Hueco Tanks State Park outside of El Paso is famous for its Native rock art and its world-class rock climbing. Each year, thousands tour the ancient paintings, and climbers from across the globe come to scale the park's boulders.
But the rugged outcropping contains a feature most visitors overlook. It's one that's found across our region.
High on boulders, polished surfaces catch the light.
They're “rubbing rocks.” They were buffed smooth by now-vanished Ice-Age animals.
LeRoy Unglaub, of Las Cruces, New Mexico, is a passionate student of rock art. In 50 years of exploration, he's photographed more than 70,000 images.
His studies have taken him to isolated mountains across the region. Over the years, Unglaub noted a curious feature at these sites. Outlying boulders often had polished areas – 10 feet or more above the ground.
He could tell they weren't created by erosion.
“It's just on the front surfaces of rocks,” Unglaub said of the smooth surfaces. “It's not at the bases, it's not in the cavities. If it was by flowing water or wind, the whole rock would be polished. And what they were, they're called rubbing rocks, because a rubbing rock is a rock that has been polished over a long period of time into smooth, highly polished surfaces.”
Unglaub reached a conclusion that's endorsed by other researchers: these rocks were worn smooth my massive Ice-Age creatures, as they groomed and scratched themselves.
The Pleistocene Epoch began about 2-and-a-half million years ago. It ended less than 12,000 years ago. Glaciers extended across much of North America. The Southwest was cool and wet – good country for large herbivores.
The rubbing rocks often contain tiny gravel scratches – identical to markings found where African elephants groom themselves today.
The largest of our region's Ice-Age herbivores was the Columbian mammoth. It had a shoulder height of 12 feet – and weighed more than 20,000 pounds. The mammoths may have covered themselves in mud – and rubbed the rocks smooth as they scratched the mud off.
Other animals could have rubbed the rocks. Pleistocene West Texas was home to mastodons and camels, to elephant-like creatures called gomphotheres, to ground sloths that weighed 5 tons and to giant bison – with horn spans of 8 feet or more.
Hueco Tanks is just one of a dozen sites where Unglaub has documented rubbing rocks. There's Akela Flats, west of Las Cruces.
“It's flat desert, except for one pile of rocks,” he said of the site. “It looks like [the work of] a giant dump truck. The whole desert for miles around there is flat, except for this one pile, and it turns out there's polished rock around the base and the south end.”
In our region, rubbing rocks were first identified at a place called Cornudas Mountain. It's a striking volcanic formation in New Mexico, on private land just across the state line from Dell City, Texas.
Walter P. Lang was a scientist with the U.S. Geological Survey who studied Cornudas Mountain in the 1940s and wrote of “the repeated occurrence of large highly polished patches of rock.” He said there was “convincing evidence that the polished surfaces of rock about Cornudas Mountain are of animal origin.”
Unglaub has counted 34 rubbing rocks at the site.
Rubbing rocks are found in the Doña Ana Mountains near Las Cruces and at Providence Cone, also in New Mexico. And recently, Unglaub documented rubbing rocks just west of Sierra Blanca.
On boulders near Interstate 10, Unglaub found Native American petroglyphs – and smooth surfaces testifying to Pleistocene creatures.
Springs, tinajas and dry lake beds occur at many of the sites. In lusher periods, these water sources would have been prolific. It's clear why ancient herbivores, and, later, Native American artists, were drawn to these places.
With the close of the Pleistocene, the massive herbivores vanished. But their traces remain.
The megafauna were food for the region's earliest inhabitants – archaeologists find spearpoints amidst mammoth bones. The remains of a ground sloth were found in a crater west of El Paso – with hair and tendons intact.
But the evidence is not just underground. At rocky outcroppings and boulders, look overhead for polished surfaces. These rubbing rocks are calling cards of a vanished age.