Aerial photo of Rainbow Bridge
The Geology of Rainbow Bridge
Rainbow Bridge is but one of the endlessly fascinating landforms found on the Colorado Plateau and the story of its formation is an intriguing one. Natural bridges are rare, and differ from arches in that they form when a watercourse breaks through rock. Arches are far more common across the Colorado Plateau, although both are SHAPED by the same erosional processes.
The rock formations which comprise Rainbow Bridge are hundreds of millions of years old, deposited in a time when the climate and terrain were very different from what they are today. The base of Rainbow Bridge is composed of Kayenta Sandstone, reddish-brown sands and muds laid down by inland seas and shifting winds over 200 million years ago. The bridge itself is composed of Navajo Sandstone. This slightly younger formation (about 200 million years old) was created as wave after wave of sand dunes were deposited over an extremely dry period which lasted millions of years. These dunes were deposited to depths of up to 1000 feet (305 meters). Over the next 100 million years, both of these formations were buried by an additional 5000 feet (1,524 meters) of other strata. The pressures exerted by the weight of all these materials consolidated and hardened the rock of these and other formations.
The Colorado Plateau
The landscape that we know as the Colorado Plateau is, geologically speaking, a relative newcomer to the Southwest. The Colorado Plateau is an area of uplifted land, located generally around the Four Corners (the intersection of Colorado, Utah, Arizona, and New Mexico), with the largest sections of the plateau being found in Utah and Arizona. 60-80 million years ago, this area looked very different. It was a relatively stable, flat area. Then, geologic forces began to push the land upward. The greatest and most rapid uplift, however, did not take place until about 5.5 million years ago--a mere breath in geologic time. During this last uplift, the plateau rose some 3000 feet (915 meters) above the surrounding landscape. The uplift buckled the surface of the land. Mountains began pushing up and the earth warped and undulated like an ocean of rock. It began to resemble the fascinating assemblage that is so familiar to us today. But one key ingredient was still to come into play.