This is Cretaceous seabed. The sand that became the Elbe Sandstone Mountains was laid down under shallow water, lithified, lifted, and then cut apart along joints by rain and frost and running water over millions of years. Wherever the cementing was weaker, the rock went first. What remains standing is the harder material - the towers, fins and, in one place here, a bridge of stone left behind after everything beneath it was removed. The arch is not stable in the way a masonry bridge is stable. Sandstone spalls, freezes, absorbs water and sheds grain, and the lower section carries cracks that are measured rather than ignored. The formation is protected as a Czech national nature monument, a category that acknowledges what is obvious on the walk up: this is a temporary object working on a very long clock.

The Romantics found it before the tourists did, which is the usual order of events in this corner of Europe. Hans Christian Andersen made the walk in 1831. The painters Ludwig Richter and Carl August Richter came too, and their pictures did what pictures do - turned a piece of remote border forest into a destination. Infrastructure followed the reputation. A small bark hut went up in 1817 for forestry workers and fire watch, and in 1826 it was licensed as an inn, which made it the first place near the arch where a walker could buy something. Then, in the summer of 1882, the Hotel Falkennest was inaugurated below the span - Sokolí hnízdo in Czech, the Falcon's Nest in both languages - built for Prince Edmund of Clary-Aldringen as a fifty-bed hotel and a summer residence. It is still the strangest building in Bohemian Switzerland: a timbered Swiss-chalet fantasy wedged under a natural arch.

For a century and a half, the whole point was to walk across it. Steel stairs and railings were fitted, and generation after generation of visitors went up and over. Sandstone does not survive that. By the early 1980s, surveys found that boot traffic had removed a layer sixty to eighty centimetres deep from the crossing - not chipped, not vandalised, simply worn away by people enjoying themselves. In 1982 the arch was closed. The stairs and railings came out, and the deck has been out of bounds ever since. You now pay at a gate, walk the terraces below and beside it, and look. It is a small, unglamorous piece of conservation arithmetic: the thing everyone came to do was the thing destroying what they came for.

In 2005 the arch had a second career. Landscape sequences for The Chronicles of Narnia: The Lion, the Witch and the Wardrobe were shot here, and the rock made the transition to fantasy without much effort - it already looked like the entrance to somewhere else. But the actors are not on it. Because the deck had been closed for over twenty years by then, the shots in which characters appear to run across the span were filmed in a studio and composited in afterwards. The most photographed natural bridge in central Europe played the part of a bridge that nobody was allowed to walk on.

The arch belongs to a landscape, not a car park. The long-distance mountain path from Eisenach to Budapest runs past it, and the ordinary approach is a climb of an hour or so up from Hřensko, the village at the lowest point in the Czech Republic where the Kamenice meets the Elbe. The terrain has been in private hands since the hotel was privatised, so there is an entry fee and opening hours, which surprises walkers who have spent the morning in an otherwise wide-open national park. Come early. The stone takes light well at the beginning and end of the day, going from grey to a warm ochre, and the gap under the span frames the wooded ridges running north toward the German border and Saxon Switzerland beyond.