The Morrison Formation

 

The Morrison Formation

To see it is to be centered in a section of time. The road cuts through the high desert of the Colorado Plateau, a landscape of heroic scale and subtle color, and there it is: a particular set of bands, a sequence of muted purples and battleship greys, decorating the hills. It’s a marker bed. Geologists know it as the Brushy Basin Member, the most widespread and recognizable face of the Morrison Formation. But for anyone with a passing interest in the deep past, it is something more. It is a signal, a signpost in stone. When you see it, you know, with a certainty that resonates deeper than academic knowledge: here were dinosaurs.

Rabbit Valley, Colorado  Red, Purple, Gray Mudstones and Bentonite Paleosols (ancient soils) of the Brushy Basin Member of the Morrison Formation

The story of the Morrison is written in mud, but it begins with fire. Picture the world 150 million years ago, in the waning days of the Jurassic Period. The map of North America is a half-formed thought. The Atlantic is a nascent rift, widening slowly, pushing the continent westward. From the north, a shallow, speculative finger of the ocean, the Sundance Sea, probes down into what will one day be Wyoming and the Dakotas. And to the west, along the continental edge, a chain of volcanoes, immense and active, stands as the consequence of a tectonic collision, a subduction zone where one plate dives beneath another. These volcanoes are the engines of the Morrison.

For millions of years, they pump ash into the sky—a fine, glassy dust that drifts eastward on the prevailing winds and settles like a blanket over a vast, low-lying basin. This basin is not a desert but a sprawling alluvial plain, a depositional apron sloping gently toward the distant Sundance Sea. It is a landscape defined by water. Great river systems, ancestors to none we would recognize, meander across the plain, carrying sediment from highlands that are now long gone. They are the circulatory system of this world, and their primary cargo is that volcanic ash.

The ash itself is geologically unstable, a temporary substance. Over the immense span of time, as it is buried, soaked, and compressed, its glassy silica structure breaks down. It alters. It devises a new identity, transforming into a peculiar type of clay mineral called bentonite. This is the material that forms the "candy-striped hills" near Hanksville, Utah, the pastel badlands that look so much like a "Mars almost like landscape." Bentonite clay has a singular, dramatic property: it shrinks and swells with phenomenal force. When wet, it becomes a slick, impassable gumbo; when dry, it cracks into a popcorn-textured crust. This constant turmoil in the soil makes it profoundly difficult for plants to gain a foothold, which is why, to this day, the exposures of the Brushy Basin can be so "completely devoid of vegetation." The chemistry of a long-extinct volcano dictates the botany of the present.

The rivers, though, were not merely passive conduits for ash. They were dynamic, energetic systems, and they left their own signatures in the rock. A river is a sorting machine. In its main channel, where the current is strongest, it moves sand and gravel. During flood stages—"higher pulses of energy," as a geologist might say—it can roll pebbles the size of a fist. In calmer times, it lays down fine sand. As the river snakes across its floodplain, it abandons old channels and carves new ones, leaving behind lenses of sandstone and conglomerate embedded within the finer muds of the floodplain.

Morrison Formation, Rabbit Valley  Remnant River Channel Sands of the Saltwash Member

These sandstone bodies are the architectural bones of the Morrison today. Being harder and more resistant to erosion than the surrounding bentonitic clays, they form cliffs, ledges, and caprocks. You see a feature that locals call a "toadstool," a whimsical balanced rock, and you are looking at a cross-section of an ancient river. A durable cap of sandstone, the fossilized riverbed, protects the soft pedestal of clay beneath it from being washed away. Look closely at the side of that sandstone. You might see fine, angled lines cutting across the main horizontal layers. This is cross-bedding, the preserved structure of ripples and sandbars that migrated along the river bottom 150 million years ago. The rock remembers the direction of the current.

It is within these river channel deposits—the sandstones and conglomerates—that the Morrison yields its most famous cargo. The formation is, by a wide margin, "the most fertile source of dinosaur fossils anywhere in North America." The bones are not found in the altered volcanic ash of the floodplains. An animal that died on the plain would have its bones scattered, weathered, and ultimately dissolved. But an animal that died in or near a river channel stood a chance of being quickly buried by sediment, its bones protected from the elements, beginning the long process of fossilization. A river in the Jurassic was both a giver of life and a potential tomb.

