Devonian geology in the Ardennes
During the Devonian, the Ardennes and Eifel (indicated in the figure on the right by a cross) were situated on the southern margin of Euramerica or “Old Red Continent”. In the south was Gondwana, which in part belongs to present-day northern Africa.
At the beginning of the Devonian period a major marine transgression reached the Ardennes which were levelled by extensive erosion. Sedimentation in this region was dominated by clastic facies resulting from the erosion of the Caledonian chain. The region was soon flooded and only several ancient massifs stood up against the battering waves. Remnants of the steep cliffs and rocky beaches are still present at the bases of ancient massifs, such as those of Rocroi and Givonne. The remains of such a massif can be seen at the famous "Roche à l'Appel" near the village of Muno (Florenville area). This small mountain is mostly comprised of the "Poudingue de Fépin" and gives a glimpse of an ancient beach line that existed here hundreds of millions of years ago near the Silurian-Devonian border.
On the Gedinnian, Siegenian and Couvinian
The Belgian Lower Devonian is divided (bottom up) into the Lochkovian, Pragian and Emsian. Officially, terms such as “Gedinnian” (Gedinnien, Gedinnium) and “Siegenian” (Siegenien, Siegenium) are no longer widely employed. Note that the lower and upper boundaries of these units do not necessarily correlate with those of the Lochkovian and Pragian, respectively. The positioning of the lower boundary of the Devonian is approximate because of the absence of graptolites and the limited conodont data available.
Some confusion exists around the “Couvinian” period, a term that is no longer used at present. It is occasionally claimed to be consistent with the Eifelian period, but in fact, it stretches from the upper Emsian to lowermost Givetian when compared to modern stratigraphic divisions. In 1950, Asselberghs and Richter & Richter correlated some stratigraphic units at an international level. Asselberghs' conclusion therein is unambiguous: "Le Couvinien n'est donc pas synonyme d'Eifélien" [The Couvinian is thus not synonymous with the Eifelian]. Calceola sandalina, a rugose coral that was formerly believed exclusively to be of Eifelian age, is in fact found as early as in the latest Emsian and continues to occur until in the lowest Givetian.
Map of the Devonian with the Ardennes and Eifel indicated on it.
The ever-tranquil Semois river near Herbeumont, southern Belgium