It was a trip of many firsts for Greg Stock. As Yosemite’s park geologist, Stock was doing the first inventory of rock types on El Capitan, with the help of two climbing rangers. The weather was clear and mild that October 2008 day on Mescalito (5.9+ A3). He stopped at the top of each pitch to break out his scientific instrument: a whiteboard with a ruler glued to it.

Dangling hundreds of feet above the ground, he snapped a photo of the granite next to the whiteboard. These photos and others shot on separate El Capitan routes, and some from the ground, would later be used together with laser scans of the cliff to create a map of its rock types.

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It was also his first time climbing El Capitan—a dream since he started climbing at 14. Stock grew up near Lake Tahoe and first climbed in Yosemite at 16. His early adventures outdoors led to undergraduate and graduate degrees in geology. Now the park’s first geologist, Stock was excited to both climb and study the iconic granite domes. The climbing rangers, Jesse McGahey and Lincoln Else, aided up the sheer, blank early pitches of Mescalito as he followed. “It was steep and amazing,” he says.

At the end of the second day, the team radioed the park dispatch office. An evening ritual, the call was intended to let the office staff know they were clocking out for the day. But, instead, an official there relayed some bad news: rocks had showered down from Glacier Point that afternoon, landing near Half Dome Village (then called Curry Village), a tourist-filled area with hundreds of tent cabins.

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The rockfall had not damaged structures nor injured anyone. Still, Stock’s enthusiasm for the climbing trip waned. That night, perched on a portaledge 15 pitches up the wall, he barely slept. Early the next morning, he turned on his radio and heard rangers discussing a second, larger rockfall that had just hurtled through Half Dome Village. In the background of the radio chatter, he heard sirens.

Boulders and smaller “flyrock” from that morning’s rockfall smashed cabins and injured visitors. One 9-foot diameter rock rolled through three empty tent cabins. Stock worried and felt guilty he wasn’t there to help. But he was stuck—suspended 1,500 feet above the ground. A couple hours later, his supervisor called with a frank message: “We need you down here.”

A helicopter landed atop the cliff and lowered a rope and a search and rescue officer down to Stock, and the two were whisked to the base of El Capitan. “So I got rescued to go to work,” laughs Stock, 44. “It’s probably a first.”

The geologist sped off to survey the wreckage. In the hectic haze that followed, he forgot to take his harness off the entire day.

Stock’s rescue took just 20 minutes. But those 20 minutes—hovering in the air, away from the dust clouds and chaos at Half Dome Village—were crucial for creating a course of action for the next four years. It was time, Stock thought, for a new approach to protecting visitors from the unstable granite cliffs that form the Valley’s walls, cliffs that slough off great slabs of rock without a second’s notice.

“I just had this very clear thought: We have to do this differently; we have to do a valley-wide rockfall assessment,” he says. “By the time my feet hit the ground, I had a plan.”

After the dust settled and the visitors evacuated, Stock got to work mapping the potential for rockfalls throughout Yosemite Valley. That fateful day in 2008 launched a decade of collaborative work between Stock, fellow geologists, and climbers. Their work has gleaned new knowledge on rockfalls, helped avoid casualties, and created bridges between climbers and rangers. Stock is discovering new insights into rockfall—not just where, and how often, the cliffs dump debris, but what triggers it. He’s inching closer to answering the surprisingly elusive question: “Why do rocks fall?”

Geologist Greg Stock surveying the scar of a 2015 rockfall on Half Dome Eric Bissell, National Park Service

Revealing rockfall rules

Between nineteen and fifteen thousand years ago, Yosemite’s glaciers retreated and revealed a gaping, kilometer-deep rift. Since then, chunks of rock have ripped free from the cliff faces and pounded the valley, creating talus slopes and boulder fields. The climber-favorite boulders in Camp 4 and Half Dome Village are all, in fact, the product of rockfalls.

