The Colorado Plateau Story: the real Jurassic Parks

Today we make a return to our series on the geologic story of the Colorado Plateau. The Mesozoic Era left a stunning variety of rocks across the region. The Triassic story, with the Moenkopi and Chinle formations has been told, but today we explore a few of the real Jurassic Parks, the ones in which the scenery is formed by rocks laid down during the Jurassic Period. The scenery makers today are the Wingate Sandstone and the Kayenta Formation, which can be seen especially well in three parks: Capitol Reef National Park, Colorado National Monument, and Vermilion Cliffs National Monument. The Wingate appears in a prior post involving Canyonlands National Park. I notice that Geology Happens is a fan of the Wingate as well.



The Wingate Sandstone formed in early Jurassic time as sand dunes migrated across the increasingly arid region. The layer tends to be bright orange-red, and has the crossbedded structure typical of rocks formed from dunes. Dune environments are not generally conducive to the preservation of fossil material, but reptile trackways are occasionally discovered.

The Kayenta Formation lies above the Wingate and is most often exposed as a series of colorful ledges and slopes. It formed as a series of rivers flowed across the dune fields from the Wingate deserts. It contains numerous trackways of dinosaurs and other reptiles. Fairly easy to erode, it is not well-known as a scenery-former but it plays a critical part in the dramatic scenery at Zion National Park (to be covered in a near-future post).

Colorado National Monument lies just south of Grand Junction and Fruita, Colorado (the top picture, and this one from a previous post). It is one of our lesser-known parks, but is well worth a visit if you travel through the region. The park preserves a spectacular eroded monocline, which is a folded sequence of rocks where the layers are horizontal at either side of the flexure, much like a rug thrown over a stairstep. A marvelous paved road snakes in and out of canyons, mostly within the Kayenta Formation just above the Wingate cliffs. And the Wingate cliffs are incredible, a sheer drop just a few steps beyond the edge of the road. Although such roads are not being built in national parks today as a matter of preservation policy, I have to appreciate the effort that went into the construction of this one. There are many, many wonderful overlooks, a modest campground, and a decent visitor center. Lots of services are available in the nearby towns. Some very important dinosaur discoveries were made just outside the park (in younger formations), and signs point to the quarry sites.



Capitol Reef National Park is illustrated in the second picture above, and in the uranium post a few days ago. The park lies in the practically unhabited region of south-central Utah (Torrey and Hanksville are the main urban centers with a combined population of about 500). The park preserves another spectacular monocline, the Waterpocket Fold, that runs nearly 100 miles in a north-south direction. The "reef"of the park name refers to the difficulty settlers had in crossing the rugged eroded rocks of the monocline. Even today, a single paved highway crosses the structure, and a few unpaved gravel roads. A Mormon settlement (Fruita) was established here in the 1800's, and the fruit orchards are still maintained by the park service and volunteers. A favorite park activity is picking cherries and other fruits in season. The region was utilized in earlier times by the Fremont people, a lesser known culture than the Ancestral Puebloans of Mesa Verde and other areas to the south.


Vermilion Cliffs National Monument is at the same time one of the more familiar and least-visited monuments in the United States. The interior of the park is rarely visited, and there are few facilities of any sort for tourists; a true wilderness park. On the other hand, hundreds of thousands, maybe millions of people drive past the park on Highway 89 from Jacob Lake to Lees Ferry and Navajo Bridge, where the Vermilion Cliffs cannot be missed or ignored. The sheer cliffs rise 3,000 feet above the highway, and are just beautiful. California Condors have been re-introduced into the region, and the lucky traveler might just catch a glimpse.


Even with my more or less yearly visits to the region, I have never been in the monument, with one huge exception. Most of the park is a vast high plateau, but the northeast portion of the monument is dissected by an incredible gorge carved by the Paria River. The headwaters of the Paria lie to the north at Bryce Canyon National Park. Where the river reaches the Navajo Sandstone, it has cut an extremely deep canyon, the Narrows, with a major tributary, Buckskin Gulch, which is one of the longest deepest slot canyons in all of the Colorado Plateau.


I was in the Boy Scouts, and one of the last trips I took as a scout was a six-day backpack down the Paria River to Lees Ferry. It was one of the greatest adventures of my life, and probably had a lot to do with my eventual choice to become a geologist. Just the same, I knew little of geology at the time, and I would dearly love to repeat the adventure one day. I will search for my pictures of the event, but I had a camera I had no idea how to use, so the pictures that came out at all were over- and under-exposed. Plus, I dropped the camera in quicksand, and it was never the same after that (i.e. it never worked again at all).


Have you ever been in real quicksand? It's not very much like Hollywood quicksand, but it was an invigorating experience sinking to my knees most unexpectedly. I had walked across about 50 feet of it before actually breaking through, and it was lots of fun getting back out!

