St Pietersberg mountain is on the south side of Maastricht. It sits at the northern end of a limestone plateau that continues well into Belgium. This limestone (marl) has been used in construction for centuries. We can still find traces of it.
The Romans used marl to construct wells and burial vaults. In all likelihood, they relied chiefly on surface mining, in an open quarry. From the late Middle Ages, marl was mined underground in corridors. We can work out the timeline of marl extraction from inscriptions: many rock cutters and owners of the land above left their names in the mountain.
We can also deduce much from the structure and depth of the corridors. The corridor system expanded over the centuries, and much of it has now been mapped. The arrival of cement producer ENCI (Eerste Nederlandsche Cementindustrie) in 1926 marked a turning point in the development of the mountain. Surface extraction of marl laid waste to much of the corridor system and connecting tunnels. Marl extraction ceased in 2018.
One of the geological layers of St Pietersberg contains so many unique elements that it has been named after the city: the ‘Maastrichtian.’ This is internationally recognized as the last phase of the Cretaceous period.
Marl mining activities unearthed fossils in the corridors and the quarry. The remains of a Mosasaurus had been found way back before the time of the French, and were sold to France. Later on, the remains of other Mosasaurs were found in the quarry. It is standard practice to name the animal after the person who found it, Carlo and Lars being two such discoveries. Bèr is an exception: he was found by amateur palaeontologist Ruud Dortangs. During the excavation work, one of the quarry workers named the Mosasaurus Bèr, derived from the Maastricht name Lambert. And it stuck. Bèr remained Bèr, not Ruud.