Rainbow Bridge in Utah is the highest natural arch on Earth.
It is 94m (309ft) high, 10m (33ft) wide and 13m (42ft) deep at its peak. It stands above a canyon with an 85m (278ft) span.
The arch is composed of pink sandstone, and its apparent color varies as the sunlight changes. The stone is tinted dark lavender under a bright desert sky. It is splashed with a variety of reds and browns in the late afternoon sun.
The sandstone's subtle coloring caused the local Paiute and Navajo Indians to think of the arch as a rainbow that had been turned to stone.
Formation of Rainbow Bridge
Around 80 million years ago, many streams began to wind their way across a sandstone plateau from the northern slopes of Navajo Mountain, a peak 3,166m (10,388ft) high.
15 million years later, upheavals in the Earth's crust caused the entire region to be lifted up into a dome. The streams carved deeper and deeper courses for themselves.
In the stream known today as Bridge Creek, a large but isolated buttress of rock jutted out from the side of the canyon, forcing the waters to flow around it.
Alternate extremes of daytime heat and nighttime cold sides caused the sides of the sandstone rock to flake. It was thinned to a narrow window. Rock debris carried by the stream battered the base of the buttress and weakened it.
The water from the stream forced its way through, and the window widened.
As the arch became exposed to the wind, it was polished. At the same time, the stream continued carving into the canyon below, making the arch appear to grow even higher.