Yosemite’s Fearsome Wonders

Every so often, the granite cliffs that soar above Yosemite Valley decide to slough off a chunk of their legendary profiles. Most times, these rockslides produce only a rumble and a puff of dust, and anyone who witnesses it feels lucky.

Nature’s timeless way also can have a more devastating effect, as the swath of fallen trees, crushed rock and an inch-thick coating of dust on 50 acres of the valley floor Thursday showed in graphic relief.

A Southern California man was killed and about a dozen people were hurt when a huge granite arch near Glacier Point broke loose before dusk Wednesday, unleashing a cascade of debris on the Happy Isles area 2,400 feet below. A thick swirl of dust rose from the valley floor like an explosive cloud, and after it settled an hour later, park rangers found a quarter-mile-long piece of forest flattened by the impact.

Work crews in surgical masks scoured the dusty rubble Thursday but did not expect to find anyone trapped. “Everyone has been accounted for,” said Scott Gediman, a spokesman for the park service. “We don’t have anybody who’s asking for someone. We don’t believe there are any more victims.”

The one known fatality was Emiliano Morales, 20, of Montebello, who was pinned beneath a tree at an ice cream stand in the Happy Isles area. His body was recovered late Wednesday night, shortly before rescue workers called off their search for the night because of darkness and falling rock.

Two people from Whittier who were with friends celebrating their graduation from Pioneer High School were the most seriously injured. Hisano Hamada, 18, was in critical condition at Doctors Medical Center in Modesto with a broken arm and leg and neck injuries. Kelly Booth, 17, who was injured by a falling tree, was in fair condition at Memorial Medical Center in Modesto after surgery to repair cuts.

About a dozen other hikers and campers, most of them suffering from dust inhalation, were treated at the Yosemite Medical Clinic and released during the night.

The sudden rockslide was a spectacular example of what geologists call exfoliation, the shifts and collapse of rock that have reshaped this place through the ages. Like the seasonal floods and fires, rockslides are a reminder that Yosemite may be one of the wonders of the world, but it is not hazard-free.

In 1980, three hikers were killed by falling rock near Yosemite Falls. Rockslides often occur in early spring or fall when snow is either melting or accumulating, but can happen anytime and anywhere.

“There’s no way of predicting them,” said Connie Rothell, a Mariposa school secretary who lives at Yosemite with her husband, a park ranger. “It’s part of the natural cycle. It’s part of the history of Yosemite Valley.”

As search crews picked through the shattered rock and splintered forest Thursday with the help of dogs and fiber-optic equipment, park rangers said it could have been much worse.

The piece of granite, estimated to be more than 200 feet wide, created such a blast of wind as it fell that it leveled hundreds of pine trees and left a thick blanket of dust. One park official who observed the scene from the air said the damage reminded him of Mt. St. Helens after it blew its top. Another said it looked like a test site for a new kind of bomb.

Luckily, by the time it shook this valley with a magnificent roar, the clock read 6:46 p.m. Most hikers had left the trails, and a small nature museum and snack shop at Happy Isles had closed for the night.

The affected section of the valley is at the head of the John Muir Trail and the trail to Vernal and Nevada falls. The area is closed to vehicles except for park shuttle buses, another factor that probably limited casualties.

“I’ve been watching rock falls and slides up here for almost three decades, and we’re very fortunate that it happened when it did,” said Jim Snyder, a park service historian.

“If it had been two or three in the afternoon, I don’t think we ever want to see that at Happy Isles. It gets pretty crowded.”

Yosemite is known for its dramatic topography, especially its deep glacier-carved valley, really a canyon with 3,000-foot-high walls of granite. Snyder theorized that the recent wet winters may have contributed to the wearing away of the granite slab high above Happy Isles.

“That arch of rock has water running beneath it, and water reduces the friction that helps hold it in place,” he said. “You tend to see these rockslides in wet years.”

Robert Merrill, a geology professor at Fresno State, said a number of rockslides have occurred in the Yosemite area over the past 20 years, but he could not remember one that was as big or as dramatic.

“This shifting and falling is a natural reshaping of the landscape in areas where you have extremely steep slopes, he said. “But it’s the size of this thing that’s unusual.”

Visitors who were caught near Happy Isles when the granite arch roared down the mountain said it was the most frightening moment in their lives.

“It looked like a tornado,” said John Lydon of Torrance, describing the dust cloud hundreds of feet high that blotted out the sun. “You could see it coming toward you. . . . I didn’t know if it was the end of my life.”

Greg Harris of Texas was rafting down the Merced River with his 14-year-old son when he heard what sounded like an explosion. A few minutes later, he saw the river change color, as if someone had taken a giant stick and was stirring it up.

“It got real silty and turbid, and I didn’t know what was going to happen next,” he said. “We got out of there real fast and then we volunteered to help with the rescue.”

What many people who were there will remember is the noise and dust.

“The dust was so bad you couldn’t breathe,” said Keith Buttleman of Santa Clarita, who was camping with his family. “We had everybody wet their T-shirts and put them around their noses and mouths.”

The roar of the falling rock and toppling trees was variously compared to that of earthquakes and aircraft. “When we first heard it, it sounded like a bunch of jets coming by,” said Richard Madrill of Reno.

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Nature’s Power at Yosemite

Yosemite Valley’s dramatic scenery is sculpted by powerful geological forces. The latest reworking occurred Wednesday night when a rock shelf gave way and plunged 2,400 feet to the valley floor. Rockslides are common in Yosemite and surrounding areas. Here is a representation of how the slide happened.

ROCK FORMATION PROCESS

1) About 100 million years ago, a chain of volcanoes turns flat land into irregular peaks. Some molten rock never reaches the surface, forming bubbles, or plutons, that harden into granite.

2) Over the next 80 million years, erosion of the surrounding soil and rock exposes the granite. It rises and tilts with stretching and thrusting of the Earth’s crust.

3) During the ice age 2 million years ago, glaciers carve sheer cliffs through the granite. Water soaking into cracks in the granite continually freezes and thaws, weakening exposed layers until they crack and peel like layers of an onion. The process, called exfoliation, is usually on a small scale, and the peeling creates picturesque arches and spires on the cliff faces.

4) Carved into a 3,500-foot cliff, an overhanging arch, 200 feet wide and 50 feet high, breaks off between Glacier and Washburn points.

5) The falling rock--at least 25,000 tons of it, by one estimate--is pulverized on the way down as it crashes against the cliff.

6) A huge blast of small rocks, dust and compressed air continues to sweep down the slope and onto the valley floor, knocking down hundreds trees along a path extending a quarter-mile or more from the base of the cliff.

Sources: Yosemite National Park and David Howell, U.S. Geological Survey

Researched by DAVID FERRELL and VICKY McCARGAR / Los Angeles Times