Part 11

The brightly colored sandstone, mudstone, and claystone in the Indian Meadows and Wind River Formations (lower Eocene) in the eastern part of Jackson Hole were derived from variegated Triassic, Jurassic, and Lower Cretaceous rocks exposed on the adjacent mountain flanks. Fossils in these Eocene Formations show that it took less than 10 million years for the uplifts to be deeply eroded and partially buried in their own debris.

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.

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.

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.

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).

Trigonias early rhinoceros Perchoerus early peccary Mesohippus 3-toed horse Aepinacodon remote relative of hippopotamus Archaeotherium giant piglike mammal Protoceras bizarre horned ruminant Hesperocyon ancestral dog Hyracodon small fleet-footed rhinoceros Poëbrotherium ancestral camel Hypisodus very small chevrotainlike ruminant Ictops small insect-eating mammal Brontotherium titanothere Protapirus ancestral tapir Glyptosaurus extinct lizard Hoplophoneus saber-toothed cat Subhyracodon early rhinoceros Merycoidodon sheeplike grazing mammal Hyaenodon archaic hyenalike mammal Hypertragulus chevrotainlike ruminant

Early in the Oligocene Epoch, between 30 and 35 million years ago, the climate in Jackson Hole became cooler and drier, and the subtropical plants gave way to the warm temperate flora of oak, beech, maple, alder, and ash. The general land surface rose higher above sea level, perhaps by accumulation of several thousand feet of Oligocene volcanic rocks (fig. 52) rather than by continental uplift. Titanotheres (large four-legged mammals with the general size and shape of a rhinoceros) flourished in great numbers for a few million years and then abruptly vanished. Horses by now were about the size of a very small modern colt. Rabbits, rodents, carnivores, tiny camels, and other mammals were abundant in Jackson Hole, and the fauna, surprisingly, was essentially the same as that 500 miles to the east, at a much lower elevation, on the plains of Nebraska and South Dakota (fig. 51).

The Miocene Epoch (15 to 25 million years ago) was the time of such intense volcanic activity in the Teton region that animals must have found survival very difficult. A few skeletons and fragmentary parts of camels about the size of a small horse and other piglike animals called oreodonts comprise our only record of mammals; nothing is known of the plants. Farther east the climate fluctuated from subtropical to warm temperate, gradually becoming cooler toward the end of the epoch.

Fossils in the Pliocene lake deposits (8 to 10 million years old; see description of Teewinot Formation) include shallow-water types of snails, clams, diatoms, and ostracodes, as well as beavers, mice, suckers, and frogs. Pollen in these beds show that adjacent upland areas supported fir, spruce, pine, juniper, sage, and other trees and shrubs common to the area today. Therefore, the climate must have been much cooler than in Miocene time. No large mammals of Pliocene age have been found in Jackson Hole. The record of life during Quaternary time is discussed later.

Volcanoes

Volcanoes are one of the most interesting parts of the geologic story of the Teton region. Although ash from distant volcanoes had settled in northwestern Wyoming at least as far back in time as Jurassic, the first nearby active volcanoes (since the Precambrian) erupted in the Yellowstone-Absaroka region during the early Eocene, about 50 million years ago. From then on, the volcanic area grew in size and the violence of eruptions and volume of debris increased until Pliocene time. This debris had a profound influence on the color and composition of the sediments and on the environment and types of plants and animals.

The color of the volcanic rocks and the sediments derived from them varies significantly from one epoch to another. For example, the middle Eocene rocks are white to light-green, red, and purple, upper Eocene are dark-green, Oligocene are light-gray, white, and brown, Miocene are dark-green, brown, and gray, and Pliocene are white to red-brown.

As mentioned earlier, it is probable that the vast outpouring of volcanic rocks during late Tertiary time in the Teton region and to the north and northeast is directly related to the subsidence of Jackson Hole and the rise of the Tetons.

The spectacular banded cliffs of the Wiggins Formation on both sides of Togwotee Pass (fig. 52) and farther north in the Absaroka Range are remnants of Oligocene volcanic conglomerate and tuff that once spread as a blanket several thousand feet thick across eastern Jackson Hole and partially or completely buried the nearby older folded mountain ranges.

