Geologic Formations

Columbia River Basalt Group

The Tualatin Mountains are formed of a thick sequence of basaltic lava flows that are broadly designated as the Columbia River Basalt Group. These lava flows and the underlying older marine sedimentary rocks arch upward into a broad fold (an anticline), forming the current mountain feature.

Individual flows may reach 100 feet or more in thickness. The total thickness of the basalt within the park area is in excess of 1,000 feet, made up of several separate flows.

Portland Hills Silt

Portland Hills Silt, a thick deposit of yellowish-brown fine-textured soil, covers much of Forest Park. These soils are thought to have originated from out-wash deposits along the Columbia River Basin to the east, which were later wind-deposited over much of the park.