Hikers pausing to view a rock formation known as The Wave in the Vermilion Cliffs National Monument in Arizona. AP Photo/Brian Witte

The Vermilion Cliffs are the second "step" up in the five-step Grand Staircase of the Colorado Plateau, in northern Arizona and southern Utah. The Vermilion Cliffs are made up of deposited silt and desert dunes, cemented by infiltrated carbonates and intensely colored by red iron oxide and other minerals, particularly bluish manganese.

The rock unit called the Navajo Sandstone features prominently in much of this landscape, and contains some of the largest and most abundant iron concretions found anywhere in the world. The Navajo Sandstone was named for the “Navajo country” of Arizona, Utah, and New Mexico. The red rock country on the Colorado Plateau where the Navajo Sandstone and other related  rock formations are prominently exposed is centered around the Four Corners region where the states of Utah, New Mexico, Colorado, and Arizona meet. This story of the red rocks started millions of years ago.

This story begins millions of years ago in a world and landscape  very different from today: during the Jurassic Period (144-206  million years ago) when the North American continent was at a different latitude, and this area was close to the equator in a belt of strong trade winds. These winds moved quartz sand to build dunes that covered an area bigger than the Sahara Desert. An accumulation of desert sand dunes is called an "erg" or "sand sea."  The largest erg to ever exist in North America is preserved in the Jurassic-age Navajo Sandstone  (approximately 180-190 million years old) that is up to 2,500 feet (750+ m) thick.

The Navajo Sandstone was deposited over a broad area of the Colorado Plateau and is now well exposed in national parks and monuments  such as Zion, Capitol Reef, Arches, Canyonlands, Grand Staircase-Escalante, and a number of surrounding areas, including near here. Other rock formations such as the Wingate Sandstone and Entrada Sandstone (see figure of Jurassic units) are also ancient sand dune deposits that show similar coloration and iron concretions. However, the Navajo Sandstone  is especially unique because it displays such a wide range of color (from white to many shades of red) and contains some of the greatest variety of iron concretions found anywhere in the world.

Brett Pelletier Photography

Blood of the Living Rocks

What colors the sandstone red?  The red color is caused by a union of iron and oxygen (an iron oxide) known as hematite (Fe2O3), a mineral named from the Greek word for blood. Iron is a powerful pigment present in many sediments and rocks, thus it commonly imparts color to the  rocks. Although red is the common pigment color, not all iron oxides are red; some  are brown or yellow (minerals - limonite or goethite), and some are black (mineral - magnetite). 

Some iron minerals are metallic yellow (mineral - pyrite consisting of iron sulfide) or green (minerals - chlorite or clay consisting of iron silicate). Although geologists have long understood that sandstone coloration is a function of varying amounts of iron, it is only recently that scientific studies detail how this happens.