Vagabonding across the 39th Parallel: Having a "Swell" Time on the Reef!

If you thought I was talking about sipping pina coladas on a beach in the Caribbean, well, think about nearly polar opposites. Hot, yes, but not tropical. We were in a very dry and remote place in central Utah. This is a continuation of our July journey through the American West, vagabonding across the 39th parallel. The swell is a huge domal uplift, and the reef is a long monocline. If there is one thing to say about driving along the 39th parallel, it is that the landscape never stays the same for very long.

Having crossed the transitional zone of the Wasatch Mountains, we had left behind the Basin and Range Province, and were now on the relatively stable crust of the Colorado Plateau. The plateau, as the name suggests, is mostly made of high flat mesas and buttes of horizontal sedimentary layers representing some 500 million years of earth history (with much older stuff buried beneath). Although mostly flat, the plateau region has been faulted here and there, and in places has been warped into a series of basins and broad domes. The San Rafael Swell is one such dome, and it is huge, 130 miles long and 70 miles across. From space, it is one of the most obvious structures in all of Utah (above).

A dome in this case refers to the shape of the sediments. They've been pushed up like a blister, but erosion has sheared off the upper layers, exposing the older layers underneath. As we left Salina, heading east on Interstate 70, the signs noted that there would be no services for the next 100 miles. That is the kind of sign that makes you look at the temperature and gas gauges...

At least the road was not as lonely as some of the others that we had been traveling. And because of the beautiful scenery, there are a number of turnouts with nice photo opportunities. The rocks are striking. They represent the entire Mesozoic Era (the age of the dinosaurs, basically), which lasted from 245 to 65 million years ago. Upper Paleozoic rocks are exposed in the core, while early Cenozoic rocks are found around the margins of the dome. All in all, some 8,000 feet of sedimentary layers can be seen while crossing the San Rafael Swell (the Grand Canyon has only 4,000 of Paleozoic layers, in comparison).

At the top of the post, I said we weren't on a tropical beach. That's not entirely true, it's just that the timing is off a little bit. Some of the oldest rocks exposed on the Swell were deposited on a tropical beach, and we had arrived just 300 million years too late to listen to the crashing waves.

The road passes through an hour of horizontal Permian and Triassic rocks, but at the eastern end of the Swell, the landscape goes crooked. The margin is marked by a massive monocline, an odd fold that has flat layers on one side, a flex in the middle and flat layers at a lower level. The effect is something like a stack of rugs draped over a step. We had reached the San Rafael Reef.

I know this photo below looks cockeyed, but if you look at the telephone poles on the ridge line, you'll realize the camera was level. It was the landscape that was bent. These monoclines were called reefs because they were barriers to east-west travel, just as a coral reef can block access to a harbor. Resistant sandstone layers eroded into teeth-like spikes separated by narrow slot canyons that made passage difficult. The freeway could only be constructed here by blasting a passage through the rock (if you click on the opening photo, you can see the cut at the bottom, with a red car for scale).

I heard braying while exploring the last rest stop, and saw a small herd of wild burros in the canyon below.

I pulled over as we passed the base of the monocline, and got some shots of the reef looking off to the south. One can see how difficult it might have been to find a passageway for a wagon or a Model T. The jagged white rocks are the Navajo Sandstone, the remains of a vast Jurassic dune sea that covered several states (the Navajo forms the cliffs of Zion National Park).

I've heard there are some nice canyons to explore in the area, as long as it is not raining. The slot canyons are notorious for trapping hikers.

The monoclines and domes of the San Rafael Swell were formed by compression of the crust, which was related to the Laramide Orogeny that took place around 80 to 50 million years ago. This event also had a lot to do with the formation of the Rocky Mountains. The monocline hides a large thrust fault in the deeply buried ancient crust, but the overlying sedimentary layers folded rather than split when the fault was active.

