37 these facets of the Green River’s arid landscape can be pondered in Tony’s artworks, with their bits of maps and marginalia, including sketches of tiny herring that remind us of a moment in time, long ago, when lakes teeming with life once stretched to the horizon that is now the backdrop for desert and canyonlands. As in Moran’s paintings, many of the reaches of the Green River are lined with horizontally bedded rocks (their original depositional position), but in some places the beds are broadly warped and folded, often tilting to the north. It is a geologic maxim that sediments are deposited horizontally and each higher layer is younger than that below it. Given the down- stream slope of the Green River as it flows southward to its confluence with the Colorado River, a person traveling along the river goes deeper into the rock record and farther back in time with each passing mile. It is a form of time travel. The steeper the north- ward tilt of the beds, the faster one races back in time along the river. At the Green River’s confluence with the Colorado River at Cataract Canyon, for example, the oldest rocks are nearly 300 million years old. If a paddler starts in the river’s headwaters in Wyoming and floats to the confluence, they will have traversed about 250 million years of Earth history in 720 miles. At this pace, the explorer goes back in time about one million years for every two to three miles paddled. Perhaps artists captivated by the stunning views of multihued cliffs and vast expanses of desert terrain in the Green River landscape—unobstructed by the heavy vegetation and thick soils typical of wetter climes—are drawn in by a sense of the immensity of space and time, of life and evolution, of past environ- ments and changing climates. Perhaps it is not just the striking rock features or the remarkable lighting that catches their attention. Only in landscapes with deep chasms, finely layered fossiliferous rocks, and little vegetation are we able to see back in time so readily and fully. Only in such places, as along the Green River with its near continuous grinding by a ribbon of water for tens of millions of years, does one have a vantage point of deep time in so much colorful glory. All of Tens of thousands of rock samples with fossil fish from the Green River Formation are now housed at the Smithsonian Museum, testifying to the allure and significance of these fossiliferous beds. Indeed, the most common fish fossil collected from the Green River Formation (and the most commonly excavated fossil in the world) is that of the extinct genus Knightia, a relatively small fish named for Rocky Mountain paleontologist Wilbur Clinton Knight (1858–1903). In the same family as herring and sardines, this Wyoming state fossil looks much like its modern descendants. Imprints and organic films from bodies of bony, finned fish preserved as fossils in the Green River Formation, we now know, were laid down in quiet-water lakes between 53.5 and 48.5 million years ago. Radiometric dates of volcanic ash inter- spersed throughout the sedimentary sequence yield the time span and fall within the Eocene Epoch (56 to 33.9 million years ago), with the Greek name “Eocene” referring to the “dawn of modern fauna.” A formation has a particular meaning to a geolo- gist, and it must go through a careful process of formal naming and acceptance. It is a rock unit so distinctive that any geologist trained in mapping can distinguish it from other formations of rock above and below. Some of the layers in the Green River Formation are shades of green, others oddly purple, and others various hues of red to brown and beige. The formation’s most notable distinction, however, is that it contains thou- sands of cyclical layers of mud, limey mud, and sand turned to shale, limestone, and sandstone, with each of the fine laminae representing a single year of depo- sition. Individual annual layers averaging only a few tenths of a millimeter in thickness became darker during growing seasons when both organic matter and sediment washed into lakes. OVERLEAF Tony Foster, Turk’s Head Ledges Looking Upriver, 2022 (detail, p. 56)
