One Best Hike: Yosemite's Half Dome. Rick Deutsch

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      Present day Yosemite

      Yosemite is undergoing change even today. Being relatively young geologically, the area is still settling in. Earthquakes have been reported since records were kept in the mid-1800s. Rockfalls are very common at Yosemite. Since 1857, more than 500 rockfalls in Yosemite Valley have killed 14 people and injured more than 60, more than at any other national park. They are the result of quakes and a process of exfoliation, that is, large sheets of granite peeling off in vertical patterns similar to an onion skin. Rockfalls also occur when rainwater gets into cracks and freezes. When water freezes, it expands. The small but constant expansion may cause the granite to open slightly. Lichens can grow here, and when they die, they provide a soil structure in which larger plants can germinate. In time, the growing plant exerts forces that can lead to a flaking off. Given enough time, these processes can take their toll.

      In the 1980s a major rockfall occurred off the Three Brothers formation. In 1996 and again in 1999, enormous sheets of granite dropped near Happy Isles; each event killed one person. The impact produced massive shock waves that literally snapped thousands of trees. The 1996 rockfall was comprised of two slabs (totaling 80,000 tons) bigger than two football fields and three stories tall. They free-fell 1,800 feet and had reached a velocity of 270 miles per hour when they impacted. In 2008 a rockfall off the Glacier Point wall behind Curry Village took more than 200 tent cabins and wooden cabins out of service. You can still see the talus fields from this fall. In 2009 a rockfall even larger than the 1996 Happy Isles fall occurred at Ahwiyah Point near the eastern side of Half Dome’s face. An estimated 115 tons of rock crashed down onto the southern portion of the Mirror Lake Loop Trail. When you are on top of Half Dome, you can carefully crawl out to the edge and look down at the fresh white stone talus field at Ahwiyah Point.

      Flooding has also changed the landscape of the park. The Merced River flood of 1997 set records, and the high water marks are still visible. It caused $178 million in damage in Yosemite Valley. As a result, the valley was closed for more than three months, and vast changes were made to the area’s management plan. Today numerous structures have been relocated, roads have been rerouted out of the flood plain, and nearly 50% of campsites have been removed.

      In 2006 spring rains created a huge rockslide southwest of El Portal that dropped tons of earth from Ferguson Ridge onto CA 140, closing that road for part of the year. Today travel is only possible via two one-way Bailey bridges, which take traffic across the Merced and onto the remnants of the old Yosemite Railroad track, then back 0.25 mile later. The train ceased operation in 1945, and the track was sold off for scrap.

      Ferguson Ridge rockslide

      Occasional fires ravage parts of the park with the loss of thousands of acres of forest. Lightning is most often the cause, but man-made fires also take their toll. An out-of-control prescribed burn in August 2009 in the Big Meadow area near Foresta devastated nearly 8,000 acres. These unpredictable phenomena are reminders of Yosemite’s dynamic state.

How Half Dome Was Formed

      Half Dome is estimated to be nearly 90 million years old. It is the signature landmark of Yosemite and is truly an American and a global icon. (It was the inspiration for the logo of the North Face company.) It is one of the planet’s most vertical walls, with the face rising 2,000 feet straight up. Its peak reaches an altitude of 8,842 feet. The distinctive face reflects Earth’s primordial power. Unlike most domes in the park, such as Liberty Cap, it is not a true spherical dome. When viewed from the west, it appears as a rounded table rather then the half-ball image it presents from other angles. It’s estimated that only 20% of the original dome is gone.

      Only 20% “missing”

      The exact method of Half Dome’s formation is not totally understood. When I moved to California, I was told straightaway that it was cut in half by glaciers. That explanation makes sense, and it certainly looks that way, but that is mostly wrong. It is true that Half Dome’s shape was influenced by glaciers; the northern and southern sides were affected about halfway up. Even John Muir believed it was glacial in origin. Yosemite likely has more granite domes than anywhere else on Earth, and most were indeed formed by glaciers. However, today’s geologists feel certain that Half Dome stood above the glaciers, as did many of the higher peaks in the park. No classic striations are on its surface or upper slopes. Another telltale sign is the erratic that rests at the Diving Board (western edge of the face) at 7,500 feet. An erratic is a large boulder that the glacier carries along and then gently puts down as it retreats. This boulder has the same material as the Cathedral Peak granite and is not the same as the Half Dome granodiorite that makes up the Diving Board. Clearly, this erratic was deposited by a glacier at that point.

      When viewed from the air or from Washburn or Glacier points, Half Dome appears as part of a ridge along Tenaya Canyon. Scientists call this ridge a vertically oriented joint. The internal compression pressure in the rock causes it to develop joint plates that align themselves with the surface, regardless of the slope of the surface.

      Glaciers did carve the lower ends of Half Dome, up to about 700 feet from the top, resulting in overhangs. These overhangs were released by the exfoliation action of the joints over time. So, it is safe to say that we don’t know definitively how Half Dome got its shape, but the face was probably caused by rockfalls and exfoliation along a prominent vertically oriented fracture on the famous north face, and glaciers probably did most of the sculpting of the smoother south face. Regardless, it is unique and beautiful.

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      Humans in Yosemite

      Climb the mountains and get their good tidings. Nature’s peace will flow into you as sunshine flows into trees. The winds will blow their own freshness into you, and the storms their energy, while cares will drop off like autumn leaves.

      —John Muir

Native Americans

      The earliest Native Americans arrived in the area about 8,000 years ago. By the time of the European explorers, the Native American tribes in the greater California region included about 90 distinct entities. The Spanish were the dominant foreigners, and they focused their activities on religious conversion and raising cattle. The burgeoning Mexican territory meant they were spread thin in the west. The Native Americans living here were mostly agrarian and did not unify into a strong fighting machine, as did the Sioux, Cherokee, or Lakota of the plains. They did not rebel against the Spaniards. From 1769 to 1823, the Spaniards began a rigorous mission construction program, wherein each of 21 California missions was located about a day’s walk from the next. The missions ranged from Mission San Diego de Alcalá in the south to Mission San Francisco Solano, in Sonoma, to the north. The goal of converting the locals consumed much of the daily life for the Spaniards, and they did not venture deep into the Sierra but instead built settlements in the coastal and central valley areas. In the mid-1800s some of the tribes in the Sierra area were the Po-ho-nee-chees, Po-to-en’-cies, Wil-tuc-um’-nees, Noot’-choos, Chow-chil’-las, Ho-na’-ches, Me’-woos, Monos, and the Chook-chan’-ces. Today some of these names live on at Native American casinos in the area.

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