YELLOWSTONE NATIONAL PARK. A United States Government reservation set apart as a public pleasure ground and game preserve. It occupies a reetanguhlr area in the northwest ern corner of Wyoming, overlapping the territory of the States of and Idaho on the north and west by strips of land about two miles wide (Map: Wyoming, D 3). The park measures 54 miles from east to west, and 62 miles from north to south; its area is about 3350 square miles. The Yellowstone Park Forest Reserve, occupy ing a strip 24 miles wide adjoining the park on the east and a strip 10 miles wide on the south, is practically a portion of the park, and increases the area to 5500 square miles.
Yellowstone Park lies in the very heart of the Rocky Mountains. It consists of an elevated plateau basin with a mean altitude of 8000 feet, and surrounded on all sides by lofty, snow-clad, and exceedingly rugged mountain ranges. On the eastern boundary runs the Absakora range, a northern extension of the Shoshone Mountains, whose spurs cover the eastern forest reserve with an almost impassable complex of peaks, ranges, and deeply eroded river valleys. Several peaks in the reserve are more than 11,000 feet high, and the Fortress and Dead Indian Peaks rise respectively to altitudes of 12.073 and 12,253 feet. The southern forest reserve is oc cupied by northern outliers of the lofty Wind River and Teton ranges, the latter running also up the western boundary. The Gallatin range on the northwest and the Snow Mountains on the north complete the barrier. The central basin itself consists of more or less level or un dulating lava fields, but spurs from the sur rounding mountains run into it froM all direc tion., volcanic cones and other peaks as well :is sinall isolated mountain groups project from the plateau door in numerous places, while most of the larger streams have cut their way through the lava in yawning callous, so that the surface is extremely diversified, and the plateau cut up into a number of separate fields, basins, or peaks. There are 21 peaks within the park proper whose altitude exceeds 10.000 feet. The loftiest as well as the most rugged portion lies east of Yellow Lake. where Mount Humphreys and Turret Mountain rise respectively 10 altitudes of 11,000 and 1'1.142 feet. Directly on the northern boun dary, however, Electric l'eak towers to a height of 11,155 feet, and affords one of the widest views of the park and its surroundings to the north.
The Continental Divide crosses the park in an irregular line from the middle of the western boundary to near the southeastern corner, so that the waters of the park are sent to both oceans. South of the Divide the upper course of the Snake River receives some of its head streams from Shoshone, Lewis, and heart. Lakes. North of the Divide all the streams are tributary to the Missouri. The most important is the Yellowstone (q.v.), which traverses the park from the southeastern corner to the middle of the northern boundary, and expands into Yel lowstone lake, one of the central leato reS of the region. The Madison River, one of the head:streams of the .Missouri, rises on the cen tral plateau. while the eastern mountains give rise to the Shoshone. an affluent of the Big Ilorn.
The original geological structure of the park is almost completely hidden by a great Tertiary lava flow, the easternmost extension of the Snake River basalt plains, which are connected to the westward with the vast Oregon lava fields. The underlying sedimentary rocks are chiefly Cre taceous, probably with some upturned Paleozoic strata and Archcean rocks, but the present pla teaus and mountains, with the exception of the Gallatin range. are composed of lava, and though there have been no recent outpourings of molten rock, the whole region is still actively volcanic. See hot Springs and (tensors below.
Owing to the high elevation, the climate 4; rigorous, but not excessively cold compared with the lower surrounding regions. The sum mer is short and characterized by great diurnal extremes of temperature, frosts occurring at night even in midsummer. The mean tempera ture for July is 62', and the average maximum 7S°, the mercury rising occasionally as high as 9(1°. The growing season does not begin until May, and vegetation reaches its greatest luxuri ance near the end of Jul•. At the end of August there is an abrupt change from summer to winter, and snow begins to lie on the ground by the mid dle of September. In January the mean tempera ture is about 20°, and the average minimum about 12°, with an occasional fall to 30° below zero. The average annual rainfall on the plateau is about 20 inches, sufficient to support the for ests and mountain vegetation.