After a major marine transgression at the beginning of late Cretaceous, vast depressions in the Bohemian Massif were filled with successions of shallow marine sediments. These are dominated by sandstones, claystones and spiculitic marlstones and reach a thickness of up to several hundred metres. These sediments host rich communities of marine organisms, notably bivalves, echinoderms, sponges and cephalopods. Rarely present are also remains of vertebrates and terrestrial flora.