Johnny thought of the baby moose and cringed inwardly.

“During the last ice age, glaciers advanced over much of the known land masses of the earth. They are now in recession. Look,” she said, pointing. “Glaciers leave rocks behind, every size from sand to boulder. What’s easiest to grow on rocks?

Come on, we were talking about this on the hike up.“

“Lichens,” Betty said.

“Mosses,” Vanessa said thickly, wrestling the peanut butter into submission.

“Very good. Yes, mosses and lichens, which begin the process of breaking down the rocks to form soil. Not much, at first, but some, enough for-what, to take root?”

“Flowers!” cried Andrea Kvasnikof.

“And grasses,” Johnny Morgan said.

“Like lupine,” said Andrea, who had her eye on Johnny Morgan, if only Vanessa Had-no-right-to-exist Cox would get out of her way.

“Yes, like lupine,” Ms. Doogan said. “Talk to me about lupine. Anybody.”

“They’re purple,” Andrea said after a brief pause.

“They’re members of the legume family,” Vanessa said.

“Which means?”

“Legumes fix nitrogen in the soil.”

“And?”

“Nitrogen makes the soil more habitable for more complex plants,” Betty said.

“Like?”

“Like shrubs.”

“Give me an example of shrubs.”

“Willows.”

“Alders.”

“Cottonwoods!”

“Cottonwoods are trees, doofus.”

“And after the shrubs, what?”

“Trees!”

“Spruce trees!”

“Hemlocks!”

“Birches!”

“Christmas trees!”

Ms. Doogan waited for the laughter to die down. “Think about this, boys and girls,” she said, waving a hand at the glacier.

“Seventy-five years ago? This little strip of beach we’re picnicking on was under the glacier. That’s right, under a big slab of ice just like that one. Your grandmas and grandpas couldn’t have had a school picnic here.” Eyes widened, measured the distance between the face of the glacier, a wall of ice a hundred feet high, and their beachfront picnic site. “Mother Nature doesn’t waste time in the Kanuyaq River basin. How many of you remember last summer, when Grant Glacier thrust forward right over the lake?”