Boulders, Braids, and J Harlan Bretz
Earth's history is something like that of a foot soldier---days of boredom punctuated by moments of terror. The planet feels neither boredom nor terror, but its past is indeed composed of long periods of monotony broken by sudden upheavals.
One such break in earthly monotony appears in Cataclysms on the Columbia, a book sent by a geologist who knows my fondness for reading about catastrophe, and who also understands that my readings in popular geology should extend beyond books by the estimable John McPhee. Written by two geologists and a professor of English, Cataclysms on the Columbia describes the great outburst floods that inundated and shaped thousands of square miles in the Pacific Northwest.
The authors approach their subject by treating it almost as a detective story, one following the career of their chief sleuth. J Harlen Bretz (they neither put a period after his first initial nor explain why it isn't there) was a superb field geologist, one determined to see what was there rather than what theory predicted should be there---a rare and valuable talent in any scientist.
Two of the clues with which Bretz worked typify his logical approach. First was the matter of what geologists call "erratics"---solitary boulders that don't seem to belong where they are found. Erratics arrive by many routes. Sometimes they even fly through the air---in Alaska, boulders originating in the throat of Mt. Wrangell can be found south of Glennallen, products of that volcano's prodigious belches. But the boulders from Mt. Wrangell are the same composition as the rock of Mt. Wrangell, so their source can be identified easily. The erratics in the Columbia and Willamette river valleys were not so easily explained. They were not eruptive products; they weren't meteorites. They were not rounded, as if washed up by some erratic swoosh of the rivers.
It's said that when you've eliminated the impossible, then what's left---no matter how improbable---is the answer. Bretz eliminated the impossible, and what was left to explain the scattered rock chunks up to 20 feet in diameter and 200 tons in weight was---icebergs. He reasoned that the boulders had been frozen into glaciers, then transported on water until the ice melted sufficiently to drop the erratics far from the mountains where they originated. This does happen; for example, undersea erratics from Canada's arctic islands can be found where ice dropped them north of Alaska.
Even in the 1920s, when Bretz began his research in eastern Washington, speculation about great floods in the area had been around for more than 40 years. Still, people couldn't accept icebergs borne on giant rivers: Bretz had realized that the so-called "channeled scablands" in the arid region northeast of the Columbia River Gorge were actually the remains of river beds, ones braided and interlaced like so many of Alaska's rivers today but on a far grander scale than even the Yukon in flood.
The outline Bretz eventually created explains the peculiar geology in that part of the world. Fingers of the ice sheet covering the middle of North America dammed rivers, producing huge lakes. Eventually the rising water lifted the ice, breaking the dam and sending hundreds of cubic miles of water surging over the countryside. The water carved away soil, left gigantic sandbars, and devastated all life in its path: at the present mouth of the Columbia Gorge, floodwaters crested at about 1000 feet.
Ice not swept away on the flood settled down, recreating the dam. And so it went, with outburst floods occurring about every 50 years for perhaps 2000 years.
Bretz's remarkable work was built painstakingly over years, but he had to fight for its acceptance over decades. Finally, in 1979, the geological establishment acknowledged his views by awarding him the prestigious Penrose Medal. Bretz was in his late 90s, and had been holding the line for more than 50 years---but both, after all, are eyeblinks (blinks of the eye?) on the geologic timescale.