Salt Flats: The Polygons on White Ground

When the water is gone, the surface of a salt flat is not smooth. It is divided into rough polygons, most of them five- or six-sided and around a metre or two across, each edged by a ridge of salt a few centimetres high. Walk in any direction for an hour and the pattern keeps going with the same spacing, which is why aerial photos of these places look like a tiled floor.

For a long time the ridges were put down to the crust simply drying and cracking. A more recent explanation looks underneath instead: salty water in the ground below slowly turns over in cells, heavier brine sinking and lighter water rising, and salt builds up where the rising water reaches the surface. The size of the polygons then follows the depth of that circulation, which would explain why it stays so even across such distances.

Photographing it brings its own problems. The ground is so bright that a camera's meter treats it as grey and the whole picture comes out dull, so it usually needs a stop or more of extra exposure. There is also nothing to measure by, no tree or rock of known size, and the net of ridges is often the only thing telling the eye how far away the horizon sits. On a screen that same net does a quiet job, giving a pale image just enough structure to hold its place behind everything else.