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.

The Real Jurassic Parks: Goblin Valley and the Entrada Sandstone

This paleogeographic map is courtesy of Ron Blakey at Northern Arizonal University

Welcome to one of the most unusual (and most isolated) parks in the American Southwest, Goblin Valley State Park. It is located in south-central Utah, miles from anywhere (the relative metropolis of Green River, population 973, is about 50 miles away). It is a fine introduction to the Entrada Sandstone and Summerville Formation, two of the more scenic layers within the San Rafael Group.

In the latest part of the middle Jurassic (around 165 million years ago) huge changes were taking place across the American west. A subduction zone had formed off the coast of California, and compressional forces had caused the faulting and uplift of a range of mountains across Nevada. A shallow sea occasionally extended into the region from the north, leaving vast deposits of mud, silt and sand deposited in tidal flats, while in other areas river floodplains and desert dunes predominated (the region still lay across the subtropical belt, much like the Sahara today). The sediments, which have a distinctive red-brown color, are known today as the Entrada Sandstone.

In other regions, the Entrada is primarily a cross-bedded sandstone that forms distinctive cliffs, but here at Goblin Valley, the formation is more variable, with alternating layers of sand, silt and shale that erode in a differential manner, forming slopes and ledges.

The Goblins are short spires called hoodoos, which resulted from the erosion along joints and fractures in the rocks. If you are a fan of the movie "Galaxy Quest", will immediately recognize the park, and will probably be looking for cute little purple cannabilistic space aliens and immense rock monsters. You will probably be disappointed in your search, but your imagination will probably conjure up even more fantastic creatures as you wander through the goblins.

The park includes a unique campground that is nicely developed with showers and flush toilets but very little shade (there is a nice group campsite too). During the summer we make a point of arriving in the late afternoon when the sun is approaching the western horizon. The views east are expansive, and few parks in the American west offer better star-gazing. The San Rafael Swell, a huge monocline, lies just to the west. The Henry Mountains, one of the last mountain ranges in the United States to be given a name, lie just to the south. Reaching over 11,000 feet, they are a series of laccoliths, a unique type of mushroom-shaped intrusion, almost, but not quite volcanoes (although volcanoes may have once existed above them). The region is also a great place for seeing pronghorn antelope!