Teewinot Lake was formed on a down-faulted block and was dammed behind (north of) a fault that trends east across the floor of Jackson Hole at the south boundary of the park. Lakes are among the most short-lived of earth features because the forces of nature soon conspire to fill them up or empty them. This lake existed for perhaps 5 million years during middle Pliocene time; it was shallow, and remained so despite the pouring in of a mile-thick layer of sediment. This indicates that downdropping of the lake floor just about kept pace with deposition.
Figure 50. Restoration of a middle Eocene landscape showing some of the more abundant types of mammals. Mural painting by Jay H. Matterness; photo courtesy of the Smithsonian Institution.
| Uintatherium | 6-horned, saber-toothed plant eater |
| Stylinodon | gnawing-toothed mammal |
| Palaeosyops | early titanothere |
| Helaletes | primitive tapir |
| Sciuravus | squirrel-like rodent |
| Smilodectes | lemurlike monkey |
| Trogosus | gnawing-toothed mammal |
| Hyrachyus | fleet-footed rhinoceros |
| Ischyrotomus | marmotlike rodent |
| Homacodon | even-toed hoofed animal |
| Orohippus | ancestral horse |
| Patriofelis | large flesh eater |
| Mesonyx | hyenalike mammal |
| Helohyus | even-toed hoofed mammal |
| Metacheiromys | armadillolike edentate |
| Machaeroides | saber-toothed mammal |
| Hyopsodus | clawed, plant-eating mammal |
| Saniwa | monitorlike lizard |
| Crocodilus | crocodile |
| Echmatemys | turtle |
Other lakes formed in response to similar crustal movements in nearby places. One such lake, Grand Valley Lake ([fig. 49]), formed about 25 miles southwest of Teewinot Lake; both contained sediments with nearly the same thickness, composition, appearance, age, and fossils. Although these two lakes are on opposite sides of the Snake River Range, the ancestral Snake River apparently flowed through a canyon previously cut across the range and provided a direct connection between them.
Development of mammals
The Cenozoic Era is known as the “Age of Mammals.” Small mammals had already existed, though quite inconspicuously, in Wyoming for about 90 million years before Paleocene time. Then about 65 million years ago their proliferation began as a result of the extinction of dinosaurs, obliteration of seaways that were barriers to distribution, and the development of new and varied types of environment. These new environments included savannah plains, low hills and high mountains, freshwater lakes and swamps, and extensive river systems. The mammals increased in size and, for the first time, became abundant in numbers of both species and individuals. The development and widespread distribution of grasses and other forage on which many of the animals depended were highly significant. Successful adaptation of herbivores (vegetation-eating animals) led, in turn, to increased varieties and numbers of predatory carnivores (meat-eating animals).
During early Eocene time, coal swamps formed in eastern Jackson Hole and persisted for thousands of years, as is shown by 60 feet of coal in a single bed at one locality. Continuing on into middle Eocene time, the climate was subtropical and humid, and the terrain was near sea level. Tropical breadfruits, figs, and magnolias flourished along with a more temperate flora of redwood, hickory, maple, and oak. Horses the size of a dog and many other small mammals were abundant. Primates, thriving in an ideal forest habitat, were numerous. Streams contained gar fish and crocodiles ([fig. 50]).
Figure 51. A typical Oligocene landscape showing some of the more abundant types of mammals. Mural painting by Jay H. Matterness; photo courtesy of Smithsonian Institution.