_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._]
_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.