The Laramide Revolution in the area of Grand Teton National Park ended during Eocene time between 45 and 50 million years ago, and as the mountains and basins became stabilized a new element was added. Volcanoes broke through to the surface in many parts of the Yellowstone-Absaroka area and the constantly increasing volume of their eruptive debris was a major factor in the speed of filling of basins and burial of mountains throughout Wyoming. This entire process only took about 20 million years, and along the east margin of Jackson Hole it was largely completed during Oligocene time (fig. 48). However, east and northeast of Jackson Lake a Miocene downwarp subsequently formed and in it accumulated at least 7,000 feet of locally derived sediments of volcanic origin. [Illustration: Figure 48. _Teton region near the close of Oligocene deposition, between 25 and 30 million years ago, showing areas of major volcanoes and lava flows. See figure 41 for State lines and location map._] 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. [Illustration: 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. [Illustration: 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._]