Yellowstone Wildlife: Ecology and Natural History of the Greater Yellowstone Ecosystem

1. History of the Greater Yellowstone Ecoregion

features and the cause of Yellowstone’s frequent earthquake activity. From the Yellowstone Plateau the Bridger Plateau extends southeastwardly like a narrow forefinger as the Owl Creek and Bridger Mountains, which approach the “thumbprint” of the Bighorn Mountains. Between them the Bighorn River flows northward through the Bighorn Basin to join the Missouri River. To the south of the Owl Creek and Bridger Mountains is the Wind River Basin, where the Shoshone and Arapahoe Indians now live in relative obscurity and where the great Shoshone chief Washakie lies buried on the parched and shrub-covered hillsides. The middle “finger” of the landscape’s topography is made by the Wind River Range, the longest and highest of the Wyoming ranges. Along its crest runs the Continental Divide, and over such passes as Union Pass and South Pass came the earliest explorers, mountain men, and finally emigrants on their way west. At its western base is the Green River Basin, whose waters drain into the Colorado River and whose sedimentary rocks bear the fossil imprints of Eocene fish, reptiles, birds, and mammals interred there some fifty million years ago. The fourth landscape “finger” is formed basally by the jagged upthrust of the Teton Range and farther south by the more gently folded overthrust ridges of the Wyoming and Salt River ranges, which nearly parallel the Wyoming-Idaho border. Finally, protruding at nearly right angles westward from the Tetons and associated mountains is a small fifth “finger.” This is the Centennial Range of the Idaho-Montana border , along which the Continental Divide continues westwardly. History of tHe Greater yellowstone ecoreGion 4 Of all these mountain groups in the Yellowstone region, by far the most recent is the Teton Range, whose dramatic eastern face was exposed less than ten million years ago by a fault in the earth’s crust, where the mountains were tilted upward and the adjoining valley floor of Jackson Hole dropped downward in a relatively rapid series of earthquake tremors. The Tetons are thus among the youngest and most spectacular of all the Rocky Mountain ranges, with their peaks and ridges having been subsequently eroded and sculpted by a variety of glacial processes, especially by ice. As the eastern escarpment of the Tetons rose and the floor of Jackson Hole dropped, rock strata that were deposited over long periods of geologic time came into view. Indeed, the slopes of the range thus exposed to view provide a sequence of rock layers representing more than half of the earth’s geologic history. The most ancient of these strata are banded Precambrian layers more than 2.5 billion years old, some of the oldest exposed rocks on the North American continent. Above these archaic rocks are sedimentary deposits less than a billion years old, which formed from materials deposited along the margins of Paleozoic seas that then inundated the area. On the northern slopes of the Tetons and adjacent Gros Ventre Mountains the reddish sandstones, blue-gray limestones , gray dolomites, and black and green shales lie stacked on top of one another where layer after layer of deposits were added to older strata below, interring with them the remains of Paleozoic animals such as trilobites and brachiopods. As the Paleozoic seas gave way to those of the Mesozoic era about 200 million years ago, soft reddish, iron-rich sediments as much as 1,000 feet thick were laid down and now may be seen on the northern flanks of the Gros Ventre Mountains. These more brightly colored rocks were subsequently covered by a much thicker layer of dull-hued silt, sand, and clay toward the end of the Mesozoic era, leaving a flat and featureless floodplain as the Mesozoic sea finally retreated eastwardly. At the very end of Mesozoic times, starting about sixty million years ago during the Eocene epoch, mountain-building in the area began as the Wind River Range was thrust upward and westward. During the last sixty-five million years, the Cenozoic era, massive mountain-building occurred in North America, and most of the modern groups of birds and mammals evolved. Uplifts in several areas of what is now Wyoming (the Laramide mountain-building period) produced the first of the Rocky Mountains, while erosion simultaneously began to bury the adjacent basins. A new geologic element was added to the massive and generally widespread forces of mountain-building and basin-filling when volcanic eruptions from the Yellowstone and...