Rainbow Bridge is made from sandstone formed during end of the Triassic and the Jurassic. Extreme fluctuations in climate during the Triassic and Jurassic periods—the region was alternately a sea and desert on par with the Sahara--produced layers of sandstone with different levels of hardness. Water had the last word in the Jurassic, and the great sea returned to cover these layers of sandstone and compress them so tightly that they would persist until the present day.

As Bridge Creek flowed toward the growing Colorado River during the last ice age, it carved first through softer rocks and veered away from the harder Triassic and Jurassic sandstones, eventually creating a wide hairpin bend that flowed around a solid “fin” of sandstone that would become Rainbow Bridge. The previous course of the creek is visible above the bridge in this image. Water flows back on itself at bends and wide spots, creating swirling eddies along the banks. As the creek flowed around Rainbow Bridge fin, eddies formed on both the upstream and downstream sides. The sediment in the creek eventually scoured the softer layers of sandstone away, leaving the harder layers behind. 

Courtesy: NASA