SARATOGA SPRINGS, Utah — As a lover of ancient rock art, Steve Acerson usually roams Utah’s backcountry searching for images of hunters and rams carved on boulders and canyon walls. But one morning, on a hillside speckled with those prehistoric petroglyphs, he was also finding signs of a younger civilization: Shotgun shells. Bullets. Shredded juniper trees. Exploded cans of spray paint. “It’s all been shot,” he said. “It’s just destroying everything.” America’s cultural divide over guns has gone into the woods. As growing numbers of hikers and backpackers flood national forests and backcountry trails searching for solitude, they are increasingly...