The falling debris ranges in size from baseball to bus—sometimes bigger. Large rockfalls, the volume of 10,000 washing machines (or the same number of cubic meters), tumble down about once a year. Stock documents a rockfall every week on average, often brought to his attention by visitors or rangers and later verified by a site visit. But he estimates up to 90 percent go undetected.

“Even 100 washing machines—that can go undetected,” he says, if the rockfall is at night or in a remote location. While most rockfalls are smaller, the height of the cliffs gives rocks time to accelerate enough to deliver a deathly blow to anyone in their way; a rock falling 1,000 feet reaches a speed of about 172 miles per hour.

Since 1857, when good note-taking began, rockfalls have killed 16 people and injured more than 100. In comparison, the Merced River and car accidents have both claimed far more lives. As of 2007, there were 162 car accident and 154 drowning fatalities reported. But, occasionally, rockfalls are catastrophic; a 1980 fall on the upper Yosemite Falls Trail killed 3 and injured 19. In September 2017, an estimated 1,300 tons of rock rained down from 650 feet up El Capitan’s southeast face, killing one climber and injuring another. Remarkably, only a small portion of natural rockfall deaths are climbers, although rocks dislodged by climbers have taken many lives.

A white scar marks the site of a large rockfall in September 2017 on El Capitan's southeast face National Park Service

“They’re fascinating, powerful, complicated, and dangerous,” says Stock. “So learning something about them can be useful.”

Stock started studying the cliffs in 2006, originally pulled in during a lawsuit against the Park Service over the rockfall death of Peter Terbush. There, he met Brian Collins, a U.S. Geological Survey civil engineer who was also brought into the case for his technical expertise. Later, when Stock conceived of the Valley-wide rockfall assessment, he enlisted the help of Collins and other experts from USGS, the Research Institute for Geo-Hydrological Protection in Italy, and the Georgia Institute of Technology.

The team first mapped how far the talus fields stretched from the base of the cliffs and how far boulders had bounded into the Valley. They dated rocks with radioactive Beryllium 10— a chemical signature left behind by cosmic rays on the freshly exposed faces of rockfall boulders. Additionally, they created a 3-D map of the Valley by beaming lasers down from a Cessna.

They used their observations of the geometry of the Valley, and the dates and locations of rocks, to create a computer simulation able to generate a “hazard zone” map in which they expect most rockfalls to occur. This hazard zone let park officials decide how to move buildings, close them, or change their use to avoid danger.

To test how effective their map was, Stock and Collins first estimated the pre-assessment risk by considering how each structure in the Valley was occupied and how often, and how close it was to the hazard line. They compared this assessment to the risk after moving the structures. The result? A 95 percent reduction in rockfall hazard risk. For example, if rockfalls before the assessment struck 100 buildings every 100 years, now that rate is more like 5 buildings in 100 years. But the stat Stock likes better is that twice now, boulders have landed in the footprints of former Half Dome Village cabins.

USGS published the study in 2012, titled Quantitative Rock-Fall Hazard and Risk Assessment for Yosemite Valley. Since then, Stock and Collins have continued their journey of studying the Valley’s complicated cliffs. They started looking at the rockfall database—an inventory started in the 1980s with over 1,000 entries dating back to 1857—to see what trends they could find. At the time, it was known that winter rains triggered rockfalls—usually many small rockfalls early in the wet season. But they noticed something weird: an unexpected amount of rocks fracturing off cliff faces in summer as well.

Brian Collins (left) and Greg Stock (right) download data at a flake near Royal Arches. Valerie Zimmer, National Park Service

Naturally, they needed to climb up a cliff to investigate. They spent the next three and a half years climbing an alternative start to the Royal Arches route every 35 days, year-round, and in all weather conditions. There, they checked their “crackmeters,” placed behind a big flake about 50 feet above the ground. The car jack-like devices were fitted with a transducer that recorded the distance they expanded or contracted throughout each day.

“I think just about any geologist would say that field work is key to understanding the process,” says Collins. “And for rockfalls, fieldwork can be rock climbing.”

