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It was the slowest trip I ever took. But I felt as though I was seeing Glacier's runoff for the first time, too, watching through their eyes as the park, my Montana backyard, softened into the long-light days, liquefying more than half a year's hoard of crystals to swell rivers bound across North America for the Pacific, the Gulf of Mexico, and Hudson Bay.

This is where, around 75 million years ago, tectonic forces buckling Earth's crust sent a slab of rock miles thick sliding eastward on top of the strata next door. The overthrust block is a layer cake of cream-colored limestone and red and green mudstones formed between 1.5 billion and 800 million years ago in a shallow inland sea. Many rocks reveal patterns of ripples, mud cracks, and even raindrops, as if raised from the shores only yesterday. A few preserve mats and mounds of cyanobacteria mixed with silt, all turned to stone. As fossils go, these microbe colonies, termed stromatolites, aren't especially striking—they look like smooshed cabbage heads—but they represent some of the oldest organisms on record. More important, they may have been the first to practice photosynthesis, manufacturing sugar for food with the help of energy captured from the sun.

The waste product given off by this invention was oxygen. As it got pumped into early skies rank with carbon dioxide and methane, photosynthesis began to change the atmosphere. That in turn transformed the course of life. Ultimately, the humble clumps of cyanobacteria and their successors made inhaling sweet air possible—made you and me possible, and the mountain-goat nannies with young single-filing along a blade-thin ridge, and the hoary marmots eyeing my trail snacks from nearby boulders. It's the sort of thing you think about at 9,000 feet (2,700 meters) with your rump on an overhang built from primeval ocean bottom and nothing around the rest of your body but blue, blue breathable sky.

Eon after eon, weather scuffed the uptilted stone layers and poked at their weak spots. Then came the ice ages, and erosion bent to its work. Over the past two million years, glaciers as much as a mile thick flowed over all but the highest points, reaching tentacles of frost into the smallest crevices of rock formations and pulling hefty chunks away as the white juggernauts growled and oozed relentlessly on. The Rockies meanwhile kept rising; not much, maybe the thickness of a snow bunting's wing each year, but ceaselessly. And when the tide of ice finally withdrew, the Crown of the Continent emerged spectacularly cut and polished, its peaks taller than ever and their sides far steeper, soaring above valleys rasped into broad U shapes with open vistas where narrow canyons had been.

Small alpine glaciers persisted on summit shoulders. Others lay cupped in cirque basins or stretched beneath headwalls too high and sheer for the sun to climb over even in midsummer. About 150 glaciers existed when the park was established in 1910. Today, with human activities spewing carbon dioxide and methane as if we were intent on re-creating Earth's ancient atmosphere, a warming climate has reduced the number of moving glaciers to fewer than 30. Dan Fagre, a U.S. Geological Survey ecologist working in the reserve, says, "The last one will probably disappear by the year 2030, tops." How does Glacierless National Park sound?

Fagre has more pressing questions as he documents decreasing snowpacks, earlier spring runoffs, longer growing seasons, and tree lines marching uphill into former subalpine and alpine habitats. For example: What's in store for the wildlife communities this reserve is supposed to protect? What happens downstream to farmers and ranchers dependent on irrigation, to communities needing drinking water, to fishermen—not to mention the fish—even to distant barge operators, once the glaciers and permanent snowfields that reliably provided water through late summer are gone?

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