White Sands National Monument
The Chihuahuan Desert also harbors another geological marvel, the blinding white dunes of White Sands National Monument in New Mexico. These dunes are made of gypsum left behind by the evaporation of a once-vast Ice Age lake. The gypsum crystals are soft and easily scratched, which means that the tiny crystals reflect the light that strikes them like so many microscopic diamonds. That accounts for the blinding white color of the sand dunes. By contrast, most sand dunes in other deserts are made from crystals of quartz or tiny rock fragments, which do not scratch easily and therefore reflect far less light.
This area was once submerged in shallow seas, like the rest of the Chihuahuan. But some 250 million years ago an arm of that inland sea was cut off from the ocean, causing it to slowly evaporate as the Red Sea in Israel is doing today. The sea probably advanced and retreated repeatedly as climate shifts and continental drift caused sea levels to rise and fall. Each time a fresh ocean evaporated from the warm shallows, the water left behind its load of salts and minerals. Eventually, this created layers of gypsum up to 500 feet (150 m) thick.
Some 23 million years ago, fresh shifts in the crust caused the broad Tularosa Valley to fracture and drop between two adjoining, mountainrange-uplifting faults. That exposed rock layers loaded with gypsum from that ancient, vanished sea that gradually eroded out of the surrounding cliffs and mountains and ran down into the low-lying valley in between.
During the last Ice Age, these deposits ended up in Lake Otero, which formed at the low point in the valley floor. When Lake Otero finally evaporated 12,000 years ago, it left behind the fine deposits of gypsum grains. The restless winds driven by the desert's strong thermals soon gathered up the gypsum and created the awe-inspiring dune fields of White Sands. Every spring, winds from the southwest reach the steady, 15-mile-an-hour force necessary to create and sculpt a sand dune. Here, the wind has created a virtual encyclopedia of sand dunes, all made of the stark white gypsum. In the areas with the strongest wind, the sand forms dome-shaped dunes. In other places, the wind creates crescent-shaped barchan dunes, with horns pointing away from the wind and a downwind slope between the horns much steeper than the gradual, upwind slope. Sometimes barchan dunes form a chain of transverse dunes running across the wind direction that march along at a steady rate as a result of avalanches of sand down the steep slope facing away from the wind. Finally, the dune fields also include U-shaped parabolic dunes that are concave on the windward side, their long arms anchored by vegetation.
These dramatic, constantly shifting dunes host their own unique set of exquisitely adapted creatures. Many of the dune species are white versions of other, more common animals found elsewhere, since evolution would favor the shift to a white color to help creatures blend in with the sand and so survive to pass along their tendency toward white. A notable example is the White Sands swift, a lizard that is pure gypsum white except for a vivid blue throat patch and a light blue stripe on its belly. Coyotes, rabbits, and pocket mice here tend to have lighter coats, a striking example of how quickly evolution helps creatures adapt to their surroundings.
The gypsum dunes create often-surprising conditions. For instance, cottonwoods remain the only trees that grow in the monument, and they grow only on the margins of the dunes. That is because cottonwoods originally evolved to quickly colonize sandbars along desert rivers. As a result, they are fast growing and send strong, quick-growing roots questing out along the surface of the sand. That proved just the right tactic for getting a roothold in the soft, shifting sand of the dunes. The cottonwoods take root in the shallow, low-lying depressions that form in the dunes and grow fast enough to avoid inundation by taking advantage of the way in which the gypsum holds moisture several feet beneath the surface to create a saturated area that the stream-adapted cottonwoods tap into.
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