Danish Faxe and the fossil sea

Walk down into Faxe Kalkbrud and you are standing on a sea floor from about 63 million years ago, a few million years after the asteroid. Bright chalk walls, blue-green water in the pits, and the whole thing wide open to the sky.

What built this rock is not the warm shallow reef people picture. It was a cold-water coral mound complex, grown in the dark by azooxanthellate corals of the genus Dendrophyllia, which need no sunlight and no symbiotic algae. Those mounds are interleaved with bryozoan mounds and together they make up the Faxe Formation. They appeared in mid-Danian time, shortly after the mass extinction at the Cretaceous-Tertiary boundary, which makes this a record of how quickly reef-building life came back after the worst day in the history of animals.

The preservation is what specialists come for. In the Baunekule facies the aragonite shells recrystallised to calcite early, and the result is a fauna of around 220 species in exceptional condition: small gastropods, polychaete tubeworms, bivalves and brachiopods, animals that lived in the gaps between dead and living coral on the flank of a growing mound. It is a conservation lagerstatte, meaning things survived here that normally leave nothing at all. Vertebrates turn up too, with thirteen species of marine fish, fifteen of sharks, and crocodiles.

For a visitor the appeal is simpler. Fossil hunting is the main event at Faxe Kalkbrud, entry is free, and the quarry is open landscape with the Prismet viewpoint above it, not a roped exhibit. Bring something to tap with and something to carry finds in, and check the posted rules at the site, because a working quarry sets its own conditions and they can change.