Oregon Public Broadcasting

New maps indicate that Portland was built on geologic catastrophes.

mthood_small.jpgMt. Hood snow captured by Web cam.

State geologists have used laser technology to determine that 80 percent of the greater Portland area's top soil is young. Much of it came to the Rose City by way of floods and landslides. Researchers said young soils are more likely than older layers to liquefy in an earthquake, or give way in a landslide.

Chief geologist Ian Madin said that's less true of some young soils, like the gravel deposits near the Clackamas River. And he says while the Columbia and Willamette river banks could be riskier, it depends on how deep the top layer goes. He said more research is needed to answer that because the new maps don't go that deep.

Madin said one clear surprise from the maps released Monday is the number of historic landslides.

"Of course, we knew that there were landslides here and there throughout the Portland area, but we didn't know there were hundreds, or thousands of these landslides, which is what we've ended up mapping," Madin said.

Madin said the new maps aren't just about risk. He said they can also point out interesting geologic features such as old volcanic cones scattered throughout the area.

This story originally appeared on Oregon Public Broadcasting.

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