur-Saône

A long and flat stage took us past the twin saltworks of Salins-les-Bains and Arc-et-Senans, home to a long history of salt production.

The Jura region was underwater some 200 million years ago, and as those seas dried up, they left behind extensive deposits of salt. This salt dissolved in groundwater and bubbled to the surface in salt springs. Since at least the 8th century, the inhabitants of Salins-les-Bains starting collecting this brine and boiling it over an open fire using wood from the nearby forests.

The saltworks at Salins-les-Bains (source: Jean Housen, CC-BY-SA 4.0)

By the 18th century, the salt content of that brine was starting to drop, and the forests arounds Salins-les-Bains were depleted, slowing the production of salt. The French monarchy was heavily dependent on salt production through the tax known as the gabelle, so the Ferme Générale, the king’s personal tax service, started investigating means of improving salt production. In 1771, King Louis XV appointed the architect Claude-Nicolas Ledoux the Inspector of Saltworks in Franche-Comté, and Ledoux set about designing his dream salt factory. The site chosen for the new Royal Saltworks was the town of Arc-et-Senans on the Loue River and, more importantly, south of the great Forest of Chaux that would provide a seemingly unlimited supply of wood for the burners. A 21 km pipeline would be built from Salins-les-Bains to carry the brine to the new factory.

The Director’s Building at the Royal Saltworks at Arc-et-Senans (source: Rolf Süssbrich, CC-BY-SA 3.0)

Ledoux was not content with building any old saltworks. He was determined to make the Royal Saltworks the center of an Ideal City built to Enlightenment principles of order and rationality. In his plan, the saltworks occupy the center of a circle consisting of outbuildings for the factory workers while outside the circle lies the planned city. The factory was constructed by 1779 and started producing salt. However, the French Revolution in 1789 put a halt to Ledoux’s plans for the city.

Ledoux’s plan for the Royal Saltworks at the heart of his Ideal City (source)

The Royal Saltworks continued production until 1895, when rail travel made it easier to bring sea salt into the interior of the continent and evaporating brine was no longer worth the cost. The Salins-les-Bains managed to hang on until 1962, when it was shut down after nearly 1200 years. The two sites were inscribed on the UNESCO World Heritage list in 1982 and are open to the public today.

The underground gallery at Salins-les-Bains dates to the 13th century (source: Jean Housen, CC-BY-SA 4.0)