The mountain scenery of the Park is not so imposing as that of Colorado and some other parts of the Rocky Mountain region; but it is more varied and beautiful. The eye is not wearied with the constant sight of vast and bare mountain cliffs, but finds relief in attractive lakes, streams, glades, parks, forests, and every combination of effects that helps to produce a beautiful landscape.
Geology of the Park.
Nature seems, from the first, to have designed this region for a mountain park. In geological chronology it was near the close of the Cretaceous Period, that the lifting of the great mountain systems of the West into their present positions was practically finished. In the formation of these mountains, the general outline of the Yellowstone Park was already marked out, probably in much more striking features than at present. A vast rim of mountains, visible now in the Absaroka, Snowy, Gallatin, Teton, and Snake River Ranges, hemmed in the extensive area which has since become so famous. Subsequent events have greatly modified its original form, but the grand outlines at first determined are still distinctly visible.
In the Tertiary Period, which was next in order of time after the Cretaceous, changes of the greatest importance occurred, consisting principally in the outpouring of enormous masses of volcanic material. The origin of these lava flows has been traced to a few craters, one of which was near Mt. Washburn, another in the Red Mountain Range, and a third near the sources of the Lamar River. Mt. Washburn has long been recognized as part of the rim of an ancient volcano. Both it and Mt. Sheridan, the two mountains which bore the principal part in working out the present features of that country, still remain the most prominent peaks from which the modern visitor can contemplate the work they have performed.
The outpourings at first consisted of andesitic lavas. They largely changed the appearance of the mountain ranges and to some extent filled up the interior basin. The flows were not continuous but were separated by long intervals of quiet, during which vegetation and the agencies of erosion were actively at work.
After the cessation of the andesitic eruptions, a quiescent period of great length ensued. Then came the period of rhyolitic flows, the centers of volcanic activity being as before Mts. Washburn and Sheridan. These flows built up the present Park plateau, and constitute the great bulk of the rocks which the tourist now sees.
Following the period of rhyolitic eruptions, orographic agencies were active in producing extensive faults or displacements, which in certain localities radically changed the relative positions of the rocks.