The Rocks You’ll See on Your River Trip
The Secrets of the Sandstone: Aeolian and Alluvial Rock in the Colorado Plateau
Many years into his infamous odyssey, just after escaping death by the ferocious Cyclops, Odysseus encountered a god-king named Aeolus. Aeolus, taking pity on the lost King of Ithaca, gave him a priceless gift: the wind.
Aeolus stuffed three of the cardinal winds, the North, East, and South, into a bag and gave the bag to Odysseus. The fourth wind, the West, was left to blow their ship home, finally, to Ithaca. Of course, this being a Greek myth, things didn’t work out quite as planned.
Just as they caught sight of their homeland, Odysseus’ curious and reckless sailors took his magical bag while he was sleeping. They opened it, and the furious winds burst forth, sending them all the way back to Aeolus’ island. Aeolus, fickle as the winds he controlled, refused to help them again.
While this Aeolus is one of the lesser known characters of Greek Mythology, his name does end up playing center stage for some geologists. And a float down the Colorado River might give some insight into why that is.
The Aeolian Deposits of Utah
The towering, red cliffs that you might see as you float down the Colorado River have something in common with those towering cliffs in Zion National Park and the famous pyres of Capitol Reef National Park: Navajo Sandstone.
200 million years ago, in the age of dinosaurs, there was an enormous desert known as an ‘erg’ that made up a vast expanse of the supercontinent Pangea. Wind would blow in from the arid center of the continent, or from the nearby coasts, and tiny particles of sand and other sediment would land in the erg, forming enormous dunes. Think of the Sahara, supersized.
After millions of years of compaction, erosion, and uplift, these sand dunes have become one layer in the famous red rock cliffs of Utah. Because the dunes were originally formed by wind, the sandstone is considered an ‘aeolian’ layer, named, of course, after Odysseus’ would-be savoir, Aeolus.
It’s fitting, perhaps, that a myth like the Odyssey, which was originally carried across Greece in the oral tradition and now sits in our own culture like a bedrock, should give the name to these rocks, which were once just wind-swept sediment but now make up some of the most magnificent and enduring wonders of the world.
But Aeolus isn’t the only god who’s handiwork has created sandstone rocks.
Alluvial Sedimentary Rocks
Poseidon, the Greek god of the oceans, may have had a greater hand in creating the incredible red rock formations of Southern Utah than even Aeolus. Take a close look at some of the layers of the dry sandstone, and you’ll see that it wasn’t always dry at all.
For instance, in the Moenkopi formation you’ll find alternating layers of sandstone, siltstone and mudstone that were creating by rising and retreating tides of ancient seas. You’ll see that the stone has ripples that were cast long ago. There are examples of ‘bioturbation’, burrows that have been left from ancient sea worms, and ‘smears’, where small reptile claw marks, perhaps those of a Eocyclotosaurus, can be seen in the sandstone.
Although this sandstone was also created in the time of dinosaurs (a long period of time, even by geological standards), it was not created by dry winds and sand dunes. It was forged on the mucky shores of riparian environments.
While the difference between aeolian and alluvial sandstones may seem like a matter of history, one that only the most specialized geologists would really care about, it can actually tell us a lot about why we find the rock formations of Southern Utah so incredibly beautiful.
The Beauty Revealed by Erosion
One aspect of the Navajo Sandstone formation that is so striking is its incredible ‘waves’ or ‘stripes’. In fact, one of the most famous rock formations in the world, named simply, The Wave, is cut from the Navajo Sandstone. The Wave’s beautiful pattern of variating red and white sandstone was created by the rather mundane changing wind patterns millions of years ago, during the Jurassic period. So, next time you curse the wind whipping your hair around, think about how that same wind might be stacking up a sand dune that will become a wonder of the future world.
Alluvial formations also contain their secrets. The Morrison formation, which can be seen throughout the Colorado Plateau, is one of the most recent sandstone formations in the area. This rock was originally laid down in a large swath of late Jurassic riparian land, and the many examples of fossilized flora pay testimony to the abundance of life at the time.
But it’s not the plant life that makes the Morrison formation famous. It’s the dinosaurs. The Morrison formation has the largest concentration of dinosaur fossils of all rock formations in North America. Famous and rare dinosaurs like the Allosaurus, Stegosaurus, and Supersaurus are found in the formation, making it a veritable goldmine for paleontologists.
Next time you are looking out your car window while passing through Green River Utah, remember that the mounds of eroding sandstone may contain the skulls of some of the largest, most spectacular creatures ever to walk the earth.