YELLOWSTONE Geological History of the Yellowstone National Park

The purpose of this paper is not so much to elucidate any special problem connected with the many interesting geological questions to be found in the Yellowstone Park, as to offer such a general view of the region as will enable the tourist to understand clearly something of its physical geography and geology.

The Yellowstone Park is situated in the extreme northwestern portion of Wyoming. At the time of the enactment of the law establishing this national reservation the region had been little explored, and its relation to the physical features of the adjacent country was little understood. Since that time surveys have shown that only a narrow strip about 2 miles in width is situated in Montana and that a still narrower strip extends westward into Idaho.

The area of the park as at present defined is somewhat more than 3,300 square miles.

The Central Plateau, with the adjacent mountains, presents a sharply defined region, in strong contrast with the rest of the northern Rocky Mountains. It stands out boldly, is unique in topographical structure, and complete as a geological problem.

The central portion of the Yellowstone Park is essentially a broad, elevated, volcanic plateau, between 7,000 and 8,500 feet above sea level, and with an average elevation of about 8,000 feet. Surrounding it on the south, east, north, and northwest are mountain ranges with culminating peaks and ridges rising from 2,000 to 4,000 feet above the general level of the inclosed table-land.

SKETCH MAP OF THE YELLOWSTONE NATIONAL PARK. A topographic map of the park on a scale of 2 miles to the inch may be purchased from the Director of the Geological Survey, Washington, D.C., for 20 cents.

For present purposes it is needless to confine ourselves strictly to legal boundaries, but rather to consider the entire region in its broader physical features.

South of the park the Tetons stand out prominently above the surrounding country, the highest, grandest peaks in the northern Rocky Mountains. The eastern face of this mountain mass rises with unrivalled boldness for nearly 7,000 feet above Jackson Lake. Northward the ridges fall away abruptly beneath the lavas of the park, only the outlying spurs coming within the limits of the reservation. For the most part the mountains are made up of coarse crystalline gneisses and schists, probably of Archean age, flanked on the northern spurs by upturned Paleozoic strata. To the east of the Tetons, across the broad valley of the Upper Snake, generally known as Jackson Hole, lies the well-known Wind River Range, famous from the earliest days of the Rocky Mountain trappers. The northern end of this range is largely composed of Mesozoic strata, single ridges of Cretaceous sandstone penetrating still farther northward into the regions of the park and protruding above the great flows of lava.

THE ABSAROKA RANGE ALONG THE EASTERN EDGE OF THE PARK.

Along the entire eastern side of the park stretches the Absaroka Range—so called from the Indian name of the Crow Nation. The Absaroka Range is intimately connected with the Wind River Range, the two being so closely related that any line of separation must be drawn more or less arbitrarily, based more upon geological structures and forms of erosion than upon physical limitations.

The Absarokas offer for more than 80 miles a bold, unbroken barrier; a rough, rugged country, dominated by high peaks and crags from 10,000 to 11,000 feet in height. The early trappers found it a forbidding land; prospectors who followed them, a barren one.

At the northeast corner of the park a confused mass of mountains connects the Absarokas with the Snowy Range. This Snowy Range shuts in the park on the north and is an equally rough region of country, with elevated mountain masses covered with snow the greater part of the year, as the name would indicate. Only the southern slopes, which rim in the park region, come within the limit of our investigation. Here the rocks are mainly granites, gneisses, and schists, the sedimentary beds, for the most part, referable to the pre-Cambrian series.

THE GALLATIN RANGE IN THE NORTHEASTERN PORTION OF THE PARK.

The Gallatin Range incloses the park on the north and northwest. It lies directly west of the Snowy, only separated by the broad valley of the Yellowstone River. It is a range of great beauty, of diversified forms, and varied geological problems. Electric Peak, in the northwestern corner of the park, is the culminating point in the range, and affords one of the most extended views to be found in this part of the country.

Archean gneisses form a prominent mass in the range, over which occur a series of sandstones, limestones, and shales, of Paleozoic and Mesozoic age, representing Cambrian, Silurian, Devonian, Carboniferous, Trias, Jura, and Cretaceous. Immediately associated with these sedimentary beds, are large masses of intrusive rocks, which have played an important part in bringing about the present structural features of the range. They are all of the andesitic type, but show considerable range in mineral composition, including pyroxene, hornblende, and hornblende-mica varieties. These intrusive masses are found in narrow dikes, in immense interbedded sheets forced between the different strata, and as laccolites, a mode of occurrence first described from the Henry Mountains in Utah, by Mr. G. K. Gilbert, but now well recognized elsewhere in the northern Cordillera.

