As we walk through the misty forest, Jon throws out numbers, casual as baseball statistics, for whoever will listen. Glacier numbers: Seventy-five percent of the glaciers in the Lower 48 are in the North Cascades, 312 in the national park alone, at least according to last count; researchers estimate that since the late 1800s, the park has lost 40 percent of its total ice pack. Climate numbers: Over the past 2 million years we've had several long ice ages, with intervening warm periods lasting 10,000 years or so. Now it's been 11,000 years, which means it's time to start cooling, but we're warming instead. The kids nod solemnly. They know this, at least in theory, and they're learning more about it each day they're here.

After three miles and lunch, we reach a stretch of trail atop a lateral moraine. The ground drops steeply at our toes, loose rock and dirt skittering into a wide cobbled creek bed, the space where the glacier used to be. Suddenly the number that stands out most starkly, of the many numbers, is the smallest: 100. That's how long ago the glacier was here -- right here! -- less than 100 years. It's not hard to imagine how it looked, white and full. Confronting its absence is like staring into an empty swimming pool.

The group settles on rocks along the trail, and Jon begins his spiel: The more the climate warms, the more the glaciers melt, which causes oceans to rise, which causes the landscape to change. As he speaks, climbers scurry past hauling packs the size of Igloo coolers and a wet film settles on jacket sleeves, not quite rain.

When it's time for questions, a student pipes up.

"How did you end up here?" she asks.

She sweeps her arms wide toward the summit hovering in the cloud and tiny rivulets trickling through the meadows, the pink phlox and the green trees, the impossibly gorgeous whole of it. Other kids crane their necks and grin.

All eyes are fixed on Jon.

He took a year off after high school, he explains, and worked in a garment factory for $2 an hour, hard boring work in Wisconsin. During that year he also read a book called Sand County Almanac

"Aldo Leopold," he says. "That book changed my life."

Now the kids nod exuberantly. They know this book, or at least some of them do, and they like Jon's story. No matter what happens, this month in the North Cascades -- camping, hiking, canoeing -- will change their lives.

Everyone is silent for a moment as the clouds lift and the lower glacier is visible in a swatch of sun: blue-tinged ice and smooth sloping snow and distant jagged crevasses like scratches on a mirror. 

Glaciers move. That's what differentiates them from snowfields. The heavy ice on top pushes out the ice underneath, like toothpaste. Even as they are melting, they are moving. The Easton, Jon says, moves five or six inches a day. From here, that's hard to believe. The glacier seems so settled, so permanent, so static. But it's not, of course. Nothing is.