Pondering puzzling patterns in sandstone

By Jeff Mitton

The multitude of parks on the Colorado Plateau is testimony of our fascination with red rock country: Colorado National Monuments, Arches National Park, Canyonlands National Park, Capitol Reef National Park, Bryce Canyon National Park, Zion National Park, Grand Staircase of the Escalante National Monument, Paria Canyon-Vermillion Cliffs National Monument, Grand Canyon National Park and Glen Canyon Recreation Area.

Wind and water create stunning colors, patterns and forms in sandstone. Alternating bands of white and red strata, in particular, draw people to colorful hotspots such as The Wave and White Pocket.

Two weekends ago, I was on a camping trip with Gary Clendening, who shares my fascination with sandstone and penchant for photography. We were hiking toward Colonnade Arch on a bench of white sandstone, about 600 feet above Utah's Green River, when we came across patterns that I had never seen before.

It puzzled me that we did not see these forms or shades of burgundy anywhere else — what caused these colors and patterns?A specific layer of the white sandstone had burgundy pigments in many forms. Gary was facile at naming the figures — the one in the photo he called "brain," but others were snail, teapot and dragon. All of these were approximately a foot across.

But other forms, in the shape of ribbons or intricate swirls, were 10 to 15 feet long. It puzzled me that we did not see these forms or shades of burgundy anywhere else — what caused these colors and patterns?

Approximately 200 million years ago, wind pushed sands into dunes, and some were buried and stabilized to form sandstone. Molecules of iron were in the sand, and oxygen interacted with the iron to form oxides of iron — the same process that turns steel into rust. Iron oxides precipitated and gave the sandstones hues of red and orange.

If red sandstones were immersed in anoxic water, the precipitated iron oxides were modified (reduced) to become soluble in water, and due to the porosity and permeability of sandstone, the soluble iron molecules moved easily through the stone.

This process essentially removed the red pigments, bleaching red sandstone to white.

When the bleached sandstone was once again immersed in oxygenated water, the soluble iron precipitated again.

As the iron precipitated, it acted as a glue or cement to bind sandstone into round concretions. Some are small, dense and dark, ranging in size from peas to baseballs, and are colloquially referred to as hematite or iron nodules, iron sandstone balls and Moki marbles. The red pigments in these formations are so concentrated that the spheres look black.

Depending on how the oxygenated water mixed with anoxic water, other shapes of precipitated pigments would be possible. The ribbons and intricate swirls could be formed if oxygenated water mixed with a stream or current of soluble iron.

I suspect that the bizarre forms that captured our imagination — brain, teapot, snail and dragon — were formed when oxygenated water diffused into a pocket containing a diffuse solution of iron, allowing the precipitating iron to form swirls, concentric circles and bubbles.

All of this would have happened millions of years ago, hundreds or thousands of feet below the surface.

Now I understand why we found these burgundy patterns only in white sandstone. The iron bleached from the red sandstone was concentrated to form nodules, swirls and patterns that tweak the imagination.

Years ago, while exploring a formation called Yellow Rock in the Vermillion Cliffs, I was astounded by bands of lavender and burgundy that seemed to flow as a stream through yellow rock.

To see a fossilized stream of iron pigments, go to flickr.com/photos/jeff_mitton/9507133426.

The diffusion of soluble iron through porous stone, when precipitated by oxidation, produces beautiful patterns.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado.

Sept. 12, 2015