The Wall of Bones: A Geologic Story
Visitors to the Quarry Visitor Center at Dinosaur National Monument are immediately confronted with a dramatic display of dinosaur fossils at a steep angle as soon as they enter the building. The quarry wall makes a perfect viewing angle for this Jurassic mural of death. But how did the quarry face become so steep?
The answer comes from geology, the study of the solid earth and how it changes. Studying geology is detective work, it requires keen observation in order to figure out a sequence of events for a certain landscape to appear as it does. As a visitor looks to the east from the Quarry Visitor Center parking lot, one sees a landscape of steeply tilted rocks. All of these rocks are sedimentary rocks, meaning they are made of smaller particles of preexisting rocks. Rocks are constantly undergoing weathering, or chemical and physical processes that break them down. To form sedimentary rocks, theses particles are transported by wind or water to a basin, where they are hardened into rock in a process called lithification. Any subsequent chemical, physical, or biological change to the rock after lithification is known as diagenesis.
As past environments in ancient Dinosaur National Monument changed over the course of hundreds of millions of years, from deserts to river systems, to shallow seas, the kinds of rocks being formed changed as well. These environments in different sequences of time are reflected by twenty-three different rock formations that can be seen in Dinosaur National Monument. Often the landscape was not ideal for sediment to be collected. The land is often rugged and elevated, and no deposition is occurring. But the landscape shifts again and becomes ideal for deposition. Thus, gaps in the rock record form called unconformities.
This package of sedimentary rocks that the quarry is found in were all deposited horizontally. This is a basic principle of geology called the Law of Original Horizontality. So any tilting, therefore, must have happened after the rocks already formed.
But how do they get tilted? The answer, as with many other questions in geology, is attributed to plate tectonics. Plate tectonics is the theory that the exterior of the Earth is composed of rigid plates, which fit together like a jigsaw puzzle, called the lithosphere. These plates are constantly in motion on top of the more ductile asthenosphere.
As the plates move (very slowly, only centimeters per year), they exert stresses on other rocks. Given enough time, these stresses caused by tectonic movement can cause rocks to deform, just like a manufacturing robot can contort a steel beam. Certain movements of tectonic plates are what cause mountains to form, an event known as an orogeny.
So how does all of this have anything to do with the quarry wall? Well, like many of the other formations of sedimentary rocks, the formation that the dinosaur fossils are found in, the Morrison Formation, was once a relatively flat landscape of rivers, lakes, and floodplains. The bones of dinosaurs were deposited in the bottom of a river and covered in sediment, which would be lithified into rock. The rocks containing the dinosaur fossils of the quarry were under an inland sea when a mountain-building event, the Laramide Orogeny, began. Tectonic pressures during this event caused the previously horizontal sedimentary layers to deform and fold. These folds in the rocks are known to geologists as anticlines and synclines. Synclines resemble a “U” shape, and anticlines resemble an upside down “U”.
As plate tectonics uplifted the package of folded sedimentary layers, the broke along tremendous lines of movement called faults. As the Yampa and Green Rivers flowed through the area, they eroded away the sedimentary layers, and because the great variety of layers have different resistance to weathering, dramatic hogback ridges, and stairstep canyons formed. That brings us back to the quarry, which is on the edge of an upside down, rocky “U”, called the Split Mountain Anticline.
The weathering that carved out the canyon country of the Monument was the same process that exposed the eight vertebrae of an Apatosaurus that Earl Douglass would discover, beginning the rich paleontological history of the Carnegie Quarry.