天生桥国家保护区中的sipapu桥，犹他州 Sipapu Bridge in Natural Bridges National Monument, Utah (© Fyletto/Getty Images)

天生桥国家保护区中的sipapu桥，犹他州 Sipapu Bridge in Natural Bridges National Monument, Utah (© Fyletto/Getty Images)

Bridges to the past

The story of this sandstone formation in southeastern Utah's Natural Bridges National Monument begins around 10 million years ago. That's when tectonic shifts began slowly lifting the 130,000 square-mile Colorado Plateau above the surrounding plains. In ensuing eons, the Colorado River's many streams eroded the elevated land, threading it with deep canyons. When water broke through one canyon wall into another canyon, sometimes a natural bridge like this one remained above the breach.

The national monument—Utah's first, proclaimed by President Theodore Roosevelt on April 16, 1908—protects three major bridges: Sipapu (pictured, and the largest), Kachina, and Owachomo. But even the president's pen can't stop the slow ravages of time. In a 1992 rockfall, Kachina slimmed down by 4,000 tons—and the remains of many fallen bridges dot the monument, hinting at the main attractions' eventual fate.