Yosemite National Park ( yoh-SEM-i-tee) is an American national park in the western Sierra Nevada of Central California, bounded on the southeast by Sierra National Forest and on the northwest by Stanislaus National Forest. The park is managed by the National Park Service and covers an area of 748,436 acres (1,169 sq mi; 3,029 km2) and sits in four counties: centered in Tuolumne and Mariposa, extending north and east to Mono and south to Madera County. Designated a World Heritage Site in 1984, Yosemite is internationally recognized for its granite cliffs, waterfalls, clear streams, giant sequoia groves, lakes, mountains, meadows, glaciers, and biological diversity. Almost 95% of the park is designated wilderness.On average, about four million people visit Yosemite each year, and most spend the majority of their time in the seven square miles (18 km2) of Yosemite Valley. The park set a visitation record in 2016, surpassing five million visitors for the first time in its history. Yosemite was central to the development of the national park idea. Galen Clark and others lobbied to protect Yosemite Valley from development, ultimately leading to President Abraham Lincoln's signing the Yosemite Grant in 1864. John Muir led a successful movement to have Congress establish a larger national park by 1890, one which encompassed the valley and its surrounding mountains and forests, paving the way for the National Park System.Yosemite is one of the largest and least fragmented habitat blocks in the Sierra Nevada, and the park supports a diversity of plants and animals. The park has an elevation range from 2,127 to 13,114 feet (648 to 3,997 m) and contains five major vegetation zones: chaparral and oak woodland, lower montane forest, upper montane forest, subalpine zone, and alpine. Of California's 7,000 plant species, about 50% occur in the Sierra Nevada and more than 20% are within Yosemite. The park contains suitable habitat for more than 160 rare plants, with rare local geologic formations and unique soils characterizing the restricted ranges many of these plants occupy.The geology of the Yosemite area is characterized by granitic rocks and remnants of older rock. About 10 million years ago, the Sierra Nevada was uplifted and then tilted to form its relatively gentle western slopes and the more dramatic eastern slopes. The uplift increased the steepness of stream and river beds, resulting in the formation of deep, narrow canyons. About one million years ago, snow and ice accumulated, forming glaciers at the higher alpine meadows that moved down the river valleys. Ice thickness in Yosemite Valley may have reached 4,000 feet (1,200 m) during the early glacial episode. The downslope movement of the ice masses cut and sculpted the U-shaped valley that attracts so many visitors to its scenic vistas today.