Legzira had two arches. One of them came down into the surf in 2016, on the Atlantic coast of Morocco's Guelmim-Oued Noun region, ten kilometres north of Sidi Ifni. What is left stands on the sand under rust-orange walls, an opening about twenty-seven metres across that reduces anyone beneath it to a speck. The reports said Legzira had lost the larger of the pair, and they had it backwards: the surveyors who catalogue arches measured the fallen span at about eighteen metres, so the one still standing is the bigger of the two. The colour is hematite, iron oxide cementing the grains of a rock that rivers laid down in the Lower Cretaceous, building a delta the sea later advanced across. You can read the proof in the wall itself: the pebbles set into the cliff are angular rather than rounded, so no wave ever polished them, and they arrived down a river.
The tide sets the timetable. For a few hours either side of low water the sand runs clear through the opening and you can wade the whole way under it on a film of water. At high tide the ocean is back against the foot of the rock and your way through is gone. The year's biggest tides come around the March and September equinoxes, draining the most sand and holding the gap open longest. Nobody swims here for long. The Canary Current drags cold water down this coast and pulls more up from below, and that is why a beach at twenty-nine degrees north never feels tropical. Handlers walk camels and horses along the tideline, where the wet sand reflects everything standing on it. The cafés built into the slope push their tables out over the surf for a tray of mint tea, bread and olives. And the mechanism that took the first arch has not stopped. Waves hollow out the base until the roof can no longer carry its own weight, and the one you walk under is on exactly that schedule.