PSU Magazine Spring 1999

, pour, the land "failed" in more than 700 sites in the Portland tri-county area, says Bums, who recently completed a year– long study of the damage for Metro. Those sites included 17 homes that were "red-tagged" as damaged beyond fixing. Sixty-four others got yellow tags-uninhabitable until repairs were made. The inventory mapped fo ur areas that are highly susceptible to sliding earth and identified the causes of specific slides, including road cuts on steep slopes and improperly designed fills. Almost 10 percent of the slides undermining houses happened because of overflowi ng gutters, broken drain pipes or poor drainage systems, Bums' study points out. F or deeper causes, look to ancient geology. When you're considering the purchase of a home, what happened 50 million years ago is the last thing you're likely to think about; but that's when the tory of the "solid" earth under our present-day community begins. Think of the ground underlying much of western Washington and parts of northwestern Oregon as a giant, two-layer cake and the story of its making as a three-act play. In the first act, the underlying layer, known as the Cowlitz Formation, took shape when the region lay submerged under a shallow, inland sea. Accumulating over 10 million years, fine silts, sands, and clays settled on the ea beds and salt marshes. Now and then, volcanoes spewed sheets of lava on top, creating a sturdy base of shale, sand– stone, and basalt. Thrust out of the sea some 35 million years back, this giant slab eroded over additional eons. Then, in Act Two, the ancestral Columbia began dumping trillions of tons of gravel over this slab, forming the upper leve l of the cake. Called the Troutdale Formation, it's almost The Aldercrest development in Kelso, Washington, was built on an old slide that is again on the move. 2,000 feet thick beneath Portland. In Act Three of the earthy drama, some two million years ago, the Cascade Range began rearing up, lift– ing much of the land beyond flooding range and wrenching the horizontal geological layers into bends, folds, and hills. The once-flat layer cake came to resemble a froth of frosting. SPRING 1999 PSU MAGAZINE 15