Land of the Sleeping Rainbow

The Capitol Reef area was called the “Land of the Sleeping Rainbow” by the Navajo because of the brilliantly colored canyon walls. The rocks of Capitol Reef are mostly sedimentary rocks which are a drab off-white color unless the rocks contain small amounts of impurities which act as pigments. Iron (“Nature’s Paintbrush”) in its various chemical states is the most common coloring agent found in rocks.
 

COMMON ROCK COLORS

Red to reddish brown to purplish rocks contain hematite (ocher) which is simply rust or iron oxide (Fe2O3). Less than half of a percent by weight of hematite is enough to color a rock brilliantly red. Red rocks were deposited under oxidizing conditions. A Capitol Reef example: the Moenkopi Formation.

Yellowish to orangish to rusty brown rocks are colored by limonite (rust containing water: FeO OH nH2O). Limonite forms under oxidizing and hydrating conditions such as in well-drained nonmarine or transitional environments that are barren of vegetation. Geothite, which is a mineral similar to limonite, forms brown concretions in some sandstone layers such as in the Navajo Sandstone. A Capitol Reef example: the Dakota Sandstone.

Light blue, greenish gray, and off-white rocks show the true colors of the sedimentary particles themselves. These persist in environments with neutral to slightly reducing conditions. A Capitol Reef example: the Shinarump Member of the Chinle Formation.

 
Dark green rocks contain minerals which contain reduced (ferrous) iron, and were deposited in stagnant marine basins, swamps, bogs, and lakes. A Capitol Reef example: the Morrison Formation.
 

Dark gray, brownish gray to black rocks contain incompletely decomposed organic matter which is preserved under reducing conditions such as in stagnant marine basins. A Capitol Reef example: the Mancos Shale.

Bright white rocks may consist of the mineral gypsum (CaSO4 2H2O) which is an evaporite mineral found in some rock layers including the Moenkopi Formation and the Carmel Formation. Gypsum also occurs as clear crystals known as selenite. Thin white veins of gypsum were deposited by circulating groundwater and are common in Capitol Reef.


 

Last updated: February 24, 2015

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