In one of my insomniac browsings of Wikipedia, I came across the interesting factoid that the valley of the River Meuse in Belgium is perhaps the second oldest river valley in the world, after an occasionally flowing river in the Australian desert.
I began to wonder if this could possibly be true. The argument is that between Charleville-Mezieres and Namur, the river cuts through Paleozoic rocks which were raised up to the surface between 320 and 340 million years ago, in what is called the Variscan or Hercynian orogeny, the process which created the Pyrenees, the mountains of southwestern Ireland, Cornwall, Devon, much of Wales, Brittany, the Ardennes, the Massif Central, the Vosges, Corsica, Sardinia, the Eifel, the Hunsrück, the Taunus, the Black Forest and the Harz Mountains. The Appalachians were being formed at the same time.
But the source cited by Wikipedia, Environmental History of the Rhine-Meuse Delta by P.N. Nienhuis, doesn’t say this at all. It says only that the river “transects the Paleozoic rock of the Ardennes Massif”. The Paleozoic era is basically anything before 250 million years ago. But the fact that the river cuts through rock of a certain age shows only that it is younger than those rocks, not that it is the same age.
Now, there is a thing that needs to be explained. The river has eroded its way through the Ardennes, producing an impressive gorge, and also terraces higher up the valley showing where the water level once was. In particular, it winds through the Rocroi Inlier, a chunk of ancient rock which the Franco-Belgian border winds through, all that is left of one of the offshore islands of the ancient lost continent of Avalonia.
The Rocroi Inlier is not soft rock; it’s hardened and mostly igneous, though crushed and faulted. So on the face of it, it seems odd that the Meuse flows across it, rather than turning west and feeding the Oise to join the Seine. The traditional theory, mentioned without adequate citation in Wikipedia, was proposed by Charles-Louis-Joseph-Xavier de la Vallée Poussin in 1875: that the river flowed north before the Ardennes ever rose and continued to erode its traditional path even as the hills rose around it. There are plenty of cases like this worldwide, the best known being the New River of North Carolina, Virginia, and West Virginia, which flows through the Appalachians and is thought to pre-date them.
Not everyone agrees that the Meuse is so old. One other explanation that I’ve seen and tend to reject is the idea that this is a case of stream capture: that the northern lower Meuse gradually eroded back across the granite to capture the southern higher waters. Stream capture is very clearly the case further up the Meuse in France, where the Moselle captured its upper streams. You can still see the old Meuse valley in the landscape west of Toul. But that’s in a flatter and more forgiving landscape than the Ardennes. I don’t see the Meuse gradually eroding southwards back through the granite, eventually breaking though to France.
There’s another problem too. It looks like the area of the Meuse valley may have been underwater during the Hettangian age, roughly 200 million years ago. That would rather kill the notion that the river could be as much as 320 million years old.
In fact, the current consensus appears to be that it is much younger. In their 2000 paper “Sediment budget and tectonic evolution of the Meuse catchment in the Ardennes and the Roer Valley Rift System”, Van Balen and four co-authors state as if it were generally accepted that “The Meuse river system developed in its current position despite the uplift of the Ardennes since the Eocene [which ended 34 million years ago]. In the Ardennes, the present-day system was to a large extent established in the Pliocene [5 to 2.5 million years ago]; only minor changes occurred in the pattern of the drainage system during the Quaternary [since 2.5 million years ago]. During the Plio–Pleistocene [the last 5 million years], the rivers incised and a terrace sequence developed[.]”
I am not a geologist, and my French is not all that good, but Francis Meilliez in his 2018 paper Le Massif Ardenno-Rhénan, un massif ancien en cure de rajeunissement also has the Meuse happily flowing north, finding its way through the faults in the crushed granite of the Rocroi Inlier, until the Ardennes and Rocroi Inlier very slowly rose in the last few tens of millions of years, the river eroding its way down to its current level. This would explain why the Meuse river terraces, showing where it was previously, are not especially ancient.
I’d love to read some more about this, but I’m satisfied for now. The Meuse is not really so very ancient as all that – certainly not as ancient as the Rhine – but these are still processes that take periods of time which are impossible for us to comprehend. It makes you feel rather small, really.
The header picture I’m currently using was taken last July on the Lesse, a tributary of the Meuse right in the middle of the Ardennes.