And what a life it was. The roster of the Morrison fauna is a litany of superlatives. This was a world of giants, an ecosystem seemingly designed to test the limits of what vertebrate life could achieve. The dominant herbivores were the sauropods, the long-necked titans that have defined dinosaurs in the popular imagination. There was Camarasaurus, the most common of them, a 75-foot-long, 50-ton behemoth built like a fortress. There was Diplodocus, whippet-thin by comparison, stretching nearly 80 feet but weighing a mere 15 tons, much of its length taken up by an incredible whip-like tail. And then there was Brachiosaurus, the giraffe of the Jurassic, its front legs longer than its back, holding its head high above the floodplain, standing nearly 70 feet tall and weighing upwards of 64 tons. These creatures, all of them, would go extinct at the end of the Jurassic, a "total wipe out" that cleared the stage for new forms to evolve.

Preying on them, or perhaps just scavenging their immense carcasses, were the large theropods. The king was Allosaurus, the most abundant large predator in the ecosystem, a 39-foot-long carnivore with a massive skull and serrated, blade-like teeth. It was the apex predator, the top of the food chain. But it had competition. There was Ceratosaurus, a medium-sized predator with a distinctive horn on its snout, and the truly formidable Torvosaurus, a massive, 33-foot theropod that may have specialized in hunting near waterways. Alongside the giants lived a menagerie of other creatures. Armored dinosaurs like Stegosaurus and the early ankylosaur Mymoorapelta browsed on lower vegetation. Bipedal plant-eaters like Camptosaurus and the small, nimble Dryosaurus filled out the middle tier of the herbivore community.

For a long time, the life of these animals was understood only through their skeletons. But in recent decades, paleontologists have begun to decipher the more subtle traces they left behind, reading the finer print in the Morrison’s pages. The story of dinosaur eggs is a case in point. For years, the utter lack of sauropod eggs from the Jurassic led to wild speculation. Perhaps, some suggested, these animals were viviparous, giving live birth to their enormous young. The discovery of soccer-ball-sized titanosaur eggs in Cretaceous rocks proved that at least some later sauropods laid eggs, but the Jurassic mystery remained. The working hypothesis now is that shelled eggs evolved independently in different dinosaur lineages, and that perhaps the Morrison sauropods laid soft-shelled eggs that were far less likely to fossilize.

Another view from the Rabbit Valley. It may not look too appealing, but here there be dinosaurs and other treasures.

The Morrison has, however, given up the oldest known dinosaur eggshell in North America. It is not from a sauropod, but from an ornithopod, likely belonging to the family of Dryosaurus. At sites like the Young Egg Site in the Salt Wash Member and the Sheets Nesting Site, discovered by a 12-year-old boy, paleontologists have found troves of eggshell fragments and the bones of baby dinosaurs. The evidence from these nests paints an intimate picture of dinosaur family life. Unlike the colonial nesting grounds of later hadrosaurs, these Jurassic dinosaurs appear to have been "solitary nesters," returning to the same favored locations year after year.

More intriguing still, the nests contain bones from two distinct size classes: tiny hatchlings and much larger "yearlings." This suggests the young were precocial—able to walk and follow the adults almost immediately after hatching, like ducklings. The theory is that the hatchlings would mimic the adults to learn what to eat, rather than being fed in the nest. The yearlings found dead at the site may have been the ones who failed to follow, waiting in vain to be led to food. The nests also contain the remains of another creature: a small terrestrial crocodile, nicknamed the "bunny croc," perhaps a nest thief, adding a layer of ecological drama to the scene.

Reading these stories requires an understanding of the grammar of the rock itself. At a certain level in the Brushy Basin Member, there is a "dramatic change." Below this line, the clays are of one type; above it, they are the puffy, swelling bentonites derived directly from volcanic ash. Geologists call this line the J-7 Unconformity. An unconformity is a break in the record, a missing piece of time. This particular line represents a major shift in the volcanic activity that defined the region, a moment when the character of the ashfall changed fundamentally. The book of the Morrison has missing pages, moments of the story that were never written down or were erased before they could be preserved.

For a visitor today, driving north of Highway 24 near Hanksville, with the solemn peaks of the Henry Mountains in the distance, this deep history is written across the land. The "candy-striped hills" are the legacy of ancient volcanoes. The "toadstool" hoodoos are the ghosts of ancient rivers. The dark patina on a rock, a desert varnish that a geologist might chip away to see the stone’s true color, is a thin veneer of the present drawn over an immense past. To stand in this landscape, to see the purplish band of the Brushy Basin, is to feel the presence of that past. It is to understand that the ground beneath your feet is not merely ground, but the compressed and altered remnant of a world stranger than any you could imagine, a world of giants that walked here when the rock was just mud.

In a future post, I'll provide a walkabout of the Colorado Walk Through Time in Rabbit Valley. There is a trail through the Morrison Fm. where you can see dinosaur bones in situ. It's a great walk to get a good feel for what is around you in dinosaur country.

Thanks for stopping by and having a read.

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