Document Actions
Geology of Forest Park
Information on the geology of Portland's Forest Park.
Summary
Forest Park is situated on the east flank of the Tualatin Mountains. The park extends upslope from the Willamette River's flood plain to the crest of the Tualatin Mountains, an average distance of 1.5 miles. Elevations vary, from 75 feet along the Willamette River to 1,100 feet at the crest of Tualatin ridge.
The east flank of the Tualatin Mountains was formed by the dipping lava flows of the Columbia River Basalt Group. Forest Park's landscape is deeply dissected by streams originating along the crest and draining eastward to the Willamette River. Stream gradients recach as much as 11.5 percent, or 600 feet per mile. Inter-stream areas range from sharp ridges to narrow elongate plateaus. Benches in the eastward sloping mountain flank are also common.
Landslides have been a major factor in the recent geologic history of Forest Park. A fine layer of Portland Hills silt overlies Columbia River basalt, creating a very unstable formation - especially when wet. Considering the somewhat soggy winter climate of western Oregon, this makes for a poor foundation material. Ironically, the propensity for landslides has also been a significant factor in the preservation of Forest Park. Throughout Portland's history, a number of development schemes have been thwarted by unstable slopes.