Studying geology is like traveling through time. Journeying down the Green River, its course carved into the cliff-forming strata on either bank, eroding ever deeper into the past, my sense of distinct human and geological timescales fades away. Moving forward with the flow that will ultimately lead to the confluence with the Colorado River and into the Grand Canyon, we slide through the layers of deep time recorded in the rocks. Past swirls into the present as the water spirals around our paddles. All that matters is the fleeting moment at hand.
I imagine John Wesley Powell, leading the first major expedition on this river 150 years ago, must have thought similarly. In 1869, however, the vastness of geologic time had yet to be measured. The radiometric dating methods that allow scientists to calculate absolute ages of rocks and minerals had not been discovered and the theory of plate tectonics would not be embraced until nearly a century later. Powell’s observations are remarkable and his understanding of natural processes and landscapes allowed him to piece together the geologic history of the region using the relative position of rocks, structures, and contacts. Field geologists use these same relationships to produce geologic maps today.
Powell was only 35 years of age when he first journeyed down the Green and Colorado rivers by boat, but his writings express a lifetime worth of experience. What drove him to explore? Scientific discovery of the unknown? The allure of beauty obscured by each sinuous bend ahead? A desire to fill in the blank spaces of the map? Perhaps some of the same reasons that draw us, a group of scientists, outdoors enthusiasts, and artists to recreate his voyage today.
The setting sun casts long shadows over the face of the Uinta Mountains as we make our way to our camp at the foot of Split Mountain within the boundaries of Dinosaur National Monument, named for the abundant fossils found nearby. I locate our position on the geologic map and notice the quarry in the steeply dipping Jurassic and Creataceous beds. (I wonder if some of the fossils discovered here will be on display at the new dinosaur exhibit opening at the National Museum of Natural History, my favorite Smithsonian back in Washington, D.C.) The map pattern reveals that Split Mountain defines an arch-shaped anticline, with older rocks exposed in the core of the range, and younger rocks on the flanks. The rich blue and purple colors on the map indicate the presence of older rocks nestled deeper within the mountains upstream. In fact, the river spans nearly a billion years as it slices through the Uinta Mountains, exposing Proterozoic through Mesozoic rocks. The record is discontinuous and punctuated by unconformities, where erosion had erased the depositional history, tectonic forces have tilted the older sediments, and new deposits have been laid down on top. For example, the K-T boundary, where an iridium-rich layer of clay and ejecta from a meteorite impact mark the mass extinction event that killed off the dinosaurs 65 million years ago, is not exposed. Tomorrow, as we start our first day on the river, our rafts will cross an unconformity out of the Cretaceous and into the Tertiary sediments of the Uinta basin, without a trace of the past geologic catastrophe, one of the five mass extinctions in Earth’s history.
The water is cold. The gaging station downriver at Jensen measures 50 degrees F. The winter snow melt feeds into the Green as it emerges from the mountains and carves its way into the Uinta basin. Here, the course meanders as it cuts into Quaternary alluvium and Tertiary sediments deposited by a lake that filled this valley when the climate was colder and wetter. The distance from the point we enter the river at Split Mountain to where we will depart measures 50 miles as the crow flies. But, because of the meandering course, there are 100 river miles to cover to reach Sand Wash. The meanders are spectacular, such as the nearly 180 degree curve at Horseshoe Bend. Where the river has downcut into the soft underlying basin sediments, the meanders become entrenched by steep cliff walls on both sides. The river gradient through the Uinta basin is subdued, with no rapids or knickpoints, averaging only a few feet per mile. With a total elevation drop of over 6,000 feet from Green RIver, Wyoming to the Gulf of California, the Uinta basin is one of the gentlest stretches of river Powell and his pals would have enjoyed. Powell’s profile of the Green and Colorado Rivers from his 1875 report shows this quite well. It is amazing how accurate his barometric measurements of altitude were for the time.
Over the next few days, we’ll paddle by historic mining claims, where prospectors searched for placer gold deposited by the Green River as it eroded the mountains upstream. We’ll see oil derricks, where petroleum and natural gas are produced from the Uinta basin lake sediments such as the Green RIver Formation, and farmland on areas marked irrigable on Powell’s map of Utah. Passing through the Ouray National Wildlife Refuge, we may observe birds with bands to track migration patterns. The legacy of the scientific agency that Powell helped to establish is present everywhere, even the geographic names on the maps. A century and a half after his initial voyage, the mission he shaped as Director of the USGS continues into the future.