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...