In 1995, Audrey and Frank Peterman pulled into Yellowstone National Park, stepped out of their truck, looked around, and wondered: Where are the people that look like us? As the black couple surveyed the lodge, Frank struck up a conversation with an older white gentleman. The man spoke wistfully of watching Yellowstone change over the years with the addition of new lodges and visitor centers each time he visited—first as a child with his father, then as a father with his children, and now as a grandfather with his grandkids. “It hit me in the pit of my stomach,” Frank recalls. “I thought I’d been a real good father. But I realized I had not given my kids the heritage of the national park, which is one of the most magnificent things that we have in America.” Frank describes that conversation as a moment of “reckoning.” Earlier that year, he and his partner Audrey had set out on a two-month, 12,000-mile road trip to experience the natural splendor of U.S...