The Devonian is a geologic period and system lying in the middle of the Paleozoic, between the Silurian and the Carboniferous. Its base is about 417.5 Mya (million years ago) and its top is about 358 Mya. The Devonian was named during the 19th century after the Devon in Southern England. However, its seven stages were defined during the same century in Belgium, Germany and France. Initially indeed, the Lower Devonian comprises the Gedinnian, Siegenian and Emsian stages, the Middle Devonian the Couvinian and Givetian stages and the Upper Devonian the Frasnian and Famennian stages. At the present time, the Emsian, Givetian, Frasnian and Famennian are still retained as valid international stages by the International Union of Geological Sciences. The name Frasnian comes from the locality of Frasnes near Couvin, on the south side of the Dinant Synclinorium whereas the Famennian is named after the Famenne region. The Givetian was originally defined in the Givet area (northeastern France) very close to the Belgian border and also on the south side of the Dinant Synclinorium. Following the decisions of the International Commission on Stratigraphy belonging to the International Union of Geological Sciences, the Gedinnian from the village of Gedinne in Belgium and the Siegenian from Siegen in Germany were replaced by the Lochkovian and the Pragian introduced in the Czech Republic, to which they correspond more or less. And according to the same authorities, the first stage of the Middle Devonian is now the Eifelian from the Eifel Hills in Germany instead of the Couvinian.
The Devonian is mainly exposed in the southern part of Belgium. The Lower Devonian is mostly represented by shales and sandstones which constitute the subsoil of the Ardenne highland. The Middle Devonian and the Frasnian are often characterized by reefal limestones with several beautiful caves. The Famennian starts with shales occurring in the Famenne, which are followed by more or less micaceous sandstones exposed in the anticlines from the central part of the Dinant Synclinorium separated by synclines with Dinantian limestones. Due to its numerous outcrops and the great diversity of its facies, Belgium has always played an important role in the investigation of the Devonian.
The Devonian is also known in two deep boreholes of the Campine Basin. In this northern area of Belgium, it is more particularly characterized by thick conglomeratic deposits whose age is Upper Givetian to Upper Frasnian.