There are many reasons to visit Zion and Bryce Canyon national parks, but what you'll find in almost all sections of both parks -- and probably what you came to experience -- are their intricately sculpted and often beautifully colored rocks, from majestic, towering formations like the Great White Throne at Zion, to the delicately carved and whimsical hoodoos at Bryce Canyon. For these we can thank the geologic processes of uplifting and erosion.

The parks are located on the northwestern edge of the Colorado Plateau, a 130,000-square-mile region that covers the Four Corners area and encompasses about half of Utah, most of northern Arizona (including the Grand Canyon), much of western Colorado, and the northwest corner of New Mexico. Named for the Colorado River, which is responsible for carving many of the area's scenic canyons, the Colorado Plateau began as the result of uplifting 10 to 15 million years ago. Large, flat beds of sedimentary rock created from different minerals -- or combinations of minerals -- resulted in layers of varying densities. Forces within the earth pushed some of these sections of sediment up out of the earth. This uplift is especially noticeable in places such as Zion and Bryce Canyon. Here, uplifts along geologic fault lines created high plateaus with large chunks of rock, where you can see the stripes of the different layers of rock.

When wind and water began their inevitable erosion, layers of softer rock, such as sandstone and limestone, wore down faster than the harder layers of mudstone and shale. This created intricate and sometimes bizarre shapes. For example, the formation called Thor's Hammer, in Bryce Canyon, owes its hammerlike shape to a variety of rock densities -- the hammer section on top is harder and more resistant to erosion than the softer handle section below. Eventually, the handle will crumble and the hammerhead will come crashing to the ground. Erosion, as well as the shifting and breaking of rock, carved the sculptures of Zion and Bryce Canyon. The process almost always involves water and gravity, although wind and temperature, primarily freezing and thawing, play an important role as well.

Although there are also hoodoos similar to Thor's Hammer in Zion -- mostly on the park's east side -- many of Zion's rock formations appear more massive and rugged. But here, too, the rocks have been carved by the forces of nature, primarily water, as can be seen clearly when looking up from the bottom of the Narrows. This incredibly slim canyon -- 1,000 feet high but less than 30 feet wide in some places -- was meticulously carved through Navajo sandstone by the North Fork of the Virgin River.

The most important of Zion's nine rock layers -- at least in the creation of those imposing rock formations -- is Navajo sandstone, which is the thickest rock layer in the park, at up to 2,200 feet. The sandstone layer was created some 200 million years ago, during the Jurassic period, when most of North America was hot and dry. Movements in the earth's crust enabled a shallow sea to form over windblown sand dunes. Then minerals, including lime from the shells of sea creatures, eventually glued sand particles together to form sandstone. Later, crust movements caused the land to uplift, draining away the sea, but leaving rivers that gradually carved the relatively soft sandstone into the spectacular shapes we see today.

By comparison, Bryce Canyon is a mere babe. Its rocks were formed some 64 million years ago, created from sediments that were left behind when ancient lakes and rivers dried up. As at neighboring Zion, these rock layers were then uplifted and exposed to the ravages of nature. Much of Bryce Canyon's rock is limestone, relatively soft and crumbly, which was easily eroded into the park's numerous intricate hoodoos. The processes that created the rock formation in both parks continue today. Even to the naked eye, it's possible to see the changes that weathering has produced on Bryce Canyon's famous Queen Victoria hoodoo over the past 25 years.

How Nature Paints the Parks

The shapes are fantastic -- towering monoliths, massive mesas of stone, delicate spires, intricately carved sculptures, and squat little toadstools -- but what first catches our eyes are the delightful colors. What seems like an infinite variety of hues elevates these parks from geologic wonders to exquisite works of art. But where do the colors come from? What dynamic forces have meticulously painted these stone sculptures so perfectly?

The answer, simply and unpoetically put, is rust. Most of the rocks at Zion and Bryce Canyon are colored by iron, or hematite (iron oxide), either contained in the original stone or carried into the rocks by groundwater. Although iron most often creates red and pink hues, frequently seen in Zion's towering sandstone faces and Bryce Canyon's spectacular amphitheaters, it can also result in blacks, browns, yellows, and even greens. Sometimes the iron seeps into the rock, coloring it through, but it can also stain just the surface, often in vertical streaks.

Rocks are also colored by the bacteria that live on their surfaces. These bacteria ingest dust and expel iron, manganese, and other minerals, which then stick to the rock, producing a shiny black, brown, or reddish surface called desert varnish. Last, but certainly not least, is the work of water, which deposits salt as it evaporates, creating white streaks.

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