Rising sharply from the plains of western Nebraska, Scotts Bluff was a critical landmark for 19th-century emigrants traveling the Oregon, California, and Mormon Trails. After weeks of navigating by rivers and rutted tracks, wagon parties would spot this towering formation and know they were on the right path.

Today, from overlooks reached by steep trails or a winding summit road, that same…

Along a trail in the woods of southern Illinois, the pathway suddenly tightens. This is One Horse Gap, a natural corridor carved into the sandstone bedrock of Shawnee National Forest.

The name hints at its dimensions. The passage is so constricted that, historically, it was said only a single horse could pass through at a time. The rock walls rise steeply on either side, creating a cool, shaded…