Iron Mountain Road Pigtail Bridges

The engineering problem Peter Norbeck and designer C.C. Gideon faced on US-16A was real: how do you gain significant elevation on a steep hillside without blasting it apart or forcing drivers through tedious switchback after switchback? Their answer was the pigtail bridge — a structure that loops the road completely over itself in a tight corkscrew, gaining height in a compact space while leaving the hillside largely intact. Three of these wooden spiral bridges sit clustered at the northern end of the road near Keystone, and they remain the clearest reason to stop rather than simply pass through.

The wood construction was deliberate. Norbeck wanted the road to read as part of the landscape, not imposed on it. Logs were sourced nearby, cut to fit, and set in place — a rustic look that required solving real structural challenges, since the bridge surfaces are neither straight, level, nor flat. The three stone tunnels along the same stretch were also placed with intention: each portal is aligned to frame Mount Rushmore at the exit, a view that only works if you're moving through at road speed or on foot, not from a pullout with a telephoto lens.

The road is not maintained in winter, so the riding window runs roughly late spring through early fall. Wildlife — including bison — can appear without warning anywhere along the corridor, so a calm pace is practical, not just courteous. The north-end pullout near the bridge cluster is where most riders stop, and it earns the detour on its own.