The oldest group of rocks in Belgium has an Early Palaeozoic age and forms a succession of several kilometers thickness, present over about 40 % of the (sub)surface of Belgium. The Lower Palaeozoic consists of three geological systems: from bottom to top, the Cambrian (541.0 to 485.4 Ma – million years ago-, ICS v2013/01), the Ordovician (485.4 to 443.4 Ma) and the Silurian (443.4 – 419.2 Ma). The names are derived from the name of the Welsh people in Wales or of the Latin names of Celtic tribes (Ordovices and Silures), living during Roman times in Wales, where rocks of that age form most of the (sub-)surface and where they were first described.
In the southern part of Belgium, the rocks crop out in the Ardenne Allochthon, below the Ardennian unconformity, in the inlier of Stavelot, of Rocroi, of Givonne and of Le Serpont (also called in litteris “massifs”). The Ordovician and Silurian are also present in the Condroz Inlier (also called Bande de Sambre et Meuse or Bande condruzienne). The three systems area also present in the Brabant Massif, in the subsurface below Mesozoic and Cenozoic cover of the central and northern part of the country and in few outcropping areas in the bottoms of valleys of the provinces of Hainaut, Walloon-Brabant and Liège. Many and deep boreholes in all these areas helped to identify these systems in the subsurface.
The sediments are mostly fine-grained with some coarser grained siliciclastic rocks, grey to black in colour with purplish and greenish colours in the lower Cambrian. The rocks are very poor in macrofossils, and have been dated by using graptolites, ichnofossils and the microfossils chitinozoans and acritarchs. The Brabant and the Ardenne inliers rocks show a low-grade metamorphic overprint ranging from epizone to anchizone.
Palaeogeographically the Lower Palaeozoic rocks of Belgium belong to the microcontinent Avalonia which rifted away from Gondwana at the beginning of the Ordovician with the Rheic Ocean forming in between, and drifted northwards towards Baltica. Avalonia docked with Baltica in the Upper Ordovician and then both docked with Laurentia in the mid-Silurian which initiated the Caledonian Orogeny. The Lower Palaeozoic rocks in Belgium formed a side branch of that extensive orogenic mountain belt.