What Remains For the past 148 years, Yosemite’s Lyell Glacier has taught us about the Earth — how it was created, where it was going, and now, how it might end.

What Remains For the past 148 years, Yosemite’s Lyell Glacier has taught us about the Earth — how it was created, where it was going, and now, how it might end.

On a cool September morning in 2014, among lodgepole pines under blue mountain sky, Greg Stock shouldered a backpack full of camping gear and scientific equipment. Boyishly slender and athletic at 45, Stock is a climber, caver, and serious reader of books about mountaineering and the natural world. He holds the enviable job title of Yosemite National Park Geologist and mostly loves the work, especially the part he was bound for that day — the study of Yosemite’s last two glaciers.

Stock and several companions started their walk in Tuolumne Meadows, the high-country jewel of Yosemite and everything that I would ever wish to find in the pastures of heaven — many square miles of grass and wildflowers surrounded by white granite domes that reflect sunshine like polished glass. Stock followed the John Muir Trail south out of those meadows into an immense U-shaped gorge called Lyell Canyon, 8 miles long and 3,000 feet deep, carved out of granite by long-vanished glaciers during dozens of ice ages. Evergreens dot the sloped walls of Lyell Canyon — straight lodgepoles down low, bent whitebarks up high. In that drought year of 2014, dry meadow grasses carpeted the canyon floor in pale gold. Down the middle, the Lyell Fork of the Tuolumne River trickled through wide, meandering oxbows. The great irrigator of Tuolumne Meadows and drinking-water source for San Francisco, that river thunders deep in spring but flows in autumn thanks to meltwater from Stock’s destination, the Lyell Glacier. Seven miles into Lyell Canyon, Stock kept an eye out for white rocks in the grass. If you didn’t know what to look for, you would never find those rocks, much less guess they marked a particular spot. When Stock saw them, he turned east off the John Muir Trail and down into the mostly empty channel of the Lyell Fork. He hopped across the shallows and then walked into the center of the canyon. Stock poked around in the grass for another pile of rocks, which marked the spot where, in 1883, a geologist named Israel Russell looked 4,000 vertical feet up to the jagged summit of Mount Lyell, 13,114 feet above sea level and the tallest peak in Yosemite National Park. Standing right there, Russell took the first known photograph of the Lyell Glacier, which John Muir had found only 12 years earlier. In Russell’s photograph, 13 million square feet of ice spread like a white shawl across Mount Lyell’s black metamorphic shoulders. Geologists and park employees have been returning to Russell’s photo point — and to the glacier itself — on a more or less regular basis ever since, replicating Russell’s images to create a scientific record 135 years old and counting. Stock has been the keeper of that tradition for over a decade, making the trip through Lyell Canyon more than 20 times to check the glacier’s vital signs. He has put gauges in runoff streams to measure meltwater trickling out of openings at the toe of the glacier. He has studied data from NASA’s Airborne Snow Observatory, an airplane outfitted with advanced sensing equipment that calculates the water volume in the Sierra snowpack and ice fields. Using much of the same technique that Muir did in 1871, Stock drove stakes into the Lyell to measure the downslope creep of ice that defines a glacier. Like everyone who has ever studied the Lyell — and pretty much everyone who has ever studied any glacier — Stock documented shrinkage. The Lyell has lost depth and retreated upslope and broken into a smattering of white Rorschach blots that, as of 2014, amounted to about 3 million square feet of ice. In 2012, Stock had collected data showing that the main lobe of the Lyell was not flowing downhill. The pleasure of working in that quiet alpine sanctuary kept him coming back in a spirit of optimism. Still, when Stock looked through his camera’s viewfinder at the largest of the Lyell’s remaining white blots, in 2014, he was surprised to see that a familiar dark patch had grown much larger. Stock led his companions farther south along the John Muir Trail to where it climbed up through forest toward Donohue Pass. At a wooden bridge across the river, Stock turned west off trail. Up rocky slopes, they came to the shores of an hourglass-shaped blue lake in a bowl of white stone — cooked dinner, slept in sleeping bags. In the morning, after breakfast, they hiked another 1,200 feet up to the main lobe of the Lyell, a broad and steep mass of ice in a quiet cirque of shattered rock. Stock felt like a man coming home after a long absence, comfortable and eager to catch up. He decided to have a close look at that dark patch. “I remember noticing that it was right under an avalanche chute in the headwall below the summit,” Stock says. Perhaps it was just rock debris from some long-ago slide, embedded in the glacier surface. The Lyell Glacier hangs at a severe angle off the mountainside. To slip and fall can mean a long, fast plummet. Stock wore crampons on his boots and carried an ice ax as he stepped onto an ice field riddled with sun cups, bathtub-sized depressions that forced him to walk along blade-thin ridges between them. Standing at the edge of the dark patch, Stock got a terrible feeling. “I just knew. That’s bedrock. Your wishful thinking that that’s debris can’t possibly be right. The next thought was, If that’s bedrock, there can’t be much glacier left.” Letting his eye roam the periphery of the ice and visualizing mountain contours beneath the main mass of the glacier, Stock struggled to form a mental model in which the glacier maintained significant volume. He could not picture more than about 20 or 30 feet of thickness. Given the Lyell’s melt rate, it would disappear in four or five more years of drought. The shock of this realization forced Stock to confront what the data had been hinting at: The Lyell was no longer a glacier at all. Put another way, the Lyell Glacier was already dead, and Stock was the last person ever to study it.