Yosemite’s granite walls have onion-like layers. The outer layers, over time, loosen and eventually break off—they exfoliate. Stock and Collins, in their research, found these flakes expand outward during the day and contract at night. During hot months, the layers stretch the furthest away from the cliff. The flake on Royal Arches moved almost a centimeter outward daily, then cooled and contracted back into place at night.

A crackmeter, which continuously measures the distance between the outer flake and the wall. Brian Collins, U.S. Geological Survey

This stretching can start to break the sheet of rock off from where it connects to the cliff, by extending the crack behind it grain-by-grain. The scientists even put acoustic sensors inside the flake on Royal Arches. “Every day, when the temperatures would warm up you’d hear this ‘cr-cr-cr-cracking,’” says Stock.

Because of their work, heat has been added to the list of rockfall triggers. Stock says it’s the climbing, in part, that fuels their discoveries (they sometimes climb just for fun, too). “With a good climbing partner, you establish this trust, and this commitment to the person, and you know how they think,” says Stock. “There’s a reason [Collins and I] have a good scientific collaboration, and it’s that we get along well in all aspects. And the fact that we’ve climbed a lot together, including at least one wall—you really bond with that person. I have no idea where we’d be if I hadn’t met Brian 10 years ago.”

Another big cause for rockfalls, attributed to 29 percent, is rain. Water slips into cracks and builds up pressure, causing the rocks to break off. This winter, in which the Valley saw about double its average annual rainfall, Stock found himself outside assessing fallen rocks almost every day. One time, he turned to drive back after checking out a rockslide on Big Oak Flat road, one of the access points to the park, only to find himself blocked by another pile of rocks that had just tumbled down.

But, there weren’t more rockfalls this year than during California’s drought years. “I get surprised all the time here,” Stock says. It’s an example to him of how, despite all the research, it’s still impossible to predict when rockfalls will happen.

There are also many other causes that make up a smaller portion of rockfalls, including earthquakes, snowmelt, freeze-thaw cycles, and lightning. Still, for 26 percent of falls documented, the geologists can’t identify a cause (although the current thinking is that heat may be at work for some of these). Even harder still is telling exactly how much heat or how much rain will stress the slabs of rock to the point that they break free.

It might be possible to predict rockfalls someday, but not for a long time. In the meantime, Collins thinks a promising route is to look at thermal maps of rock faces. Warm air can surround both sides of a detached flake and heat it up faster than the rest of the cliff. These hot areas on the cliff could be the most likely to rip off next.

Analogous to earthquakes, predicting rockfalls is, well, shaky at best. But, while forecasting earthquakes remains elusive, scientists have learned where faults are, how they work, and how often they strike—such scientific progress has helped make cities safer. In the same way, Stock says, the rockfall prediction pursuit itself is brimming with valuable information. He and Collins continue to collect data and take on new projects to tease apart these rockfall mysteries. “It’s a worthy goal, even if it’s going to take a long time to get there.”

Greg Stock hikes along the base of Middle Cathedral Rock in Yosemite. Ryan Ford

Identifying insecure rocks

For all the radiometric dating, lasers, and other high-tech research tools, a lot of Stock’s job is just hiking around, looking at fresh rockfalls and jotting down notes.

On a warm, late-spring day in May, Stock hikes a steep, rocky path up to the base of Middle Cathedral cliff, then turns right to scramble the talus field leading up to the Gunsight—a pass between Lower and Middle Cathedral. He’s hunting for a rockfall from a month ago. A climbing ranger had sent him a photo of the cliff, with a dust cloud circling its shoulder.

Not finding anything, he turns around and treks back down to Middle Cathedral. The area next to the cliff is cool and quiet—a reprieve from the warmth of the day and the chaos and crowding on the Valley floor below, thanks to the burgeoning tourist season. Small, frail trees edge the cliff and the ground is littered with rocks—many are mossy and partially buried—likely a few years old, says Stock.