At the base of the reef, horizontal layers appeared again, including the very colorful Morrison formation, the one made famous by the many discoveries of dinosaurs, including the huge brontosaurs (yes, I know they aren't really called that), the allosaurs, and stegosaurs.

A few minutes later we arrived at the settlement of Green River (population 600 or so) to look for lunch. We decided not to eat at the "Rant Lounge", and settled instead for Arby's.

Time Beyond Imagining: A Scrambled Landscape

It's kind of strange, what's happened to the Colorado Plateau. For more than a billion years, the region was one of the most stable areas on the planet, lying submerged under shallow seas, or lying exposed as desert dunes or river floodplains. Something like two or three miles of sediments accumulated, providing us today with one of the more complete records of the earth's history for the entire existence of complex life forms. Then, all of the sudden (in geological time), it got scrambled up. The Laramide Orogeny, starting a mere 70 million years ago, upended the sediments into a series of domes, basins and monoclines. The entire region was lifted around two or three miles, and the inevitable, relentless forces of erosion began to strip away the sedimentary cover. Though we have mentioned the early volcanism and the deposition of lake sediments at Bryce Canyon and Cedar Breaks in recent posts, the real story of the Cenozoic Era in the plateau region is one of subtraction of sedimentary rocks on a massive scale.

The reasons for the disruption of the crust can be traced to my home state of California, along with Washington and Oregon. A subduction zone had been present here for tens of millions of years, but aside from developing a chain of volcanoes that dropped ash all over the region, the effect inland had been muted. During the Laramide Orogeny, it is thought that a huge segment of subducted oceanic crust did not sink into the underlying mantle as would be normal, but instead got caught scraping along the base of the continent, leading to the deformation and uplift that characterized the orogeny. As the oceanic crust finally did sink, it allowed the burst of volcanism to sweep across the region, as mentioned in the last few posts. It left behind a strange erosional landscape in the American West that includes mountains completely buried in their own debris, whole swaths of deep crust forming the summits of mountain ranges, older rocks pushed over younger rocks, rivers that run nowhere, other rivers that cross mountain ranges in illogical places, and canyons whose locations make no sense at all. And there is that ultimate expression of river erosion for the world, the Grand Canyon of the Colorado River.

This is the kind of thing that brings home the concept that we are living in one of the best times to actually be able to comprehend the history of our world: the story of a billion years of geological history pushed upward and exposed for our study and understanding.

This all brings us to a sort of puzzle for the day. The photo above is a beautiful canyon in western Colorado. It has been carved through a massive uplift, and the walls along the bottom of the canyon expose Early Proterozoic gneiss and granite; it is one of the most accessible places for observing these rocks, in comparison say to Grand Canyon where one has to hike 4,000 feet downhill to see the rocks. We obviously drove to the site. The canyon is well over 2,000 feet deep, including more than 1,000 feet cut through the very resistant metamorphic rocks. The name of the canyon in the Ute language is "Canyon with Two Mouths". And that's the problem of the day. The canyon does have two mouths, but it's missing something in-between: a river. There are two pathetically small creeks flowing out of the canyon in two different directions, and in no universe could those little rivulets have carved this gorge. So, where did it come from? What carved it?

Fire Down Below - a Geological History of the Colorado Plateau

I've been seeing a fair number of geobloggers giving apologies for not blogging recently, and then I notice I haven't posted anything in nearly two weeks. It's obviously field season! I've actually been home for a couple of weeks, but other projects took precedence, not the least of which was a lot of weeding. I spent most of the last month discussing the active volcanoes of the Hawaiian Islands, and I will return to that project soon. For the moment I am taking the blogger's perogative to change the subject. I have come close to completing a year-long project on the geology of the Colorado Plateau, and there is a matter of some serious volcanic activity in Cenozoic time across the plateau country.

