The range of colors of the Navajo Sandstone –red, brown, pink, salmon, gold, and even white—results from varying amounts and forms of iron oxide within the rock, and in the case of the white upper portion of the Navajo, the overall lack of iron.
The processes behind the color variation are complex and took place in multiple phases over long periods of time. To start, the Navajo is made of grains of light-colored quartz sand, similar to those found in many modern dune or beach environments. Soon after being deposited in dunes, the sand grains were coated with a thin layer of reddish-brown iron oxide (the mineral hematite; a.k.a. rust). This was due to the chemical breakdown (oxidation) of very small amounts of iron-containing minerals within the sand, and made the earlier Navajo Sandstone a pinkish-red color overall.
Much later, ancient groundwater or hydrocarbons passed through the permeable Navajo Sandstone. The particular chemistry of the fluid altered the color of the rock layers in certain places, while they were still buried beneath the ground surface. Occasionally referred to as 'bleaching', some of these fluids removed the iron oxide, leaving the remaining rock white.
In some places iron minerals precipitated from solution to form iron concretions of various shapes and sizes (like those in the photo at left), as well as the iron-rich layers which cap hoodoos on Zion's east side (above).
These dark, iron-enriched concretions and layers are more resistant to erosion than the regular Navajo Sandstone, often leaving them standing out in positive relief against the softer sandstone.
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Last updated: June 13, 2015