About 25 million years ago, with the start of the Miocene Epoch, volcanic vents opened up within, and along the borders of, Grand Teton National Park. Major centers of eruption were at the north end of the Teton Range, east of Jackson Lake, and south of Spread Creek. They emitted a prodigious amount of volcanic ash and fragments of congealed lava. For example, adjacent to one vent a mile in diameter, about 4 miles north-northeast of Jackson Lake Lodge, is a continuous section, 7,000 feet thick, of waterlaid strata derived in large part from this volcanic source. These sedimentary rocks comprise the Colter Formation which is darker colored and contains more iron and magnesium than the Wiggins Formation. The site of deposition at this locality was a north-trending trough that represented an early stage in the downwarping of Jackson Hole.

Pliocene volcanoes erupted in southern and central Yellowstone Park. The volcanoes emitted viscous, frothy, pinkish-gray and brown lava called rhyolite. This is an extrusive igneous rock that has the same composition as granite, but is much finer grained. In several places, lava apparently flowed into the north end of Teewinot Lake, chilled suddenly, and solidified into a black volcanic glass called obsidian. Because it chips easily into thin flakes having a smooth surface, obsidian was prized by the Indians, who used it for spear and arrow points (fig. 54). Some of this obsidian has a potassium-argon date of 9 million years.

After Teewinot Lake was filled with sediment, the floor of Jackson Hole became a flat boulder-covered surface. Nearby vents erupted heavy fiery clouds of gaseous molten rock that rolled across this plain and then congealed into hard layers with the general appearance of lava flows. Under a microscope, however, the rock is seen to be made up of compressed fragments of glass that matted down and solidified when the clouds stopped moving. This kind of rock is called a welded tuff. One of these forms the conspicuous ledge in the Bivouac Formation on the north and east sides of Signal Mountain (fig. 55), and is especially important because it has a potassium-argon date of 2.5 million years. More of this welded tuff flowed southward from Yellowstone National Park, engulfed the north end of the Teton Range (fig. 53), and continued southward along the west side of the mountains for 35 miles and along the east side for 25 miles.

THE LAST HUNDREDTHS ABSOLUTE TIME IMPORTANT EVENTS OF AN INCH OF THE (Years ago) YARDSTICK 0 0 Last glaciation followed by faulting ¹/₁₀₀₀ 50000 Second glaciation ²/₁₀₀₀ 100000 ?—First glaciation ⁶/₁₀₀₀ 700000 ?—Second Quaternary lake ⁸/₁₀₀₀ 1 million ?—Tilting and faulting of southern part of Jackson Hole ¹¹/₁₀₀₀ 1.3 million ?—First Quaternary lake ¹²/₁₀₀₀ 1.5 million } Complex series of volcanic eruptions in southern Jackson Hole ¹⁵/₁₀₀₀ 1.9 million } ¹⁶/₁₀₀₀ 2 million ?—Development of Hoback normal fault ²/₁₀₀ 2.5 million Eruption of welded tuff in Bivouac Formation ²⁴/₁₀₀₀ 3 million

QUATERNARY—TIME OF ICE, MORE LAKES, AND CONTINUED CRUSTAL DISTURBANCE

The Quaternary Period is represented by less than 15-thousandths of the last inch on our yardstick of time (fig. 56) and the entire Ice Age takes up less than 2-thousandths of an inch (less than the thickness of this page). Nevertheless, the spectacular effects of various forces of nature on the Teton landscape during this short interval of time are of such significance that they warrant a separate discussion. The role of glaciers in carving the rugged Teton peaks and shaping the adjacent valleys was mentioned in the first part of this booklet, but is discussed in more detail here. The magnitude and complexity of crustal movements increased during the final 2 million years of time—so much so that the beginning of Quaternary time has not yet been identified with any single event. Figure 56 shows the major events described below.

Hoback normal fault

The Hoback normal fault, 30 miles long, with a mile or more displacement, developed in the southernmost part of Jackson Hole about 2 million years ago. This fault is on the east side of the valley. Thus, the valley block was downdropped between this fault and the Teton fault that borders the west side.

Volcanic activity