The Plateau is above all about horizontal rocks. They may form cliffs, but it a simple truth that the landscape is dominated by sedimentary layers of sand, silt, clay and lime. Throughout Paleozoic and Mesozoic time, thousands of feet of rock were laid down, but in Cenozoic time things were changing. The land was rising and buckling so that the last sediments formed on the plateau were in freshwater lake basins of somewhat limited extent that form the colorful rocks of Bryce Canyon and Cedar Breaks today. The Laramide Orogeny, which had lifted the Rocky Mountains, was also responsible for the vast changes in this region as well. The tectonic conditions responsible for the deformation was to have one other effect on the land: volcanism.

Driving across this strange and scenic landscape, one is struck every so often by the sight of rocks just sticking up into the sky. The ancients were pretty sure that these were the remains of monsters who had been frozen in stone. Although geologists have a wonderful time debating the reasons, the fact is that from about 30 million years to around 20 million years ago, volcanism swept across the region, leaving behind all manner of volcanoes, calderas, volcanic necks, and strange laccolithic mountains (see the next post).

Today's pictures include a couple of volcanic necks, including Shiprock in New Mexico. These are the cores of volcanoes that have been exposed by intense erosion.

Time Beyond Imagining - A not-so-brief History of the Colorado Plateau

I'm almost embarrassed to come back to this subject, but I have always had an unfortunate propensity to leave big projects unfinished and I would like to overcome it. I was also inspired by the reemergence of the Accretionary Wedge after a brief absence, and some of the final posts in this series will involve the newest theme of the Wedge for June. The subject of my blog is the geologic history of the Colorado Plateau, and it has been a theme of my blogs since last July, although posts have been sporadic of late. Check here to get caught up. My last post concerned the disaster that met the animals of the region at the end of the Mesozoic era. This post will seem similar, but now we are looking forward to the new world that was left to the survivors at the end of the age.

The Colorado Plateau of the early Cenozoic was a new and unique landscape, unlike anything that had passed before. Somewhat like the last post, it brings to mind another poetic passage in the Bible, from Isaiah: "For behold, I create new heavens and a new earth; And the former things will not be remembered or come to mind." The oceans, which for many eons had washed over the region in a series of transgressions and regressions, had receded far to the west and south, and never again would lay down sediments on the Plateau. The crust had been buckled by regional compression which reactivated ancient faults in the deep crust and forced the rocks into a series of structural domes and basins. This event, the Laramide Orogeny, is what raised the Rocky Mountains, and formed the broad outlines of sub-provinces of the Colorado Plateau that we see today. The edge of one of the uplifted domes is seen as the Cockscomb Monocline in Grand Staircase/Escalante National Monument. Farther north, the buckled rocks form the eastern edge of the Kaibab Plateau that is bisected by the Grand Canyon.

Rivers were established across the landscape, and some ended up with nowhere to go, forming vast lakes in what is now Utah, and Wyoming. Volcanism swept across the region as well. But more than anything else, the region was now far above sea level, so in most areas it was eroding, and rocks that had been laid down over the previous 2 billion years were now being stripped away and being carried to the sea. For the geologists of today, it meant that the evidence of the past was disappearing, and in many respects, the Cenozoic Era is one of the most cryptic time periods for us to decipher. Compared to Paleozoic and Mesozoic time, Cenozoic rocks are fairly rare on the Plateau, and thus little evidence remains to tell us what happened, or who was living here.

And that is the other part of the story...the ecosystem of the region, and indeed the entire planet had been decimated by...something. The iconic dinosaurs were gone, and many large denizens of the oceans had disappeared as well. The land-based ecosystems in the earliest part of the Cenozoic were depleted, especially of larger animals. The mammals, who for whatever reason had survived the extinction event, vied with the birds for dominance. The birds almost won! Some species of the avian clan were large flightless predators that would have done their dinosaurian ancestors proud. But by and by, the birds retreated to the environmental niche they were best adapted for, the trees and skies, and mammal-based ecosystems were established in forests and grasslands throughout the world.

Posts coming soon on the Cenozoic's unique national park on the plateau, and the evidence of volcanism in the region. And...where the heck did the Grand Canyon come from?