Today, White Sands National Monument in New Mexico has finally been accepted into the elite club of recognized U.S. National Parks. Its hundreds of thousands of miles of stark white sand dunes, whipped this and that way by the wind to constantly change their shape, appear infinite to the naked eye.
But not that long ago, it looked a lot different.
At the end of the last Ice Age, the lake to its side was just starting to evaporate. In the process, it left behind selenite crystals that eroded into the sands of Alkali Flat. When the Pleistocene came to an end, a mammoth, a human, and even a ground sloth (though we can’t tell you at what speed) made the journey on the eastern side of the lake. 10,000 years later, ground-penetrating radar picked up the tracks they left behind.
Like any other fossils, trace fossils are quite a lucky occurrence. “You need a surface that is soft enough to deform and leave an imprint, which is true for sand and mud. To preserve it, you also need the surface to ‘lock in’ that imprint somehow,” says Douglas Jerolmack, a geophysicist at the University of Pennsylvania. It would require another sediment to submerge the print faster than wind or water can modify it, cementing it into place – which is exactly what happened here at White Sands.
Researchers have known about the prehistoric tracks here for a long time, but they’re difficult to study up close. There could be millions of fossils of tracks around White Sands, but they’re often spectral – visible only in certain conditions, such as just after a rain, or at a particular angle of sunlight hitting the salts. “Most of them, you’ll only see intermittently or not at all. That’s why we call them ‘ghost tracks,’” explains Thomas Urban, a research scientist at Cornell University.
It’s easy for humans to confuse these ancient tracks from mammoths, ground sloths, canids, felids, bovids, and camelids for something else, such as a puddle that left an unremarkable dent in the earth before drying up. Though they are as big as a trash can lid, mammoth tracks “just look like a big round thing,” Urban says.
But this investigation all began In 1932, when a government trapper named Ellis Wright discovered 13 tracks “of unbelievable size” – 22 inches long and eight or 10 inches wide – imprinted in gypsum, and rounded up as many people as he could to go look explore them.
Using ground-penetrating radar machines to find these trace fossils, data comes back that can reveal the size, shape, depth, and even direction of tracks. Form this, the researchers can also infer biomechanics information, including how the creatures distributed their body weight as they strode along.
The researchers revealed that they expect to encounter more. So far, “there are always more than we thought there were,” Urban says.
For the average visitor, there’s no telling when they might cross some trace fossils here – if you can recognize it in the first place. These sands certainly hold many more stories beneath their surface.