According to the sign at the entrance: "The Natural Bridge is the longest rock arch east of the Rockies. It was formed 200 million years ago when the sea washed the sandstone away, leaving iron ore veins holding the bridge."
This was fun to explore. There was no one there except me and it took about an hour to hike the 1.3 mile trail which included three large natural caves and a mini-waterfall. My only disappointment was that I didn't bring the wide angle lens.