Rounding the Four Corners – July 2011

Don’t Judge a Rock by its Color

by Larry Larason

Some minerals appear in a variety of colors.  Fluorite, for example, may be purple, green, yellow, or transparent.  Small impurities in usually clear quartz crystals can produce amethyst [purple] or citrine [yellow to brown].   Quartz in the form of agate may be any color you can name.

Rock formations extending miles across the terrain also sometimes change their colors, usually subtly but sometimes abruptly.  Think about the White Cliffs just east of Gallup.  There the usually pink Morrison Formation sandstone is white just east of where it bends into the hogback.  Something similar happens with the Navajo Sandstone, which is red in some places, near-white in most others.

While there is no Wingate Sandstone in the cliffs north of I-40, it appears prominently in Colorado National Monument. (Photo by Gernot Keller)

Some rocks are so similar in color to others that the casual viewer may become confused between them.  Sheer cliffs of massive red sandstone are iconic to the Four Corners.  Tourists come to the region, somewhere they learn about the Wingate Sandstone, and then they think they see it everywhere.  Wingate Sandstone is pretty common west and north of the Defiance Plateau and Chuska Mountains, but we have other massive red sandstones here, as well.  Sometimes even geologists become confused.  The cliffs at Lupton, Arizona still confound them.  They can’t agree on exactly what formations are present there, although none of them believes any of those rocks are Wingate.  A tourist with a smidgen of knowledge might think otherwise, because he knows that Wingate is red and the cliffs at Lupton are certainly red.

What could be a better place to see Wingate Sandstone than near Fort Wingate, where it was named in 1884?  Unfortunately, there is no Wingate in the red cliffs north of I-40.  I’ve told the story before [GJ: July 2006] but here it is again in a short version.  Clarence Dutton named the sandstone for Fort Wingate in 1884.  In the early part of the twentieth century the US Geological Survey named a formation in Utah the Entrada.  Later, geological mapping showed that the Entrada and Wingate were the same.  Rather than continue the first name, the USGS kept Entrada.  However, there was another massive sandstone, once considered to be a member of the Wingate, that was allowed to keep the original name.

As presently defined, the Wingate Sandstone formed as sand dunes in an area west and north of the modern-day Chuska Mountains.  The massive cliffs that provide a backdrop for the small community of Lukachukai, Arizona are the Wingate Formation.  The dune field extended north beyond Moab, Utah and Grand Junction, Colorado, where it appears prominently in Colorado National Monument.  It is also seen in southeast Utah’s Comb Ridge and Canyonlands National Park.

Geologists have debated for a long time where the Wingate Sandstone fits into the geologic time stream.  Some consider it to be the upper member of the Triassic Chinle Formation.  Others see it as the first deposit of the Jurassic Period.  I think that more geologists now see it as beginning in the Triassic and crossing the boundary into the Jurassic, so it is roughly 206 million years old.  The time scheme was devised by humans, and, after all, the world doesn’t stop while we turn the page on our geologic calendar.

The Entrada Sandstone in the red cliffs east of Gallup is younger than the Wingate, but still Jurassic in age [180-140 million years ago].  Part of what confuses the observer is that the Wingate always sits atop the Chinle Formation, but in some places it is absent and the Entrada occupies that space.  In New Mexico the Entrada dune field covered about the northern half of the state, extending east to the Oklahoma Panhandle.  It also covered much of Wyoming, Colorado and southeastern Utah.

Parts of the Entrada were deposited in water and are silty rather than hard sandstone.  The silty strata tend to weather into hoodoos, as seen in Goblin Valley, Utah or the Baby Rocks east of Kayenta, Arizona.  Entrada Sandstone also is found in the fins and arches at Arches National Park

The red sandstone that forms Monument Valley is DeChelly Sandstone, the same that forms Canyon de Chelly.

A place that often confuses tourists is Monument Valley.  The spires, buttes and mesas there, with sheer cliffs of red sandstone seem to meet the definition of Wingate Sandstone.  But is it?  No.  It’s DeChelly Sandstone, the same stuff that forms the walls of Canyon de Chelly.  The DeChelly Sandstone formed in the Permian Period about 275 million years ago.  During that time North America was locked into the supercontinent of Pangea.  Mountains of the Uncompagre Uplift were shedding sediment to form the red beds so widely exposed over the Four Corners.  Dune fields east of what was then the western coast extended from the Southwest to Montana.  The DeChelly continues into New Mexico, where it is called the Meseta Blanca member of the Yeso Formation.  Like some of the other sandstones in the region, it changes color in places and appears white in some exposures in Colorado.  In Monument Valley the stately blocks of DeChelly sandstone sit on bases of Organ Rock Shale, which appears too crumbly to support them.

One of my favorite scenic drives in our area is on Navajo 12 north of Window Rock to Bluff, Utah.  Between the communities of Round Rock and Rock Point there is an area that my wife has dubbed “the mini Monument Valley.”  As the road climbs past a road cut in Chinle mudstones, Round Rock and Little Round Rock are seen to the north.  Los Gigantes Buttes can be seen to the northeast.  While the scene may mimic what you see in Monument Valley — massive sandstone “edifices” sitting on crumbly looking bases — these buttes are composed of the Rock Point Member of upper Triassic Chinle Formation at the base, capped by cliffs of Wingate Sandstone.

To summarize, the three major red, cliff-forming sandstones in the Four Corners are: [1] the DeChelly, [2] the Wingate and [3] the Entrada.  They are often confused.  But there are others, though not as red and not so likely to be confused with one another.  The Zuni Sandstone is one.  Another, the Navajo Sandstone, preserves a record of one of the largest deserts in the geologic record [150,000 square kilometers by one estimate].  During the Jurassic when the Navajo was deposited, dunes more than sixty feet high roamed the land.  Today we are living atop a huge sand pile that accumulated while this region was an inhospitable desert those many times in the ancient past.  Considering the geological record makes our recent drought seem rather insignificant.