The red of Rance is the result of marine deposits. Like the overwhelming majority of rocks in Belgium they therefore belong to the category of sedimentary rocks. These didn’t keep in our country the horizontality of their strata. These layers were twice lifted up by powerful mechanical actions. Most of the sedimentary rocks are crowded with fossils, i.e. elements formed by the presence and activity of organisms that lived at the time of sedimentation. But the flora and fauna underwent successive changes in the course of geological time. Therefore, the animal and plant remains have only a very remote relation to the animal and plant life which we are familiar with.
The Devonian grounds where we find most of the Belgian marble sorts are deposits of detrital elements of preexisting continental rocks or marine deposits. Those marine deposits, from the Devonian time, the coral reefs were built up by corals, animals living in colonies. The coral reefs appeared at the Frasnian stage, in isolated or continuous blocks. The isolated blocks are located in different places along a line between Trélon (France) and Hamoir (Belgium) passing through Philippeville, Givet and Durbuy. These cities have generally red marble at the base and top and grey marble in the middle. The outcrop of Rance is, in fact, formed by several limestone domes. The red color is due to the bacteriological activity and can vary in intensity.