The formation is a marine deposit of Ypresian age, laid down in the earliest Eocene between roughly 55.8 and 53.5 million years ago, and it reaches about sixty metres thick. It outcrops across the western Limfjord, from Silstrup in Thy by way of Mors and Fur to Ertebølle, in sea cliffs and in the quarries cut into them. Geologists split it into two members: the lower Knudeklint Member, named for the headland on Fur, and the upper Silstrup Member, named for its site in Thy. Beneath both lies the Stolleklint Clay, spanning the Palaeocene-Eocene boundary and grading up into the diatomite proper. The setting was a warm, subtropical inland sea over what is now the North Sea, its bottom water starved of oxygen. Nutrient-rich currents fed enormous diatom blooms; the blooms died and sank; nothing on the seafloor was alive to eat what arrived. That is the whole trick: anoxia is a poor place to live and a superb place to be buried.

The volcanoes were not local. They lay in the widening gap between Scotland and Greenland, erupting while the North Atlantic seafloor was pulling apart, and their plumes drifted east and settled over the Danish basin. The lower, negatively numbered layers are thin, scattered and acidic; the positive series above is thicker, closer-spaced and basaltic, the signature of volcanism accelerating as the ocean opened. Layer plus nineteen is the monster. That single eruption, 54.0 million years ago, threw out some 1,200 cubic kilometres of ash - enough to dust three percent of the planet's surface five millimetres deep - and ranks among the largest basaltic pyroclastic events in the geological record. The whole series totals roughly 21,000 cubic kilometres in 600,000 years. The same ash beds turn up in North Sea oil wells, dating the Sele and Balder Formations, and in England, Austria and the Bay of Biscay. A quarry face on a small Danish island is, in effect, a stratigraphic clock for northwest Europe.

Preservation here is not merely good. It is unreasonable. More than two hundred insect species have been recovered with colour patterning, individual eye lenses, even stridulation organs intact. Over sixty fish species turn up, many as complete skeletons, including sensational bonytongues and tarpons. Sea turtles come out with soft tissue and skin pigment readable. Land plants arrived from shore as leaves, seeds, fruits and flowers, with silicified redwood trunks up to nine metres long, some with mussels and barnacles still fastened where they drifted. The barnacles are extraordinary, and the only fossil shrimps known from Denmark come from these beds. Then there are the birds. The Fur Formation holds the earliest diverse bird fauna of the Paleogene: more than thirty species, some nearly complete, some in three dimensions, some with feathers and traces of pigment structure. Most are the earliest known members of their orders - trogons, swifts, ibises - and all were land birds, blown or washed out over a sea that would not let them rot.

Denmark has a law for this. Fossils judged to be of national importance are declared Danekræ, the state buys them, and they pass into public collections rather than private ones. Since 1990 the great majority of Danekræ finds have come from the mo-clay country of the western Limfjord, a measure of both the deposit's richness and the number of people on the cliffs with a hammer. Finds are split mainly among the Fossil and Mo-clay Museum on Mors, the Fur Museum, and the Natural History Museum of Denmark. The timing is part of the appeal. These creatures lived only about ten million years after the extinction that removed the non-avian dinosaurs and the ammonites, in a world that had just come through the Paleocene-Eocene Thermal Maximum and was still four to eight degrees warmer than today. The Fur fossils are how we know what that recovery looked like.

One last accident improved the site. Glaciers riding over the region at the end of the last ice age crumpled every exposed body of moler into a complicated pattern of folds and thrusts. Because the ash layers are numbered and traceable, geologists can follow one bed through the contortions and reconstruct exactly how the ice moved, and in what order. What would ordinarily be damage became data, making the cliffs an unusually clear teaching case in glacial tectonics. In 2026 the diatomite cliffs were inscribed on the UNESCO World Heritage List as The Bony Fish Fossils of the Western Limfjord: Evolution and Climate Adaptation in the Earliest Eocene. It is a mouthful of a title for a soft white rock quarried for a century into insulating brick and cat litter, which happens to hold one of the best-preserved early Eocene ecosystems on Earth.