Book Review

Marjorie Burns, in "Cataclysms on the Columbia" explains the geological history of this region so beautifully that I will quote two passages from the book verbatim.

p.77

The Columbia Plateau of Washington originated between 16 and 6 million years ago, when the greatest outpouring of lavas recorded in the history of North America spread out across 80 thousand square miles of western Idaho and eastern Washington and Oregon. The lava flows swept westward down a succession of broad valleys into the sea off northwestern Oregon, creating, from Astoria to Lincoln City, the rugged headlands which Oregonians call their own but which in fact emanated from the fissures as far east as Idaho! During this time, the course of the ancestral Columbia River was repeatedly shouldered to the north and west by these enormous basalt floods, finally incising its present course near the northern edge of the flows. This up to two-mile thick pile of black basalt, consisting of nearly 200 separate lava flows, is collectively called the 'Columbia River Basalt Group'. But the flows with which we are concerned in this book are called the 'Yakima Basalt'.

Weighted down by the over 90 thousand cubic miles of basalt, the earth's crust gradually sank for a period of 4 million years and, in doing so, produced the saucer-shaped Columbia Basin, which slopes inward from elevations of two to four thousand feet above sea level around the periphery to less than 500 feet at the lowest point in the Pasco Basin. About 12 million years ago, the pressure of this collapse began to warp the plateau into the numerous east-west sharp folds that characterize the western edge of the basin, just as the crust on a pudding wrinkles in cooling. Farther down the Columbia River in Oregon, the same processes of faulting and downwarping formed similar but smaller basins at The Dalles, Mosier, Hood River and Portland.

p.79

During the long two million years of the last ice age, the great Continental Ice Sheet advanced from Canada into the northern United States and retreated again, four times. The periods of ice advance have been named the Nebraskan, Kansan, Illinoian and Wisconsin glaciations, after good exposures of their deposits in those states. Each advance lasted from tens of thousands to a few hundred thousand years. During the melting and recession of the ice, long interglacial warm and dry periods intervened, some perhaps even warmer than at present. Records of the first three of these advances in the Northwest are scarce, but the record of the last glaciation, the Wisconsin, is abundant.

So much water was stored on land as ice during the glacial advances that the level of the sea was lowered by more than 300 feet, exposing wide coastal plains on the continental shelves of the world. The coastal plain off Oregon, for example, was at least 25 miles wide; off the Atlantic coast it was 150 miles wide. During the recurring warm interglacial periods, however, melting ice released its water and the sea repeatedly rose 150 feet or more above its present level. If this happened today all the major coastal cities of the world would be drowned! In the Pacific Northwest, as Thomas Condon suggested in 1871, high sea levels probably inundated the Willamette Valley with marine water during times of glacial melt, and the same would happen again if the ice sheets now covering Antarctica and Greenland were to melt once more.

During the interglacial warm periods, the climate was so arid that the glacial outwash sediments in the valleys and basins along the course of the river (mostly rock ground up by the ice) were picked up and swirled about by violent dust storms all over the Northwest. In the Columbia Basin these deposits of wind-blown glacial dust and silt (loess) created the 'Palouse Formation', with deposits up to 150 feet in thickness. It forms the fertile farmland that makes the Columbia Plateau one of the great food-producing areas of the world, its deep, rich soil created by the inhospitable agents of water, ice and wind. Farther west, the uplands around Portland are mantled with similar loess, known as the 'Portland Hills Silt'.

The last major advance of the Continental Ice Sheet began about 70,000 years ago in the Northwest, and it too consisted of several periods of glacial growth and retreat. Early progressions of ice probably resulted in large recurrent floods 60 and 50 thousand years ago. These were possibly even larger than the Bretz Floods which resulted from the last advance of ice between 15 and 12.8 thousand years ago, which were of such a magnitude that most of the evidence left by the earlier floods has been destroyed.