Geologic Formations

Rainbow Bridges’ soaring arch has captivated humans for generations. Although it may look similar to many of the sandstone arches also found in the Colorado Plateau, natural bridges are a rarer phenomenon.

 

NPS Photo/M. Prachthauser

Before Rainbow Bridge

The sandstone that makes up Rainbow Bridge is hundreds of millions of years old. Back when the sediment which makes up the stone was first deposited, the landscape was unrecognizable from what it is today. First, the area was covered by inland seas accompanied by winds that laid down the mud and sand that would form the Kayenta Formation, the stone that creates the base of Rainbow Bridge. The environment eventually changed, becoming dryer, while sand dunes were wind swept into the area. This dry dune field lasted for millions of years and is what created the Navajo Formation, which the span of Rainbow Bridge is composed of. Both of these layers would later be buried under 5,000 feet (1,521 meters) of other rock layers or strata. The weight and pressure of these other rock strata compressed and hardened the rock of the Kayenta and Navajo formations.

The land here is in a constant state of change. Around 60-80 million years ago, the region of the Colorado Plateau was flat and generally stable. Then, molten rock, or magma from beneath the Earth’s crust began to swell, forcing the Colorado Plateau upward. This uplift took place around 5.5 million years ago, causing the landscape to rise some 3,000 feet (915 meters). The topography changed greatly, yet more changes needed to occur before Rainbow Bridge was formed.

 

The Sculpting of a Rainbow of Stone

Despite the modern day arid environment, the true sculptor of Rainbow Bridge is water. Before the bridge was formed, the Bridge Canyon stream meandered through the rock taking a path that turned back on itself creating tight curves. The uplift of the Colorado Plateau caused the angle of the stream drainage to steepen. Along with higher rainfall, water started to erode the sandstone. Sandstone, being made of its namesake sand and other sediments, erodes easily as the individual grains of sand are bound together by calcium carbonate, which is water soluble. With the bonds of the stone being dissolved, the flowing water is able to sweep the individual grains downstream.

 

This illustration shows the process of the formation of Rainbow Bridge.

 

With Bridge Canyon stream now at a steeper angle, the water no longer flowed through the bends and around sandstone fins with the path of least resistance; it now was hitting the sides of the fins with more force. The stream eventually forced its way through the fin. Flowing straight through the fin rather than around oxbow bend became the new path of least resistance. The water kept flowing through this new opening, widening it and making it bigger. Eventually it began to resemble the Rainbow Bridge seen today.

This process of creating Rainbow Bridge continues to this day, the water is still eroding away the sandstone, making the opening larger. Erosion and water created Rainbow Bridge, but will eventually destroy it as well. One day the opening will become too wide or the bridge too thin, and will come tumbling down. The dynamic landscape of Glen Canyon and the Colorado Plateau never stays still for long. We are lucky to experience it during this brief moment in geologic time.

 

Did you know?

What's the difference between an arch and a bridge? How they form!

Arches and natural bridges are both rock formations characterized by their openings in the stone. Arches form through seeping moisture and frost action. Natural bridges form by running water carving a path through the rock