Glaciers used to be fun, even thrilling. It’s hard to believe now, but there was a time when geology was much like genetics today, the cutting-edge inquiry that routinely delivered breathtaking insights that captivated the educated world. Many of those insights, starting in the mid-1700s, had to do with the age of the Earth, as people looking closely at rocks found evidence that our planet was a lot older than the 6,000 years suggested by the Old Testament — perhaps many millions of years older. For this reason, the 19th century is said to have discovered “deep time,” the astronomical and geological time scales that reach into pasts so distant that our minds struggle to imagine them. An academic cottage industry sprang up to reconcile deep time with Scripture — arguing, for example, that the Book of Genesis actually described two distinct geological periods separated by an immense span of time and the original authors simply left out those middle zillion years. As for how landforms like mountains and canyons got themselves made, religiously minded geologists inclined toward so-called catastrophist explanations consistent with the Bible’s depiction of mountain ranges created in a day — massive earthquakes, cataclysmic volcanic eruptions. Across Northern Europe, geologists documented curious scouring and scraping marks on bedrock, running in a more or less north-south direction, and decided they must be evidence of the flood that provoked Noah to build an ark. Charles Lyell, a Scottish geologist and namesake of the mountain and glacier, proposed a counter-theory known as uniformitarianism, under which landscapes were shaped by extremely slow-moving forces still active in the present day, like sedimentation on seabeds — and the rock-grinding effects of glaciers. Lyell’s contemporary, the Irish geologist John Tyndall, happened also to be an important player in the golden age of mountaineering, making first ascents in the Alps. His book The Glaciers of the Alps, published in 1860, was a hit with British men and women who traveled to Switzerland, where they walked out onto glaciers to learn about how all that ice carved and sculpted the mountains beneath and how those scouring and scraping marks on Northern European bedrock were likely caused by mile-thick ice sheets that had covered much of the Northern Hemisphere during some long-past ice age. Around the same period, a militia hunting indigenous people stumbled upon Yosemite Valley. Within three years, the first tourists entered Yosemite, and word got out across the United States and Europe that a new wonder of the natural world had been found at the western edge of the American empire. Leading American geologists like Josiah Whitney and Clarence King looked all over the Sierra for glaciers and speculated about how the great cliffs of Yosemite Valley came into being. Neither found glaciers, and Whitney carried the day with the catastrophist argument that Yosemite Valley’s floor had collapsed downward, leaving behind monoliths like Half Dome and El Capitan. John Muir was an unknown Yosemite hospitality worker at the time, a college dropout and Civil War draft dodger, but he’d studied geology at the University of Wisconsin. He knew about theories of the ice age and thought it plain as day that glaciers carved Yosemite. When contemporary experts mocked him, he hiked into the high country, identified the Lyell, and drove stakes into an ice field on the upper flanks of the adjacent Mount Maclure. Over 42 days in late 1871, Muir measured the downslope movement of his stakes to prove that this ice field was, in fact, a glacier, the first confirmed in the Sierra Nevada. He published the results along with descriptions of the Lyell and neighboring glaciers in articles for the New-York Tribune and Overland Monthly that made a curiously big splash and introduced the Lyell Glacier to the American public as both a remnant of the ancient engine behind Yosemite’s creation and still more evidence in our collective awakening to the soothing eternities of deep time.