Then, “I’m pretty sure that’s a freshie,” he says, as he looks at a thin, jagged slice of rock the size of a computer monitor. A clean face is exposed—free of moss and lichen. It’s perched on the ground, not buried, and beneath it lies a green, leafy branch. Similar shards surround it. “I would guess that most of these fragments are from the rockfall,” he says.

Greg Stock stands next to fresh debris from a rockfall on Middle Cathedral. Ryan Ford

He notes the location and approximate volume of the shards. It’s hard to add them up, since it appears a slab ripped off and then shattered as it tumbled down. Here, he estimates 20 to 30 cubic meters (later, a laser scan of the cliff, compared to an older scan, will create a precise estimate). He’ll also ask the climbing rangers to warn the owner of the fixed rope hung on the cliff beneath the rockfall scar, so that the climber is aware of the incident—loose rock around the scar may still fall.

Has being in on the frontline of rockfalls changed his attitude toward climbing? “It makes me more thoughtful,” he says. He wears a helmet on approaches and on the wall, and knows how to assess loose rock. Unless he’s gearing up for bigwall climbing, he sticks to shorter cliffs (the more rock, the more rockfall potential).

Even with his increased caution and special expertise, sometimes he’s only gotten away due to luck. One day, Stock, his wife Sarah, and their then seven-year-old daughter, Autumn, were heading out the door to climb at Lower Falls Amphitheatre. Just before they left, they got a call from one of Autumn’s friends inviting her on a play date in El Portal. So they dropped Autumn off, and then spent the day climbing at Parkline Slab instead. When they got back to the Valley, they found out that several microwave-sized rocks had rained down at Lower Falls Amphitheatre that day, right into the area they had planned to climb in, Ranger Crack.

Although he describes himself as the “most paranoid guy on the planet about rockfalls in Yosemite,” Stock hasn’t stopped climbing. In April 2009, he came back and climbed El Capitan via Tangerine Trip (A2 5.8) on its southeast face. He’s since climbed the Nose, too. It’s like the relationship between surfers and sharks, he says. With sharks and with rocks, “they’re there, [but] it’s not very often that you encounter one, and even then it’s less likely you’ll be injured or killed. But they’re there, and if you’re going to do that, you have to know.”

He offers a few tips: The more overhanging a rock, the more unstable it will be, because granite gets weak when it’s freely hanging and unsupported. Reduce your time approaching and/or avoid bivying at cliff bases to stay clear of the rockfall zone. When you climb, knock on questionable blocks with an open palm to test for hollowness, and come down if you’re not comfortable proceeding. Pull down rather than out on loose or flexing holds (if you have to use them).

And, if you witness loose or falling rock, spread the word.

A rockfall on Half Dome in July 2006. National Park Service

Collaborating with climbers

In autumn 2015, Aaron Liebling and a climbing partner endured six pitches of “yucky,” dirty, and brush-covered terrain from third class to 5.8—to reach the base of Mt. Watkins. They camped there, aiming to get an early start on the cliff’s South Face (V 5.9 C2+). It was a beautiful, clear night, says Liebling.

At about 1 a.m., Liebling woke to the sound of thunder. He sat up in his sleeping bag and saw the cliff below him light up with an electric glow. Then, his partner started running away along the ledge where they were camped. He’d realized it was rockfall, and it soon dawned on Liebling too.

“It was a huge rockfall and what we were seeing were all the sparks from the huge pieces of granite hitting each other, creating this waterfall of lighting,” says Liebling.

“It was truly awe-inspiring, more than anything else I’ve ever seen.”

Liebling stayed and watched, mesmerized by the glowing rock a few hundred feet below him. His partner returned and the two sat in shock for a while before going back to sleep.

The next day, they tried to climb, after seeing that scar left by the rockfall was not on their route. But they backed down after struggling to execute a pendulum at the top of the second pitch. “Our stoke was just not there after the rockfall,” he says.

