Geological Survey Bulletin 1347 The Geologic Story of Yellowstone National Park

The vivid descriptions brought back from the Yellowstone country by the early explorers and trappers (see frontispiece), whose reputations for telling tall tales were widely accepted if not altogether deserved, fell upon the disbelieving ears of the nation for more than half a century. Yet the intriguing rumors persisted, and during the years 1869-71 several expeditions staffed partly by scientists and engineers rediscovered this unique region atop the backbone of our nation. We now know that the earliest visitors, even if prone to exaggerate, could not do justice to the long-hidden secrets of Yellowstone, for none of them saw all of the fascinating features that occur within this great National Park.

By the time the modern-day visitor enters Yellowstone National Park through any of its five entrances, he probably will have traveled through many parts of the Rocky Mountains and grown somewhat accustomed to the "lay of the land." But this will in no way lessen the exciting impact of viewing the natural wonders of Yellowstone for the first time. Immediate attention, of course, is still drawn to the remarkable array of geysers, hot springs, and other thermal phenomena which in sheer numbers and variety are unsurpassed throughout the world. But, as if these were not enough of an attraction, nature has also provided an incredible setting of sparkling rivers and lakes, thundering waterfalls and cataracts, awesome canyons and gorges, and lofty glaciated mountain peaks and extinct volcanoes. Truly this is a land apart, a spectacular masterpiece of nature that fully deserves the accolade of "wonderland" bestowed long ago by early explorers and trappers. (See figs. 1 and 2.)

YELLOWSTONE NATIONAL PARK AREA, showing rivers, lakes, landforms, roads, towns, settlements, and major geyser basins (stippled). The Park embraces 3,472 square miles (2,221,770 acres), and its boundaries traverse a distance of nearly 300 miles. Yellowstone Lake, with an irregular shoreline of 110 miles and a surface area of 137 square miles, is one of the largest natural mountain lakes in the United States. (click on image for an enlargement in a new window) (Fig. 1)

INDEX MAP showing localities where photographs (and one sketch, fig. 35) were taken to illustrate this bulletin. For photographs of distant views, arrows point in direction of view. Numbers refer to figure numbers in text. (Fig. 2)

Beyond the first stirring impressions derived from the grandeur of the vast Yellowstone wilderness and its myriad wildlife, assuredly shared by people of all ages and from all walks of life, the various aspects of the Park take on a very different meaning for different individuals. The artist sees grand vistas to be painted, the naturalist delights in the flower-laden meadows and the native habitats of many kinds of birds and animals, the engineer visualizes the amount of energy stored in the waterfalls and steaming geysers, and so on. To the geologist, in particular, who studies rocks and fossils and all of the natural processes involved in shaping the surface of the land, and to all those who would share such interests, Yellowstone takes on a very special meaning. For the Park is foremost a geological Park, created by an extraordinary sequence of natural processes and events that have combined to produce an immense outdoor laboratory for studies that have contributed to a fuller knowledge and a better understanding of the earth itself. The geological aspect of the Yellowstone country is reflected by its very name, given long ago to the river that issues from the great canyon of the "yellow rocks."¹ This report, borrowing from a century of scientific study within and around the Park area, describes the geological "how, why, and when" of this unique and fascinating region.

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¹The specific area about which the early-day Indians first used the term that is now translated as "Yellowstone" is unknown. The name may have referred to the yellowish rocks that line the banks of the Yellowstone River near its confluence with the Missouri River in eastern Montana and western North Dakota. However, in the opinion of H. M. Chittenden, who studied the question in considerable detail, there is little doubt that the name was taken from the striking yellow-hued walls of the gorge now known as the Grand Canyon of the Yellowstone.

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A geological preview

Some 600,000 years ago the rumblings of an impending volcanic eruption sounded ominously across the Yellowstone country. Suddenly, in a mighty crescendo of deafening explosions, tremendous quantities of hot volcanic ash and pumice spewed from giant cracks at the earth's surface. Towering dust clouds blackened the sky, and vast sheets of volcanic debris spread out rapidly across the countryside in all directions, covering thousands of square miles in a matter of minutes with a blanket of utter devastation. Abruptly, a great smoldering pit—a caldera 30 miles across, 45 miles long, and several thousand feet deep—appeared in the central Yellowstone region, the ground having fallen into the huge underground cavern that was left by the earth-shaking eruptions. Lava then began oozing from the cracks to fill the still-smoking caldera.

Thus, in one brief "moment" of geologic time there was launched that incredible chain of events which led to the creation of many of the natural wonders of Yellowstone National Park. Heat from the enormous reservoir of molten rock which produced the massive eruption still remains deep within the earth beneath Yellowstone, sustaining the spectacular hot-water and steam phenomena for which the Park is so justly famous. The formation of the caldera and the eruption of lavas profoundly influenced the shape of the present-day landscape. Once a land covered almost entirely by mountains, the part that collapsed—nearly one-third of the total Park area—is now characterized by low rolling plateaus formed from the thick lava flows that filled the caldera (figs. 1 and 2; see fig. 22 for the outline of the Yellowstone caldera). Moreover, the carving of the spectacular Grand Canyon of the Yellowstone (fig. 41) and the fashioning of the large interior basin now occupied by beautiful Yellowstone Lake (fig. 27) were closely related to this mighty volcanic event.

North, east, and south of the central plateaus are extensive mountain ranges and other highlands which provide much of the Park's scenic beauty (figs. 3 and 4). Formed by many episodes of intense mountain building and ancient volcanism, these uplands bear the lasting imprints of a wide variety of geological activities that date back approximately 2.7 billion years. Indeed, as we study all the features of the Yellowstone landscape, we find in them a most impressive and fascinating story of that ageless conflict between the internal forces of nature that raise the land through the upheaval of mountains and the eruption of volcanoes, and the external forces of erosion that wear the land down. It is this vast relentless interplay of giant forces that determines the appearance of any given place upon the earth's surface. And, in few other places around the globe can the processes of both building up and tearing down the landscape be illustrated more dramatically than in Yellowstone National Park.

SKYLINE OF THE GALLATIN RANGE in northwestern Yellowstone National Park, as viewed from a point on the road between Canyon Village and Norris Junction. The range consists chiefly of Paleozoic and Mesozoic sedimentary rocks and Precambrian metamorphic rocks that were uplifted by folding and faulting of the earth's crust. The dark-gray rocks along the roadcut in the left foreground are rhyolite lava flows of the Solfatara Plateau. (Fig. 3)

HAYDEN VALLEY. View north along the Yellowstone River and Hayden Valley toward the Washburn Range. Mount Washburn, part of an ancient Absaroka volcano, is the highest prominence (elevation, 10,293 feet) on the skyline to the right, and Dunraven Pass is in the notch in the center of the skyline. The foot of the range marks the north edge of the Yellowstone caldera. Hayden Valley is cut in glacial lake sediments that overlie thick lava flows covering the caldera floor. (Fig. 4)