We see then that the Absarokas rise as a formidable barrier on the eastern side of the park, the Gallatins as a steep mural face on the west side, while the other ranges terminate abruptly, rimming in the park on the north and south, and leaving a depressed region not unlike the parks of Colorado, only covering a more extended area with a relatively deeper basin. The region has been one of profound dynamic action, and the center of mountain building on a grand scale.

It is not my purpose at the present time to enter upon the details of geological structure of these ranges, each offering its own special study and field of investigation. My desire is simply to call attention to their general features and mutual relations. So far as their age is concerned, evidence goes to show that the action of upheaval was contemporaneous in all of them, and coincident with the powerful dynamic movements which uplifted the north and south ranges, stretching across Colorado, Wyoming, and Montana. This dynamic movement blocked out, for the most part, the Rocky Mountains, near the close of the Cretaceous, although there is good reason to believe that in this region profound faulting and displacement continued the work of mountain building well into the Middle Tertiary period.

Throughout Tertiary time in the park area, geological history was characterized by great volcanic activity, enormous volumes of erupted material being poured out in the Eocene and Middle Tertiary, continuing with less force through the Pliocene, and extending into Quaternary time. Within very recent times there is no evidence of any considerable outburst; indeed the region may be considered long since extinct. These volcanic rocks present a wide range in chemical and mineral composition and physical structure. They may all, however, be classed under three great groups—andesites with basalts, rhyolites, and basalts—following each other in the order named. In general, the relative age of each group is clearly and sharply defined, the distribution and mode of occurrence of each presenting characteristics and salient features frequently marked by periods of erosion.

Andesites are the only volcanic rocks which have played an important part in producing the present structural features of the mountains surrounding the park. As already mentioned, they occur in large masses in the Gallatin Range, while most of the culminating peaks in the Absarokas are composed of compact andesites and andesitic breccias. On the other hand, the andesites are not confined to the mountains, but played an active role in filling up the interior basin. That the duration of the andesitic eruptions was long continued is made evident by the plant remains found in ash and lava beds through 2,000 feet of volcanic material.

In early Tertiary times, a volcano burst forth in the northeast corner of this depressed area not far from the junction of the Absaroka and Snowy Ranges. While not to be compared in size and grandeur with the volcanoes of California and the Cascade Range, it is, for the Rocky Mountains, one of no mean proportions. It rises from a base about 6,500 feet above sea level, the culminating peak attaining an elevation of 10,000 feet. This gives a height to the volcano of 3,500 feet from base to summit, measuring from the Archean rocks of the Yellowstone Valley to the top of Mount Washburn. The average height of the crater rim is about 9,000 feet above sea level, the volcano measuring 15 miles across the base. The eruptive origin of Mount Washburn has long been recognized, and it is frequently referred to as a volcano. It is however simply the highest peak among several others, and represents a later outburst which destroyed in a measure the original rim and form of an older crater. The eruptions for the most part were basic andesites. Erosion has so worn away the earlier rocks, and enormous masses of more recent lavas have so obscured the original form of lava flows, that it is not easy for an inexperienced eye to recognize a volcano and the surrounding peaks as the more elevated points in a grand crater wall. By following around on the ancient andesitic rim, and studying the outline of the old crater, together with the composition of its lavas, its true origin and history may readily be made out. It has been named the Sherman volcano. This old volcano of early Tertiary time occupies a prominent place in the geological development of the park, and dates back to the earliest outbursts of lava which have in this region changed a depressed basin into an elevated plateau. We have here a volcano situated far inland, in an elevated region, in the heart of the Rocky Mountains. It lies on the eastern side of the continent, only a few miles from the great Continental Divide, which sends its waters to both the Atlantic and Pacific.

After the dying out of the andesitic and basaltic lavas, followed by a period of erosion, immense volumes of rhyolite were erupted, which not only threatened to fill the crater but to bury the outer walls of the volcano itself. On all sides the andesitic slopes were submerged beneath the rhyolite to a height of from 8,000 to 8,500 feet. This enormous mass of rhyolite, poured out after the close of the andesitic period, did more than anything else to bring about the present physical features of the park tableland. A tourist visiting all the prominent geyser basins, hot springs, Yellowstone Lake, and the Grand Canyon and Falls of the Yellowstone, is not likely to come upon any other rock than rhyolite, excepting, of course, deposits from the hot springs, unless he ascends Mount Washburn. A description of the rhyolite region is essentially one of the Central Plateau. Taking the bottom of the basin at 6,500 feet above sea level, these acidic lavas were piled up until the accumulated mass measured 2,000 feet in thickness. It completely encircled the Gallatin Range, burying its lower slopes on both the east and west sides; it banked up all along the west flanks of the Absarokas, and buried the outlying spurs of the Teton and the Wind River Plateaus.