Yosemite National Park

Yosemite's rock is called granite.

It began as hot melted rock deep under the ground. It cooled very slowly and turned hard. Over a long, long time, the rock on top wore away. Then the granite was out in the open.

Most of Yosemite is granite, a strong, speckled rock. It formed when melted rock, called magma, cooled slowly deep underground instead of erupting from a volcano. This happened about 105 to 85 million years ago, when one of Earth's plates was sliding under North America.

Over millions of years the rock on top wore away and left the granite at the surface. Then, in the last 20 million years or so, the land rose and tilted. Rivers ran faster down the slopes and cut deep canyons. Because granite is so strong and has few cracks, it can stand in giant cliffs like El Capitan.

Yosemite's granitic rocks belong to the Sierra Nevada Batholith, an amalgamation of plutons: bodies of magma that froze in the crust rather than erupting. The Park Service links the batholith to subduction that began about 220 million years ago, when ancestors of the Pacific Plate began diving beneath the North American Plate and the California coast resembled today's Andes. Most of the park's granites are Cretaceous, 105 to 85 million years old, and more than 90 percent of the landscape is granitic. The oldest rocks are metamorphic belts of marine sediments deposited about 400 to 500 million years ago; the youngest are small volcanic outcrops.

Subduction ended about 65 million years ago, and erosion stripped the overlying rocks to expose the granite. Within roughly the past 20 million years the range was uplifted and tilted southwest along faults on its east side, a process the Park Service says continues today; steeper streams cut deeper canyons. Granite's large interlocking crystals and lack of layering make it strong enough to hold bold forms such as El Capitan's 3,000-foot face.

Long ago, the weather got very cold.

Rivers of ice, called glaciers, slid down the valleys. They scraped and smoothed the rock. When the ice melted, it left wide valleys and tall waterfalls.

About 3 million years ago the world began to cool, and glaciers, slow rivers of ice, grew and shrank again and again in the Sierra Nevada. The biggest glaciers filled the main valleys. They dug the narrow V-shaped river canyons into broad U-shaped valleys with steep walls and flat floors.

Smaller side valleys were not dug as deep. When the ice melted, they were left hanging high above the main valley, and their streams now pour over the edge as waterfalls, such as Yosemite Falls and Bridalveil Fall. The ice also polished the granite, scratched it and dropped lonely boulders called erratics.

The Park Service dates the onset of repeated glaciation to about 3 million years ago. Most glacial features reflect the Tioga Glaciation, which began about 26,000 years ago and peaked about 18,000 years ago, when ice covered most areas above 8,900 ft (2,700 m). Thicker trunk glaciers converted V-shaped canyons into U-shaped valleys such as Yosemite Valley and Lyell Canyon, leaving tributary valleys hanging; Bridalveil, Yosemite, Sentinel and Ribbon falls are hanging waterfalls. Ice smoothed fully buried landforms such as Lembert Dome and sharpened projecting ones such as Cathedral Peak, while fractures in the granite steered erosion. Nearly all natural lakes are glacially scoured and slowly filling: Mirror Lake is nearly a meadow.

Today's Lyell and Maclure glaciers are not Ice Age remnants but products of later climate swings. They may have been near their maximum in 1872, when John Muir first measured their movement; since then they have lost more than 90 percent of their volume, the Lyell Glacier no longer moves, and the Park Service expects both to disappear soon. Rockfalls, triggered by rain, earthquakes and even solar heating, are now the park's most active geological process.

The water in Yosemite is very cold and strong.

Stay on the trails near waterfalls and rivers. The rocks can be slippery. Put all your food in a bear locker, so bears stay wild and safe.

The Park Service reminds visitors that mountain water is extremely cold, even on hot days, and that even a slow current can carry you where you don't want to go. Near waterfalls, stay on trails: the rock is extremely slick, even when dry. If a sign says no swimming, don't swim or wade.

Bears have an amazing sense of smell. Anything with a scent counts as food, from snacks to soap, and it must be stored properly. That's the law in Yosemite. After dark, food must go in a bear-proof food locker, not in your car. If you see a bear away from campgrounds and buildings, stay at least 50 yards away.

The Park Service's water-safety advice warns that hazards are often invisible: one misstep on a rock above a calm pool can sweep a person downstream, mountain water causes hypothermia quickly even for strong swimmers, and submerged branches or gaps between rocks can trap a person underwater. Visitors must obey 'no swimming' closures, such as at Emerald Pool above Vernal Fall, and stay on trails near waterfalls, where the rock is extremely slick even when dry. Anyone who falls in should float on their back, feet downstream.

Food storage rules have the force of federal law, with fines of up to $5,000: any scented item counts as food, food may stay in a car only in daylight and out of sight, and after dark it goes in a food locker. The Park Service explains that bears that get human food can lose their fear of people and may have to be killed, so storage protects bears. On the Half Dome cables, hikers need a permit and should stay on the inside of the cables, and the Park Service warns that wet rock makes the rock and cables very slick.