Petrography

A scant distance from my toes, the world drops off. I'm staring across the canyon created by the Missoula Floods and toward Dry Falls in Washington State, intense sunlight highlighting the water-formed cliffs. The sun turns the small pools of water at the canyon base to bright, improbable blue, the color of my mother's eyes. Dry Falls, I have learned, is the largest waterfall in the world. Or it was when it was wet. It hasn't been wet since the last time Glacial Lake Missoula flooded, ten thousand years ago. When it was wet, it was ten times larger than Niagara Falls—about 400 feet tall and 3.5 miles wide; Niagara is only about 165 feet tall and 2,500 feet wide. It takes some imagination to see Dry Falls overflowing with water, floating boulders as big as houses down the river, but this picture can be conjured. It just takes a fair amount of staring at the bone-dry, stone-dry cliff.

And so I stare for a while, not completely successful at imagining Dry Falls wet, the canyon in front of me filled with water and splashing over my feet. But I try.

The story of the Missoula Floods starts at the beginning of time with the geologic episodes that formed the Pacific Northwest. We need to jump to thirty-seven million years ago, when the Juan de Fuca Plate—one of the earth's tectonic plates—was shoved under the edge of the North American continent, resulting in the volcanic Cascade Range. Between twelve and seventeen million years ago the volcanoes—both mountain volcanoes and fissure volcanoes—erupted, and each eruption spread thick basalt over the Columbia Plateau. This is the next layer of the story, and the compelling nature of this story is in its layers.

As the millennia progressed, life and land flourished. Ice ages came and went. Species appeared and disappeared. Then, in the last two million years, came the glaciers that shaped the actual texture of the landscape. The volcanic activity laid down the raw materials, and the glaciers shaped it—clay and hands, so to speak. Art does not come without pain or price.

Then came the real action, the catalyst itself. Between ten and fifteen thousand years ago, during the last ice age, a finger of the Cordilleran Ice Sheet from Canada moved down and dammed up the Clark Fork River in northern Idaho—this created Glacial Lake Missoula, flooding most of western Montana. It's important to be accurate about this: the Missoula Flood was not a single event, but one that probably happened every 50 years or so, for at least 2,500 years. The ice dam would break and reform, allowing the water to build up again until the pressure of the water crushed it. They estimate that the ice dam was two thousand feet high, which created a lake that was two hundred miles long, two thousand feet deep, and contained more water than Lakes Erie and Ontario combined. More than five hundred cubic miles of water, they estimate. Obviously, the pressure of all that water would become too much for the ice dam, and the dam would break, sending all that water straight to the Pacific. And nothing in its way would stop it. Ice is not concrete, after all.

Perhaps over a period of days, the ice dam would have made noises. Creaking. Groaning. Cracking. Perhaps even some popping. All the kinds of animate noises humans attribute to ice. I'm a Minnesotan—I know what ice sounds like. These noises would have been the only warning that the dam was about to break. When it did break, it was not a small leak. The ice broke. Probably shattered into pieces from the force of the water. The water, suddenly free, rushed toward the Pacific at a speed ten times the combined flow of all the rivers in the world, shooting downriver at a speed of sixty-five miles per hour. At this speed, the lake would have drained in perhaps as little as forty-eight hours. A lake the size of Erie...