Although worldwide a new epoch began, things in southern Limburg did not change dramatically: the 'Cretaceous sea' flooded the area for another 5 million years. However, the fauna was completely different: across the KT-boundary 60% of all species died out, including Mosasaurus, which until then had been the terror of all Ammonites in the Cretaceous sea. The sea finally withdrew from southern Limburg 65 million years ago; the area became land again. A thick sequence of marly limestone ('mergel') remained, interspersed with flint. Sixty-five million years later still, 'mergel' and flint would prove to be important raw materials. The Tertiary is currently subdivided into five stages: the Paleocene, Eocene, Oligocene, Miocene en Pliocene.