Stock’s job has its mundanities, like middle-of-the-night phone calls from rangers explaining that, yet again, a boulder has tumbled onto a park road, and he has to jump out of bed and grab a flashlight and go look at the surrounding cliffs to evaluate whether more boulders might fall soon. That’s pretty much impossible with current technology, but somebody has to make a judgment — if only so rangers can know if they should close the road — so Stock does his best.

Mostly, though, Stock lives a mountain lover’s dream life. His research into the causes and patterns of rockfall allows him to make on-the-clock ascents of El Capitan and Half Dome. He also shares a cabin with his wife, Sarah Stock, who has the equally enviable job of Yosemite National Park Wildlife Ecologist — with mundanities of its own, like roadkill mammals that routinely appear in the family freezer, courtesy of well-meaning park employees who’ve heard that such corpses aid Sarah in research, which they do. The Stocks’ living room window looks directly up at Yosemite Falls. Bookshelves carry works by Wendell Berry, Henry David Thoreau, and, of course, Muir, who wrote such impassioned letters about glaciers to an older married woman he was courting that she felt compelled to tell him that ice ages horrified her and she much preferred flowers. (Muir insisted that glaciers were angels with folded wings; she replied, “My spirit was converted by your lovely sermon, but my flesh isn’t.”) The story of Muir’s insight into glaciers and Yosemite — the simple-hearted shepherd outdoing the professionals — struck such a cultural chord that it remains central to the legend of Muir as nearly every California schoolchild learns it: our very own long-bearded prophet who read the Book of Nature in the ice and then spread the word about the saving power of wilderness, ultimately convincing our forefathers to create national parks. For Stock, who grew up in a small Gold Rush town near Yosemite and read Muir in college before getting a doctorate in geology at the University of California, Santa Cruz, his work on the Lyell Glacier was more than just a chance to participate in a great research lineage. It amounted to living inside the origin myth of California’s secular religion — the faith that routine exposure to our mountains and coastline will lift our lives and spirits. The ice age, of course, by Stock’s time had long since come to be understood as not just one glacial period but a 2.5-million-year geological epoch known as the Pleistocene, during which glaciers advanced and retreated on a roughly 100,000-year cycle. Those regular cold periods seemed to have been caused mostly by standard fluctuations in Earth’s orbit and rotation and resulting changes in how much of the sun’s warmth reached the planet’s surface. Traces of at least four of those glacial cold periods had been identified in the Sierra Nevada. An early Yosemite geologist named François Matthes was responsible for identifying a more recent and peculiar glacial advance called the Little Ice Age, which started abruptly in about 1300, gave birth to the accumulation that became the Lyell Glacier, and got so cold in 1780 that New York Harbor froze solid. People walked from Manhattan to New Jersey. Sometime around 1850, two decades before Muir drove his stakes into the ice, the Little Ice Age ended with equally strange abruptness, and the Lyell began to retreat. Stock knew, in other words, that the retreat of the Lyell was the latest in a long sequence of glacial retreats reaching back through time. But he also knew that this retreat was different because the Earth should have kept right on cooling into a broader glacial advance consistent with orbital cycles. Instead, the cooling associated with the Little Ice Age, and therefore the growth and expansion of the Lyell, reversed by about 1900 — likely because the Industrial Revolution led to coal-burning and climate change. Geologists are not like wildlife biologists. Stock’s wife, Sarah, went into biology knowing that her work would involve bearing witness to innumerable deaths of individual animals, occasional exterminations of populations, and, given the age in which we live, even extinction events. Geologists, by contrast, go into their fields with a tacit commitment to the study of change but across vast time scales and involving such indisputably inanimate materials that it amounts to the study of no change in the context of no life and, therefore, of no death. “There’s this sense among geologists that you build on the work of others,” Stock says, “and others will build on your work, and that goes off into infinity.” Nothing in Stock’s professional life has prepared him to be the man who presides over the last days of a glacier, much less the end of an entire geological epoch in which glaciers have come and gone. “I suppose I was a little naive, thinking geologists didn’t have to deal with this,” he admits. “I’ve started to interact with geologists around the world, scientists who’ve dedicated their lives to studying glaciers and ice fields, and it’s tough for all of us to realize that we’re studying a system in decline, the demise of the cryosphere, that frozen part of the world.” In bedtime discussions with his wife, Stock says, “She’s reminded me that all it would take to restore the glaciers would be a change in the climate — more snow and cold enough temperatures for it to pile up. My response is, ‘Tell me when we’re going to have the next ice age.’ I feel like she’s lucky in that she and other biologists can be more hands-on in fighting to restore a species. I feel sort of helpless.”