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 7: Mountains Rise

Where did all the dirt come from??? During the previous post, we discovered that something had caused thousands of square miles of land across Arizona, Utah, and Colorado to be inundated with mud and sand more than 1,000 feet deep. These rocks formed above sea level, as deltas, floodplains, river channels, and sand dunes. In the Grand Canyon, these layers of red sediments are called the Supai Group and Hermit Shale. In southern Colorado and Utah, related rocks are referred to as the Hermosa Group, the Halgaito Formation, the Cedar Mesa Sandstone, and the Organ Rock Formation (the Organ Rock Formation makes up the base of the spires of Monument Valley).

On the other hand, today's photograph shows a canyon near the eastern edge of Colorado National Monument, one of those oft-overlooked gems that are scattered about the Colorado Plateau. The grayish scrub-covered rocks in the bottom of the canyon are composed of the same 1.7 billion year old metamorphic rocks that were seen in the Grand Canyon and Black Canyon of the Gunnison. Unlike the Grand Canyon, there is no Grand Canyon Supergroup, no Tonto Group, no Redwall Limestone, no Supai Group or Hermit Formation. None of the rocks of the Grand Canyon are found here at all. The rocks forming the canyon walls are Mesozoic in age (less than 245 million years old), with the lower slopes composed of Triassic Chinle Formation, and cliffs of Wingate Sandstone (more on these later). So we have a second mystery: why are the Paleozoic rocks missing in southwest Colorado when they are thousands of feet thick in adjacent regions?

Pioneering geologists in the region soon recognized that the source of the sediments found in Arizona were mountains that had risen in southwest Colorado during Pennsylvanian and Permian time. The mountains were pushed so high that all the overlying Paleozoic sediments and a considerable amount of the Proterozoic metamorphic rocks were washed onto the coastal plains, leaving a denuded surface of metamorphic rocks that were not covered by sediments until Mesozoic time. These mountains are referred to today as the Ancestral Rocky Mountains or the Uncompahgre Uplift. As we look at the unconformity between the Proterozoic and Mesozoic rocks, we are looking at the eroded roots of what must have been an extraordinary mountain range rising from the interior of the North American continent.

Which leads to the next vexing question: what were mountains doing within the continental interior? Mountains most often form along a continental margin (think of the Andes, Cascades, or Coast Ranges). For a long time, no particularly good explanation could be offered to explain the development of these mountains.

More in the next post! And a salty surprise!

Time Beyond Imagining - An Intro to the Colorado Plateau

I would like to use a series of posts to illustrate the incredible story revealed in the rocks of the Colorado Plateau where I held class for the last two weeks. The Colorado River and her tributaries have laid bare a wide swath of the earth's crust, exposing more than two billion years of earth history. Few places offer so much information, exposed so vividly, within an area that can be toured over a matter of days.

The principle of superposition tells us that in a sedimentary sequence, the oldest layers will be exposed deeper within the surface, presuming that no overturning has taken place. It is when we find the base rocks upon which the deepest sedimentary layers are laid that we find the truly ancient rocks. On the Colorado Plateau, this happens in a couple of places: the Inner Gorge of the Grand Canyon, at Colorado National Monument, and especially at Black Canyon of the Gunnison National Park. If you wish to pick up a few pieces of the ancient crust, you might check some of the quarries in Unaweep Canyon south of Grand Junction, Colorado and other locales on the Uncompahgre Highlands; collecting is generally allowed on Bureau of Land Management and National Forest lands.

Black Canyon of the Gunnison is extraordinary. Over time, the deep crust in this region has been lifted and pushed upwards into mountain ranges thousands of feet high, in Proterozoic time, in late Paleozoic time, and now in Cenozoic time. Mountains rose, and mountains were eroded away. Eroded so completely that shallow seas washed over the remains of the mountains and covered them with hundreds and thousands of feet of additional sediment.

Today, the Gunnison River is misplaced, in a sense. Standing on the rim, one is struck by the way the land slopes away from the canyon walls; the deep valley more or less crosses a mountain ridge. The formation of the present-day canyon, which at 2,000 feet is often deeper than it is wide, is an entire post in itself, to be saved for another time. I want to concentrate on the ancient rocks that form the canyon walls.

The Proterozoic is what we call the time period from 2.5 billion years ago to about 545 million years. It is a vast expanse of time; all the story of complex multicelled life on planet Earth took place in the most recent 545 million years. The Proterozoic was four times as long!

The rocks exposed in the canyon walls at Black Canyon are not sedimentary; they have been changed by heat and pressure into metamorphic rock: dark biotite mica schist, gneiss, and hornfels. On the Painted Wall of Black Canyon, seen in the photo above, one can also see light colored dikes of granitic rock that melted and intruded into the older rocks about 1.4 billion years ago.

The story of these most ancient rocks on the Plateau begins with tomorrow's post!