The rock is bryozoan limestone from the Lower Danian, laced through with bands and nodules of flint, and it was laid down when a warm sea covered what is now Jutland. That is the sentence that does the work: Denmark's climate today is coastal temperate, grey and windy for much of the year, and the material the wind is scouring was built by colonial animals in tropical water. The same Danian formation surfaces elsewhere in the country. It is the white face of Møns Klint, 160 kilometres to the south-east, and it comes up again in northern Jutland south of Aalborg, where it is quarried on an industrial scale as the raw material for cement. Sangstrup is the same story with none of the fame. Access is easiest at the bight about ten kilometres north of Grenaa, where the Karlby cliffs stretch north and the Sangstrup cliffs run south.

Coastal erosion keeps the face honest. Rock falls open new material regularly, which is why the fossil hunting stays good — the cliff is continually handing over surfaces nobody has examined. In places the sea has cut caves back into the wall. Mostly a narrow shore runs along the base, though a few coves are cut off and reachable on foot only at low water, and the tidal range here is slight, usually around thirty centimetres. This is an east-facing coast, sheltered from the Atlantic swell that pounds Jutland's western shore, and the difference shows in the delicacy of what survives at the waterline. Underwater the limestone keeps going: outcrops, ledges and crevasses threaded with kelp, continuing offshore as a reef called Kalkgrunden that rises to within about 1.2 metres of the surface near Grenaa harbour. Anglers, snorkellers and scuba divers know these cliffs far better than most walkers do.

Scattered across the cliff face are round, rust-stained cavities. The local account is that German warships used the cliffs for gunnery practice during the Second World War and that these are cannonball strikes, corroding outward from buried iron. It is a good story and it may well be true, but it should be flagged for what it is: an explanation passed along locally rather than one anybody has documented. The cliffs keep a certain amount of this kind of knowledge — half-remembered, unverified, and repeated because it fits.

Around the year 1150, stone was cut from a quarry near Sangstrup and shipped down the coast and inland to build the country church at Nødager, in the middle of the Djursland peninsula. Eight other churches on Djursland went up the same way. This was possible because the peninsula's geography was different then: a sound called Kolindsund cut deep into Djursland from the east, and boats could carry heavy cargo well into the interior. The estuary silted shut during the Middle Ages at what is now Grenaa Beach, and the sound became a lake for several centuries, until it was pumped dry in 1870 and turned into farmland. Drive across Kolindsund today and there is nothing to suggest a shipping route — just flat black fields, several metres below the sea that used to fill them.

The limestone had a second life as a chemical. At Glatved, twenty kilometres south of Sangstrup, an industrial kiln at the end of Glatved Strandvej burned limestone taken from the coast and the hills into quicklime; the operation ran until the turn of this century, though lime extraction along eastern Djursland goes back several hundred years. Burnt lime is what makes a traditional Danish farmhouse or village church that particular chalk white, and it is a principal ingredient of the mortar holding Denmark's brickwork together. Nødager church is both at once — built from the limestone and then whitewashed with it. Which makes the odd thing about Sangstrup odder still. The cliffs are barely known, not even reliably among residents of Djursland itself, a peninsula roughly forty kilometres square with about 80,000 people on it. Five kilometres of ancient reef, facing Sweden across a hundred kilometres of open water, and hardly anybody comes.