The First Big Lake
Teewinot Lake ([fig. 49]), the first big freshwater lake in Jackson Hole, was formed during Pliocene time, about 10 million years ago, and in it the Teewinot Formation was deposited. These lake strata consist of more than 5,000 feet of white limestone, thin-bedded claystone, and tuff (solidified ash made up of tiny fragments of volcanic rock and splinters of volcanic glass). The claystones contain fossil snails, clams, beaver bones and teeth, aquatic mice, suckers, and other fossils that indicate deposition in a shallow freshwater lake environment. These beds underlie Jackson Lake Lodge, the National Elk Refuge, part of Blacktail Butte, and are conspicuously exposed in white outcrops that look like snowbanks on the upper slopes along the east margin of the park across the valley from the Grand Teton.
Figure 49. Teton region near close of middle Pliocene time, about 5 million years ago, showing areas of major volcanoes and lava flows. See [figure 41] for State lines and location map.
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.