Almost every dune field in the world is made of quartz. Quartz is hard, chemically inert and practically insoluble: that is why it survives millions of years of transport and ends up being the only thing left once everything else has gone.
Not this one. This one is made of a mineral that comes apart in water, and that single difference explains why it is white, why it is the largest of its kind in the world and why it is exactly here and nowhere else.
01 It is not sand: it is gypsum
Gypsum is calcium sulfate, and it formed here during the Permian, when shallow seas covered this area and left beds of evaporites on the bottom. Later, tectonics lifted that sea floor, which today forms part of the San Andres and Sacramento ranges.
So far, nothing extraordinary: there is gypsum in many mountain ranges around the world. What is odd is what happens next, because gypsum has a property that disqualifies it as dune material: it dissolves.
In a normal system, rain strips it from the range, carries it dissolved in stream water, that water reaches a river, the river the sea, and the gypsum ends up in the ocean. It never gets to be sand.
In any basin with an outlet to the sea, this sand would not exist: it would be dissolved in the ocean.
02 That is why you need a basin with no outlet
The Tularosa Basin is a graben, a tectonic trough dropped between two uplifted blocks, and it belongs to the Rio Grande rift system. It covers some 16,800 square kilometres, about 240 kilometres north–south and up to 100 east–west.
And it is endorheic: no water leaves it. It is closed to the north by a mesa and to the south by a high plain. Water entering it, from rain and snowmelt off the ranges, has only two possible exits: to soak in or to evaporate.
That is the condition that makes everything else possible. The gypsum comes in dissolved and cannot leave. Each cycle of rain and evaporation leaves a little more mineral on the floor, and the process has been running since the climate dried out, some twelve thousand years ago.
03 The factory is in the southwestern corner
Some twelve thousand years ago the basin had lakes, streams and grasslands. The largest of those lakes, Otero, covered about 4,100 square kilometres of the valley floor.
As it dried out it left an enormous plain of selenite crystals, selenite being the crystalline form of gypsum, known today as the Alkali Flat. And it left an active heir too: Lake Lucero, a dry bed at one of the lowest points in the basin.
The factory is still running there. Rain, snowmelt and upwelling from deep groundwater periodically fill Lake Lucero with water carrying dissolved gypsum. When that sheet evaporates, it precipitates selenite crystals onto the ground.
And then the wind comes in, breaking the crystals, grinding them against the ground and carrying them northeastwards. That is the complete mechanism, and it is running right now.
04 How the dune field sorts itself out
That transport from the southwest explains the field's internal structure, which is sorted as if someone had classified it. On the southwestern margin there are dome dunes; in the centre, transverse dunes and barchans; along the northern, southern and northeastern edges, parabolic ones.
And there is a measurable gradient from west to east. In the west the dunes exceed fifteen metres, have little vegetation and move fast, with large grains of varied shapes. In the east they are small, heavily vegetated, slow, and the grains are small and round.
Groundwater follows the same pattern: it lies a metre and a half below the surface at the eastern end and barely thirty to sixty centimetres at the western one, where it is also older and saltier. And there is a surprising detail: the dune field is cooler and moister than the desert soil around it.
05 Forty-five species that exist nowhere else
An alkaline substrate, poor in nutrients, very salty and on the move is a hard place to live, which is why what lives here lives nowhere else. At least 45 species are endemic to the park.
Forty of them are moths. The other five include a pocket mouse, a woodrat, a bleached earless lizard and two camel crickets. And almost all of them share the same trait: they are paler than their relatives outside the dune field.
The explanation is direct: on white ground, a pale animal is less visible and less often caught, so it lives longer and leaves more offspring. The dune field has been working as a colour filter for twelve thousand years.
The plants solve a different problem, that of burial. Some grow fast enough not to be buried by the advancing dune; others form a hardened pedestal around their roots that holds them up once the sand moves on.
06 What to look at when you are there
First, the grain itself. Pick up a handful and hold it to the light: gypsum is translucent and shiny, not matt like quartz. And it does not burn: the white sand reflects so much radiation that you can walk barefoot on it at midday in summer.
Second, direction. If you enter from the east and move west, you will see the complete gradient: from low green dunes to tall bare ones. You will be travelling the system from its end to its factory.
And third, a practical precaution: the park is surrounded by a test range and closes to the public during some operations, usually for a few hours and with advance notice. It is worth checking whether it is open on the day before driving out.