When he got home, Liebling typed up an email to Stock—who Liebling had seen posting on the Supertopo forum—detailing the rockfall. Stock quickly followed up, asking for more detail on the location and time of the event. Later, he surveyed the oval-shaped scar halfway up Mt. Watkins, and estimated the rockfall had been 500 cubic meters in volume.

Stock and the Yosemite climbing rangers use information from climbers, like that provided by Liebling and his partner, to help update the Yosemite Climbing Information website. They try to share information strategically, says Stock, to avoid an overload while also keeping climbers aware of the risks at the ever-shedding cliffs.

A 200-feet-tall and 100-feet-wide sheet of rock, in yellow, broke off from Half Dome July 2015. National Park Service

It helps that Stock’s a climber, Climbing Ranger Eric Bissell says, because he can safely get up the cliffs to assess a fall. Last fall, Bissell, Stock, and another ranger, Brandon Adams, climbed the Regular Northwest Face of Half Dome to check out the margins of a massive rockfall that crashed down in July 2015; the sheet of rock, 200 feet tall and 100 feet wide, took with it two pitches of the route and left behind a smooth slab. They found the scar located at the base of a series of chimney systems above the Robbins bolt ladder. Stock looked at the freshly exposed slab and took notes about the unstable rock surrounding it.

For Bissell, it changed his relationship with the route, which he had climbed six times before. “Climbing it with Greg changed it from thinking about pitch descriptions to thinking about it in geologic terms—the exfoliation flakes, geologic time, and thinking about it as an onion,” he says. “Sometimes we think about the routes as being permanent, etched in stone, but Greg thinks about it much more fluidly.”

They shared their findings in a conditions report. It’s one example of ongoing communication between climbers, rangers, and scientists. Stock and Bissell note that such communication is helping mend historic rifts between climbers and rangers. In the '60s and '70s, park officials and climbers were on opposing sides of a nationwide cultural divide, where the young, rebellious climbers saw park rangers as “the man.” Climbers flouted park regulations and stayed beyond campsite limits, fueling tensions.

Since then, the sport’s grown more mainstream, and officials have also accepted rock climbing as a part of the park’s history and fame. The unique history, says Bissell, has helped grow programs like the Climbing Rangers, which enable the park to work with visiting climbers.

“Over the years, there’s been more and more overlap between the two groups, where there are many rangers who consider themselves first and foremost to be climbers, and a lot of us end up living and working in the park, because we are climbers and we want to be here,” says Bissell. “I think also that the modern climber is more interested in being engaged with concepts like stewardship.”

Bissell points to the growth of the Yosemite Climber Stewards program. The program consists of climbers who devote their time to educating climbers on environmental stewardship and safety in the Valley. The four rotating volunteers spend a minimum of 12 weeks fixing trails, cleaning up trash and sharing information at Camp 4 Climber Coffee events. Their newest volunteers include travelling filmmakers Vikki Glinskii and Spenser Tang-Smith of the RV Project.

Now, Stock is taking advantage of these stewardship-oriented climbers for his next project: mapping the geology of Half Dome. To undercover Half Dome’s history, he’s enlisting the help of the climbing rangers and their volunteers to bring up a small device, the size of a deck of cards. It takes just a couple minutes to scan the rock and test the strength of its magnetic field, but Stock can’t climb up all the routes himself.

The device measures subtle differences in the granite, which can tell scientists about the strength of the rock and possibly how the cliff stands so tall. The project might help answer why Half Dome has its unique shape.

“It’s the question I get the most here: ‘What happened to the other half of Half Dome?’ I don’t know! Nobody knows,” he says. “I think its probably a complicated story that took place over a long time.”

The fickle nature of the Valley’s cliffs has generated more questions than answers for Stock. “I feel like I almost know less now than I did when I started the job 12 years ago,” he says. “I don’t go into this with a strong sense of how things should be.”

But, as relationships and mindsets change, it looks like he’ll have the help of a new generation of Yosemite climbers.